Fuel, Air and Ignition Systems Lesson Test

Answer: D

Correct answer: Meter fuel into the intake port or cylinder. An injector is an electrically operated valve held shut by a spring; when the ECU energizes its solenoid, it opens and sprays a fine mist of pressurized fuel. Because fuel pressure is held steady, the amount delivered depends mainly on how long it stays open, called pulse width. Port injectors spray toward the intake valve, while direct injectors spray into the cylinder. The oxygen sensor is a common mix-up: it measures the result, while the injector carries out the command.

• A: An exhaust oxygen sensor reports this feedback to the ECU; it is not a fuel injector.
• B: The plug gap is where the ignition discharge occurs; an injector does not generate the spark.
• C: A radiator transfers heat from coolant to the surrounding air rather than metering fuel.
• D (correct): An injector is the commanded actuator that sprays a measured amount of fuel into an intake port or directly into the cylinder.

Answer: A

Correct answer: Air filter. An engine draws in a very large volume of air, and fine dust in it acts like an abrasive on cylinder walls, piston rings and valves. The air filter sits in a housing at the start of the intake path, where a pleated paper or fabric element traps particles before they reach the throttle and intake manifold. A clogged filter restricts airflow and can reduce power. The catalytic converter is tempting because it also cleans gas, but it treats exhaust leaving the engine, not air entering it.

• A (correct): An intake air cleaner traps specified particulates before they enter the cylinders.
• B: A catalytic converter treats exhaust gases after combustion, not incoming air.
• C: A spark plug initiates ignition and does not act as an air filter.
• D: A fuel pump transports gasoline rather than filtering incoming air.

Answer: A

Correct answer: Spark plug. The plug threads into the cylinder head with its tip exposed in the combustion chamber. The ignition coil sends it a pulse of many thousands of volts, enough to make current arc across the small air gap between the center and ground electrodes, and that arc lights the compressed mixture. The crankshaft position sensor is the tempting distractor: it tells the ECU when to fire, but it only sends a timing signal and never produces the spark itself.

• A (correct): The spark jumps at the plug electrodes in the combustion chamber.
• B: A fuel tank stores gasoline but has no in-cylinder spark gap.
• C: A fuel rail distributes liquid fuel to injectors, not high-voltage sparks.
• D: A position sensor supplies timing information rather than making the spark discharge.

Answer: C

Correct answer: Fuel contacting sufficiently hot compressed air. Compressing a gas quickly raises its temperature. Diesels use high compression ratios, so the air in the cylinder gets hot enough to exceed diesel fuel's self-ignition temperature; when fuel is injected near the end of compression, it ignites on its own. That is why diesels are called compression-ignition engines and need no spark plugs. A tempting error is assuming every engine uses a spark; glow plugs found in some diesels only help with cold starting.

• A: Conventional diesel combustion does not rely on a recurring gasoline spark plug.
• B: A starter rotates the crankshaft for starting; cooling-fan motion is not the ignition mechanism.
• C (correct): Diesel compression ignition relies on hot compressed air and properly injected fuel.
• D: The catalyst treats exhaust after combustion, not the primary fuel ignition in a cylinder.

Answer: B

Correct answer: Coordinate fuel and ignition using sensor information. The ECU is a computer that works in a repeating loop: sensors report conditions such as airflow, crank position, coolant temperature and exhaust oxygen; the ECU compares them with stored maps; then it commands actuators such as injectors and ignition coils. Think of it as the brain, with sensors as nerves and actuators as muscles. A common misconception is that the ECU does mechanical work itself, but it only sends electrical signals that other parts act on.

• A: Air filtration occurs in an air-cleaner element, not inside the control module.
• B (correct): The ECU interprets sensor data and sends fuel and ignition commands to engine actuators.
• C: Gasoline is stored in a tank; the ECU is an electronic controller.
• D: A mechanical valve train opens valves; the ECU can influence actuators in specific variable systems but is not a universal camshaft substitute.

Answer: D

Correct answer: MAF sensor. MAF stands for mass airflow. A common design passes intake air over a heated wire or film; the more air mass flows past, the more electrical current is needed to keep the element hot, and that current becomes the signal. The ECU uses the measured air mass to calculate how much fuel to inject for the target air-fuel ratio. Do not confuse it with the MAP sensor, which measures manifold pressure rather than airflow directly.

• A: The starter cranks the engine instead of measuring intake-air mass.
• B: A fuel-level sender indicates quantity in the tank rather than incoming air mass.
• C: An ignition coil generates high-voltage spark conditions and does not measure air.
• D (correct): MAF refers to mass airflow measurement and provides air-charge data.

Answer: D

Correct answer: High voltage to fire the spark plug. The battery supplies only about 12 volts, far too little to jump a spark plug gap under compression. A coil works as a transformer: current through its primary winding builds a magnetic field, and when that current is switched off the field collapses, inducing a very high voltage in the secondary winding, which has many more turns. That pulse arcs across the plug gap. The tempting error is linking the coil to fuel delivery, which is the job of the pump and injectors.

• A: Fuel pressure is supplied by a pump and delivery system, not an ignition coil.
• B: Coolant movement belongs to the cooling subsystem, not the ignition coil.
• C: An oxygen sensor supplies exhaust feedback; a coil is an ignition actuator.
• D (correct): A coil transfers stored magnetic energy into a high-voltage pulse that jumps the spark plug gap.

Answer: C

Correct answer: The fuel's resistance to knock. Octane rating describes a gasoline's resistance to knock. Higher-octane fuel tolerates more compression and spark advance before self-igniting; it does not contain more energy.

• A: Octane is not an energy content figure; premium does not pack more energy into the fuel.
• B: Engine oxygen comes from intake air; octane has nothing to do with oxygen content.
• C (correct): Octane rates how well gasoline resists self-igniting under compression and spark advance.
• D: Flow through injectors depends on fuel pressure and pulse width, not octane.

Answer: B

Correct answer: Distribute incoming air or charge toward cylinders. The intake manifold is a set of runners branching from a common chamber, usually fed through the throttle body, with one runner leading to each cylinder's intake port. Its job is to share the incoming air evenly so every cylinder breathes alike; in port-injected engines the injectors sit near the ends of these runners. The usual mix-up is with the exhaust manifold, which does the reverse job, gathering burned gas from each cylinder into one outlet.

• A: A tank sender measures fuel quantity, not the intake manifold.
• B (correct): An intake manifold routes the incoming charge toward intake ports.
• C: Collecting outgoing burned gases is the role of the exhaust manifold.
• D: High-voltage ignition depends on the ignition system, not the manifold.

Answer: C

Correct answer: Oxygen sensor. The sensor threads into the exhaust pipe and compares the oxygen in the exhaust with the oxygen in outside air, producing a voltage signal. Plenty of leftover oxygen means the mixture was lean; very little means it was rich. The ECU uses this feedback to trim fuel toward the ideal ratio, which also lets the catalytic converter work efficiently. The fuel injector is the tempting distractor because it acts on the reading, but it only delivers fuel and measures nothing.

• A: Injectors meter gasoline into an engine, while exhaust oxygen is reported by a separate sensor.
• B: An air filter treats incoming air upstream of combustion.
• C (correct): The oxygen sensor samples exhaust conditions for engine and emissions control.
• D: A throttle plate adjusts an intake opening rather than reporting exhaust oxygen.

Answer: D

Correct answer: Open-loop control. In open loop, the ECM sets fueling from stored tables and sensor inputs but ignores exhaust oxygen feedback. Engines run this way during cold starts and often at wide-open throttle, when a richer mixture protects the engine.

• A: Speed-density is a way to estimate airflow from MAP and rpm; it does not describe the presence or absence of feedback.
• B: Closed loop means the ECM reads the oxygen sensor and trims fuel from that feedback, which is not happening here.
• C: Knock control adjusts spark timing from the knock sensor; it does not describe how fuel is set without feedback.
• D (correct): Setting fuel from tables and inputs without oxygen-sensor feedback is open-loop control, common at wide-open throttle.

Answer: D

Correct answer: 176.4 g. Rearrange the air-fuel ratio to solve for air mass:

• A: This adds the ratio and the fuel mass, which is not how a mass ratio works.
• B: This adds the 12 g of fuel to the air mass; the question asks for air alone.
• C: This divides the fuel mass by the ratio instead of multiplying the ratio by the fuel mass.
• D (correct): Air mass equals AFR times fuel mass: 14.7 × 12 g = 176.4 g.

Answer: B

Correct answer: Absolute pressure in the intake manifold. Absolute means measured from a perfect vacuum rather than from outside air pressure. At idle with the throttle nearly closed, the pistons pull the manifold well below atmospheric pressure, so MAP reads low; at wide-open throttle it approaches atmospheric pressure, or exceeds it with a turbocharger. The ECU uses this reading as a measure of engine load. The tempting mix-up is the MAF sensor, which measures air mass flow instead of pressure.

• A: An exhaust temperature sensor, when present, measures temperature rather than MAP.
• B (correct): MAP means manifold absolute pressure and reports intake pressure.
• C: Fuel level is a different measurement from intake-manifold pressure.
• D: Spark plug gap voltage belongs to ignition diagnostics, not MAP measurement.

Answer: A

Correct answer: 14:1. An air-fuel ratio always compares air mass to fuel mass, air first, so divide the air by the fuel: 280 g ÷ 20 g = 14, meaning 14 grams of air for every gram of fuel. The grams cancel, leaving a pure ratio. A common trap is adding the masses to get 300, which is the total mixture rather than a comparison. As a check, 14:1 is close to gasoline's chemically ideal ratio of about 14.7:1, so the answer is realistic.

• A (correct): Divide air mass by fuel mass: 280 ÷ 20 = 14, giving 14 parts air to one part fuel.
• B: The 7:1 figure results from an unrelated partial division and understates the given air-per-fuel mass.
• C: The number 20 is the fuel mass in grams, not the quotient of the given masses.
• D: Adding the two masses gives 300 g of total mixture, not an air-to-fuel mass ratio.

Answer: B

Correct answer: The mixture is rich. A traditional zirconia oxygen sensor switches sharply: high voltage, about 0.8 to 0.9 V, means rich exhaust, and low voltage, about 0.1 to 0.2 V, means lean. A reading stuck near 0.85 V indicates a rich mixture.

• A: Near stoichiometric, a zirconia sensor switches back and forth rather than holding one high value.
• B (correct): A traditional zirconia sensor outputs a high voltage, around 0.8 to 0.9 V, when the exhaust shows a rich mixture.
• C: A lean reading from this sensor type is low, near 0.1 to 0.2 V.
• D: The downstream sensor is the one mainly used to judge the catalyst; a steady high upstream reading points to mixture.

Answer: A

Correct answer: Lambda is about 0.86 and the mixture is rich. Lambda compares the actual air-fuel ratio with the ideal one: actual ÷ stoichiometric, here 12.6 ÷ 14.7 ≈ 0.86. A lambda of 1 is chemically correct; below 1 means less air than the fuel needs, so the mixture is rich, and above 1 means extra air, or lean. The common trap is flipping the fraction to get about 1.17, which reverses the conclusion. A memory hook: lower lambda, less air, richer mix.

• A (correct): Lambda = 12.6 ÷ 14.7 ≈ 0.86, below 1, indicating rich operation.
• B: Reversing the fraction generates about 1.17, but lambda uses actual divided by stoichiometric.
• C: Lambda 1.00 would require actual AFR equal to the specified stoichiometric ratio.
• D: Adding the two AFR numbers does not calculate lambda or show fuel absence.

Answer: A

Correct answer: Crankshaft position sensor. The sensor faces a toothed wheel on the crankshaft, usually with a missing tooth or gap as a reference mark. Each passing tooth produces a pulse: counting pulses gives crank angle, the gap marks a known position, and the pulse rate gives rpm. The ECU needs this to time sparks and injections. A tempting mix-up is any other sensor in the engine bay, but temperature sensors report heat, not rotation, and the camshaft sensor mainly identifies the cycle phase.

• A (correct): A crank sensor sends positional pulses that support timing and rpm calculations.
• B: A radiator pressure cap regulates coolant system pressure rather than reporting crank position.
• C: The fuel filler neck accepts fuel into the tank and does not provide timing data.
• D: An intake air temperature sensor reports how warm the incoming air is, not crank angle or speed.

Answer: B

Correct answer: It helps identify the four-stroke cycle phase. In a four-stroke engine the crankshaft turns twice per cycle, so a piston at TDC could be ending either compression or exhaust, and crank data alone cannot tell which. The camshaft turns at half crankshaft speed, once per cycle, so its position marks which revolution is which. With both signals the ECU knows when a cylinder is near the end of compression and can fire the spark and time injection for the correct stroke.

• A: Sensor feedback does not enlarge gasoline storage volume.
• B (correct): Cam phase helps assign events to the proper revolution and cylinder.
• C: A sensor does not mechanically replace valves in ordinary engines.
• D: Cam position information does not chemically convert exhaust into oxygen.

Answer: D

Correct answer: Closed-loop fueling correction. A control loop is closed when the result of an action is measured and fed back to adjust the next action. Here the ECU injects fuel, the oxygen sensor checks the outcome in the exhaust, and the ECU trims the next injection richer or leaner. Open-loop control, by contrast, follows preset values without checking results, as engines often do right after a cold start before the sensor warms up. A home thermostat works the same closed-loop way.

• A: Piston compression is a physical engine stroke, not a feedback control mode.
• B: A sensor-driven ECU adjustment is not the same as a sensor-free carburetor.
• C: A fuel rail supplies fuel, not an electrical spark command.
• D (correct): Measured exhaust feedback influences a subsequent command, defining a closed-loop feature.

Answer: C

Correct answer: Near the end of the compression stroke. Burning takes a short but real time, so the spark fires a few degrees before top dead center while the charge is tightly compressed. Peak pressure then arrives just after TDC, as the piston starts moving down, giving the strongest push on the power stroke. Firing during intake or exhaust would waste fuel or ignite gas that cannot do useful work. Recognize the pattern: ignition comes late in compression, and the power stroke follows.

• A: Ignition must be timed; a continuous gap discharge at all angles is not the basic operating cycle.
• B: Ignition is coordinated with the compressed charge, not arbitrarily fixed to mid-intake.
• C (correct): Spark initiation usually occurs near the end of compression so pressure can develop for power.
• D: The exhaust stroke removes spent gases rather than initiating the main power event.

Answer: C

Correct answer: Retarded by 6 crankshaft degrees. Spark timing is counted in crankshaft degrees before TDC, so a larger BTDC number means an earlier spark (advance) and a smaller number means a later spark (retard). Going from 20° to 14° BTDC is

° later. A common trap is calling any change advance, or adding the two numbers. Technicians and engine computers retard timing to stop knock, accepting a small loss of power.

• A: Adding 20 and 14 ignores that both angles are before the same reference point.
• B: A six-degree numerical difference means the specified timing changed.
• C (correct): Moving from 20° BTDC to 14° BTDC is 6° closer to TDC, a later or retarded spark.
• D: The second event is later, not earlier, so the change is not advance.

Answer: D

Correct answer: Gasoline direct injection. When the injector sprays fuel directly into the combustion chamber, the system is direct injection. It must inject against compression pressure, so it needs much higher fuel pressure, usually from an engine-driven pump.

• A: Port injectors spray into the intake port ahead of the valve and work at moderate pressure.
• B: Throttle-body systems use one or two injectors above the throttle, far from the combustion chamber.
• C: A carburetor draws fuel into the air stream through a venturi and has no injectors at all.
• D (correct): Spraying straight into the cylinder against compression pressure is direct injection, which needs a high-pressure pump.

Answer: C

Correct answer: A supercharger. Both turbochargers and superchargers compress intake air. The difference is the drive: a turbocharger uses exhaust gas flowing through a turbine, while a supercharger is driven mechanically by the crankshaft, usually through a belt.

• A: A turbocharger is spun by a turbine in the exhaust stream, not by a belt.
• B: A MAF sensor measures incoming air mass; it does not push air into the engine.
• C (correct): A supercharger is a compressor driven mechanically by the crankshaft, usually with a belt.
• D: An intercooler only cools compressed intake air; it does not compress anything.

Answer: A

Correct answer: 25%. The fraction of time an injector is open is called its duty cycle: open time ÷ total interval. Here 6 ms ÷ 24 ms = 0.25, and multiplying by 100 gives 25%. Keep the parts straight, with the open time on top. A common slip is reporting 75%, which is the closed portion. A longer open time within the same interval delivers more fuel, which is how the ECU enriches the mixture.

• A (correct): Compute 6 ÷ 24 = 0.25 = 25%; this is an idealized timing fraction.
• B: Forty percent would require a different open-time-to-interval ratio.
• C: Four percent does not follow from the stated 6/24 ratio.
• D: Seventy-five percent is the remaining closed fraction, not the open fraction.

Answer: C

Correct answer: Catalytic converter. The converter sits in the exhaust and contains a honeycomb coated with precious metals such as platinum, palladium and rhodium. These catalysts speed up reactions that turn carbon monoxide and unburned hydrocarbons into carbon dioxide and water, and change nitrogen oxides back into nitrogen. It works best when hot and when the mixture stays near the ideal air-fuel ratio. The air filter is tempting because it also cleans, but it treats air before combustion, not exhaust afterward.

• A: An intake air filter treats incoming air before combustion.
• B: An injector nozzle sprays fuel before or during combustion rather than cleaning exhaust afterward.
• C (correct): The catalytic converter belongs to the exhaust aftertreatment path.
• D: The ignition coil pack generates spark energy and is not an exhaust catalyst.

Answer: A

Correct answer: Electronic fuel injector. In this system each part has one job: the pump creates pressure, the rail holds fuel at that pressure and feeds every injector, and the injector decides when and how much fuel goes in. Because rail pressure stays steady, the ECU controls the dose by how long it holds the injector open, measured in milliseconds. A tempting mistake is crediting the pump, but it only supplies pressure and cannot time individual cylinder doses.

• A (correct): The injector opens for commanded intervals that meter delivery.
• B: An exhaust manifold collector carries burned gas away, not timed gasoline metering.
• C: A tank stores fuel and does not individually meter cylinder pulses.
• D: The filler cap closes the refueling opening rather than timing injections.

Answer: D

Correct answer: A misfire can have ignition, fuel or compression causes. A cylinder fires properly only when three conditions meet: a correct air-fuel mixture, enough compression, and a spark at the right time. Lose any one and the symptom looks the same, a rough-running engine or a missed power pulse, so one symptom cannot name the cause. Good troubleshooting tests each system in turn, for example checking spark, then fuel delivery, then running a compression test. Jumping straight to one part wastes time and money.

• A: One symptom cannot uniquely identify a cracked spark plug.
• B: Cooling restrictions can affect operation but are not the guaranteed sole cause of every misfire.
• C: A misfire does not uniquely establish an empty fuel tank.
• D (correct): A missing combustion contribution can arise from several distinct subsystems.

Answer: C

Correct answer: Retard the spark until the knock stops. Knock is more likely with heavy load and hot intake air. When the knock sensor detects it, the ECM retards spark timing so the spark fires later, then slowly restores advance after the knock stops.

• A: Firing earlier adds to knock; too much advance is one of its causes.
• B: Cutting fuel is not the standard response to knock; it would stall power rather than protect timing.
• C (correct): The ECM delays spark timing in the affected cylinders, then gradually restores advance once knock ends.
• D: More airflow and load raise cylinder pressure, which makes knock more likely, not less.

Answer: A

Correct answer: Low pressure in the narrowed venturi. Air rushing through the carburetor's narrowed venturi speeds up and its pressure drops. That low pressure draws fuel from the float bowl through calibrated jets, mixing it into the air stream.

• A (correct): Air speeds up through the venturi and its pressure drops, which pulls fuel through the jets.
• B: A basic carburetor meters fuel mechanically and has no oxygen sensor or computer.
• C: Injectors replaced carburetors; a carburetor does not use injectors to deliver fuel.
• D: The coil supplies ignition voltage to the plug and has no role in drawing fuel.

Answer: B

Correct answer: 14.7:1. Air-fuel ratio is air mass divided by fuel mass, so 441 g ÷ 30 g = 14.7. Check it quickly:

and the remaining 21 g is 0.7 of 30, giving 14.7. This value is the stoichiometric ratio for gasoline, the mix in which the oxygen and fuel can be used up completely. Reversing the division gives fuel per unit of air, about 0.068, which is a fuel-air ratio and not what was asked.

• A: Adding one to 14.7 is not part of mass-ratio calculation.
• B (correct): Dividing 441 by 30 yields 14.7 units of air mass per unit fuel mass.
• C: The sum of masses, 471 g, is not the mass ratio requested.
• D: Reversing the masses gives about 0.068, which is fuel/air rather than air/fuel.

Answer: A

Correct answer: Lambda = 1.10, lean. Lambda shows how the real mixture compares with the chemically ideal one, so divide actual by stoichiometric:

A check:

exactly. Above 1 means more air than needed to burn the fuel, so the mixture is lean; below 1 would be rich. The common trap is dividing the other way, which gives about 0.91 and the wrong label of rich.

• A (correct): Lambda is actual/stoichiometric = 16.17/14.7 = 1.10; more air per fuel is lean.
• B: The sum of ratios is not how lambda is defined.
• C: The reciprocal of lambda is not the defined lambda for this problem.
• D: The actual AFR differs from stoichiometric, so lambda is not 1.

Answer: D

Correct answer: Oxygen sensor → ECU → injector. Every feedback system has the same three roles in order: a sensor measures, a controller decides, and an actuator acts. Here the oxygen sensor reads leftover oxygen in the exhaust, the ECU compares it with the target and calculates a correction, and the injector carries it out by staying open longer or shorter. To check any chain, ask which part gathers information, which decides, and which does physical work. Putting the injector first places the action before the measurement.

• A: A spark plug and exhaust manifold are not the described feedback controller pathway.
• B: The air cleaner, starter and radiator do not constitute an exhaust feedback fuel-control chain.
• C: An injector does not send the primary exhaust oxygen reading through the fuel cap.
• D (correct): A feedback sensor reports information to the controller, which commands fuel metering.

Answer: B

Correct answer: Ignition mechanism, not merely injection location. Where fuel is injected and how it is ignited are separate design choices. Gasoline direct-injection engines spray fuel into the cylinder but still need a spark, because gasoline resists self-ignition at their compression ratios. Diesels also inject directly but use much higher compression, heating the air enough to ignite the fuel on contact. So when the injection location is the same, compare ignition: spark ignition versus compression ignition. Both engines still need oxygen and a crankshaft.

• A: Direct injection is not evidence of carburetion in either engine.
• B (correct): Direct cylinder injection can appear in spark-ignition gasoline and compression-ignition diesel; ignition initiation differs.
• C: A reciprocating spark-ignition engine still uses a crankshaft for rotating output.
• D: Both combustion systems require oxygen to react with fuel.

Answer: D

Correct answer: 4,800. In a four-stroke engine each cylinder completes one cycle, with one power stroke, every two crankshaft revolutions. At 2400 rpm that is

power events per cylinder each minute, and four cylinders give

The usual slip is forgetting to divide by two and getting 9600, which would fit a two-stroke engine instead. A quick rule for a four-stroke: power events per minute = rpm × cylinders ÷ 2.

• A: This is a single-cylinder four-stroke rate, not the four-cylinder total.
• B: This ignores that four cylinders produce events and confuses engine rpm with total firing events.
• C: Multiplying by four without dividing by two overcounts each cylinder’s cycle.
• D (correct): Each cylinder fires once per two revolutions, so 2400/2 × 4 = 4800 events per minute.

Answer: B

Correct answer: 24 g. A 14:1 air-fuel ratio means 14 grams of air for every 1 gram of fuel, so the fuel mass is the air mass divided by 14:

Check by multiplying back:

Because air dominates the ratio, the fuel mass must be much smaller than the air mass, which rules out any answer near 336 g. Setting up a proportion,

gives the same result.

• A: This amount comes from an unrelated reciprocal-like calculation, not air divided by 14.
• B (correct): Fuel mass equals air mass divided by AFR, so 336/14 = 24 g.
• C: Adding the ratio to the air mass produces no physically meaningful fuel mass.
• D: Equal air and fuel masses would give a 1:1 ratio, not the stated 14:1.

Answer: C

Correct answer: Accurate scheduling of ignition and injection events. The crankshaft position sensor tells the engine computer where the pistons are and how fast the crankshaft is turning. The computer uses that angle reference to decide exactly when to fire each spark plug and open each injector. Normal fuel pressure only shows that fuel is available; it says nothing about timing. If the crank signal is lost completely, many engines crank but will not start because no events can be timed.

• A: Differential gear lubrication comes from gear oil in the axle housing and is separate from engine position sensing.
• B: The fuel-tank volume is a physical storage property, not a crank-sensor output.
• C (correct): Without reliable crank angle information, the ECU can have difficulty timing commanded combustion-related events.
• D: A crank angle signal does not filter intake air; the air filter element removes dust and debris.

Answer: C

Correct answer: About 1.10, lean. Lambda compares the actual AFR with the stoichiometric AFR for the fuel in use:

A lambda above 1 means lean. Comparing 9.9 with gasoline's 14.7 would wrongly suggest a rich mixture.

• A: This uses the gasoline reference of 14.7 instead of the stated ethanol value.
• B: This inverts the ratio, dividing the stoichiometric AFR by the actual AFR.
• C (correct): Lambda = 9.9 ÷ 9.0 = 1.10, which is above 1, so the mixture has excess air for this fuel.
• D: This divides the gasoline value 14.7 by the actual AFR, mixing up both the fuel and the order.

Answer: A

Correct answer: The spark command moved 8 crank degrees later. BTDC numbers count crankshaft degrees before the piston reaches top dead center, so a smaller number means the spark fires closer to TDC, which is later. The change is

of retard. The tempting trap is to read the change as a percentage or as proof of some power or emissions result; with every other condition unknown, only the timing shift itself is certain. Memory hook: advance means earlier, retard means later.

• A (correct): Retarding from 24 to 16 degrees before the same reference delays ignition by eight crank degrees.
• B: Changing spark command does not convert the engine fuel type to diesel.
• C: Ignition timing and exhaust-valve timing are different events.
• D: The timing-angle change alone cannot establish a fixed power percentage.

Answer: B

Correct answer: Treat the code as a clue and check relevant air, fuel and sensor functions by approved procedures. An onboard diagnostic code records that a monitored value, such as fuel trim, went outside its expected range; it does not name a failed part. A lean or rich condition can come from a vacuum leak, a dirty airflow sensor, weak fuel delivery or a faulty sensor, so testing is needed to narrow the cause. The common mistake is replacing whatever part the code description mentions, which often costs money without fixing the problem.

• A: Opening a pressurized fuel system during operation risks fuel spray and fire and is not appropriate diagnostic advice.
• B (correct): OBD reports monitored conditions; multiple causes remain possible and qualified diagnosis should use safe procedures.
• C: Exhaust temperature by itself does not establish fuel-pump health or fuel pressure.
• D: A code naming mixture does not uniquely prove a sensor is defective.

Answer: B

Correct answer: AFR rises from 14:1 to 21:1, so the second mixture is leaner. Air-fuel ratio is air mass divided by fuel mass, so cutting the fuel while the air stays the same makes the ratio larger. A larger number means more air for each gram of fuel, which is a leaner mixture. The tempting error is assuming less fuel gives a smaller ratio; fuel is in the denominator, so it works the opposite way. For gasoline, the chemically ideal ratio is about 14.7:1.

• A: A fixed air mass does not fix AFR when the fuel mass changes.
• B (correct): The initial ratio is 210/15=14 and the later ratio is 210/10=21; more air per unit fuel means leaner.
• C: Adding 210 and 10 gives total mass but not the air-to-fuel ratio.
• D: Reducing fuel at constant air cannot reduce the numeric air-to-fuel mass ratio.

Answer: A

Correct answer: Oxygen supplied by air. Combustion needs three things at once: fuel, oxygen from air, and ignition. Fuel and spark are present here, so the blocked intake removes the oxygen supply and the engine cannot burn its fuel.

• A (correct): Combustion needs oxygen, and an engine gets it from the air it draws in; a blocked intake cuts off that supply.
• B: The tank is full and the fuel system is unaffected by a blocked air intake, so fuel is still available.
• C: The ignition system is working, so a spark is still produced; the blockage affects only the air stream.
• D: Battery power feeds the starter, ignition and control circuits, and a blocked intake does not drain or disconnect it.

Answer: D

Correct answer: 120 per second. In a four-stroke engine each cylinder fires once every two crankshaft revolutions.

• A: This counts one power stroke per cylinder every revolution; a four-stroke cylinder fires only once every two revolutions.
• B: This is the rate for a single cylinder (900 per minute divided by 60); it was not multiplied by the eight cylinders.
• C: 7200 is the number of power strokes per minute; it still has to be divided by 60 to get a per-second rate.
• D (correct): Each cylinder fires 900 times per minute, eight cylinders give 7200 per minute, and 7200 divided by 60 is 120 per second.

Answer: C

Correct answer: Water in diesel fuel can damage the precise injection parts. A fuel filter stops solid particles, but diesel systems also face water contamination. Because water harms the high-precision pump and injector parts, a water separator removes it before the fuel reaches them.

• A: Water does not improve diesel ignition quality; the separator exists to protect parts, not to adjust a fuel rating.
• B: Small amounts of water do not overfill a tank; the concern is what water does inside the injection equipment.
• C (correct): Diesel injection pumps and injectors have very tight clearances, and water can corrode and wear them, so it is separated out first.
• D: A fuel-system water separator handles fuel, not exhaust gases; exhaust treatment happens after combustion.

Answer: D

Correct answer: 11.9:1, rich. Lambda equals actual AFR divided by stoichiometric AFR, so the actual AFR is lambda times the stoichiometric value.

Because lambda is below 1.00, the mixture has less air than needed for the fuel present: it is rich.

• A: This divides 14.0 by 0.85 instead of multiplying; lambda below 1 can never give an AFR above stoichiometric.
• B: The AFR is calculated correctly, but a ratio below stoichiometric means extra fuel, which is rich, not lean.
• C: Dividing instead of multiplying gives 16.47, and an AFR above 14.0 would be lean anyway, so both parts are wrong.
• D (correct): Rearranging lambda = actual AFR / stoichiometric AFR gives 0.85 x 14.0 = 11.9, and lambda below 1 means rich.

Answer: C

Correct answer: Fuel sprayed into the port washes over the back of the intake valve. In port injection, fuel enters the intake port just upstream of the intake valve, so it constantly washes the back of the valve. A direct injector sprays into the cylinder instead, so the intake valves lose that cleaning effect and carbon can build up.

• A: Port injection actually runs at moderate pressure; direct injection is the system that needs much higher pressure.
• B: Every four-stroke piston engine of this type has intake valves; in port injection the fuel is sprayed toward them.
• C (correct): In port injection the fuel hits the hot intake valve on its way into the cylinder, which helps keep the valve clean.
• D: The catalytic converter is in the exhaust system after the engine and has no effect on the intake valves.

Answer: D

Correct answer: To store the fuel supply safely and sealed. The fuel tank stores fuel and keeps it sealed, with vent connections that capture vapors. Pumping, filtering and metering are done by other parts, even if some are mounted inside the tank.

• A: Raising pressure is the fuel pump's job; the pump may sit inside the tank, but it is a separate part.
• B: Fuel metering is done by the injectors under ECM control, not by the tank.
• C: Removing particles is the job of the fuel filter or pump strainer, not the tank itself.
• D (correct): The tank is a storage container that holds fuel and keeps it sealed against leaks and vapor escape.

Answer: C

Correct answer: The fuel pump. Fuel delivery is a chain: the pump creates flow and pressure, the lines carry the fuel, the filter cleans it, the rail distributes it and the injectors meter it. The part that moves and pressurizes fuel is the pump.

• A: The rail holds pressurized fuel at the engine and feeds the injectors, but it does not create the pressure.
• B: The filter traps debris in the fuel; it creates no flow and actually adds a little restriction.
• C (correct): The pump, usually electric and often inside the tank, creates the flow and pressure for the whole delivery chain.
• D: The injector meters fuel into the port or cylinder; it relies on pressure supplied upstream.

Answer: D

Correct answer: It holds pressurized fuel and feeds it to each injector. The part described is the fuel rail. Its job is distribution: it holds fuel at system pressure and supplies each injector, while the injectors decide how much fuel actually enters each cylinder.

• A: Metering is done by each injector's pulse width; the pipe only supplies fuel to them.
• B: Burned gases leave through the exhaust manifold; this pipe carries fresh, pressurized fuel.
• C: The pump is normally at or in the tank; the pipe at the engine receives fuel that is already pressurized.
• D (correct): This describes the fuel rail, which distributes pressurized fuel to every injector connected to it.

Answer: C

Correct answer: The fuel pump, its relay and its fuse. Isolate the missing function first. Spark is present and the ECM sees the crank signal, so ignition and timing check out. Zero rail pressure means no fuel is being delivered, so testing shifts to the pump, its relay and its fuse.

• A: Strong spark is already present, so the ignition side has passed its test and is not the likely cause.
• B: The ECM is already receiving a crank signal, so timing information is available.
• C (correct): Spark and the crank signal are present but fuel pressure is zero, so the problem is in fuel delivery, starting with the pump circuit.
• D: The converter treats exhaust after combustion; with zero fuel pressure the engine has nothing to burn in the first place.

Answer: D

Correct answer: By changing the amount of fuel injected. A gasoline engine controls power by controlling air with a throttle, then matching fuel to it. A traditional diesel does not throttle its air for power control; it varies power mainly by the quantity of fuel injected.

• A: Diesels ignite fuel with compression heat and have no spark plugs for each power stroke.
• B: A conventional diesel has a fixed compression ratio; it is not adjusted to control power.
• C: Traditional diesels have no throttle plate for power control; that method belongs to gasoline engines.
• D (correct): A traditional diesel takes in air freely and varies output mainly by injecting more or less fuel.

Answer: B

Correct answer: 14.25:1, slightly rich. Divide air mass by fuel mass.

Since 14.25 is lower than 14.7, there is more fuel for the air than stoichiometric, so the mixture is slightly rich.

• A: The ratio is calculated correctly, but a value below 14.7 means more fuel than stoichiometric, which is rich.
• B (correct): 456 divided by 32 is 14.25, which is below 14.7, so the mixture has extra fuel and is slightly rich.
• C: This divides fuel by air; AFR always puts the air mass on top.
• D: This adds the two masses instead of dividing them; an AFR in the hundreds is a sign of that mistake.

Answer: A

Correct answer: 315 g. Rearrange AFR = air mass / fuel mass and use the reference value the problem gives.

• A (correct): Air mass equals AFR times fuel mass: 9.0 x 35 g = 315 g.
• B: This uses the gasoline value of 14.7 instead of the stated 9.0 for ethanol.
• C: This divides fuel mass by the ratio; the air mass must be much larger than the fuel mass.
• D: This adds the ratio number to the fuel mass instead of multiplying.

Answer: C

Correct answer: The sensor under-reports airflow, so the mixture runs lean. A hot-film MAF sensor measures air mass by how much the passing air cools its heated element. Contamination reduces that cooling, so the sensor under-reports airflow, the ECM calculates too little fuel and the engine tends to run lean.

• A: A coated element is cooled less by the passing air, so it reads low, not high.
• B: A MAF sensor reports airflow for fueling; a dirty element does not lock spark timing.
• C (correct): The contamination insulates the element, the ECM thinks less air is entering, and it injects too little fuel for the real airflow.
• D: Pump pressure is not set by the MAF reading, and higher pressure would not follow from a dirty air sensor.

Answer: B

Correct answer: 1500 rpm. One four-stroke cycle takes two crankshaft revolutions (720 degrees), and the camshaft turns once per cycle. So cam speed is half crank speed: 3000 divided by 2 is 1500 rpm. That is why a cam signal tells the ECM which stroke each cylinder is in.

• A: This would be true only if the cam turned at crankshaft speed; it turns once for every two crank revolutions.
• B (correct): The camshaft turns at half crankshaft speed, once per four-stroke cycle, so 3000 divided by 2 gives 1500 rpm.
• C: This doubles the crank speed; the camshaft actually turns more slowly than the crankshaft.
• D: This divides by 4, as if there were one cam turn per stroke; there is one cam turn per complete four-stroke cycle.

Answer: C

Correct answer: 10 degrees, retarded. Treat BTDC angles as positive and ATDC angles as negative: 8 degrees BTDC is +8 and 2 degrees ATDC is -2.

The spark now fires later in the piston's travel, so the timing was retarded by 10 crankshaft degrees.

• A: Subtracting 2 from 8 treats both readings as before TDC; the second reading is after TDC, so the two angles add.
• B: The size is right, but moving from before TDC to after TDC makes the spark later, which is retarded.
• C (correct): The spark moves 8 degrees to reach TDC and 2 more past it, a total of 10 degrees later, which is retarded.
• D: This both subtracts the angles incorrectly and reverses the direction; firing after TDC is later, not earlier.

Answer: A

Correct answer: From the air drawn into the engine. Gasoline's hydrogen and carbon must combine with oxygen to burn, forming mostly carbon dioxide and water. That oxygen comes from the intake air, which is why an engine needs about 14.7 times as much air as fuel by mass.

• A (correct): Air is about 21 percent oxygen, and the engine draws in many times more air than fuel by mass to supply it.
• B: Ordinary gasoline is hydrocarbons and does not carry the oxygen it needs to burn.
• C: Coolant flows in a sealed circuit around the cylinders and never enters the combustion chamber.
• D: The converter is in the exhaust after combustion; it treats exhaust gases rather than supplying the engine with oxygen.

Answer: B

Correct answer: The starter motor. Turning the key to start energizes the starter motor, which cranks the engine. The battery supplies the power, the coil and plug create the spark, and the starter itself does not ignite any fuel.

• A: The coil steps up voltage for the spark; it does not physically turn the engine.
• B (correct): The starter motor is the electric motor that cranks the engine when the key is turned to start.
• C: The battery stores the electrical energy the starter uses, but it does not turn anything by itself.
• D: The pump moves fuel to the engine; it cannot rotate the crankshaft.

Answer: A

Correct answer: Fuel injector. Ask whether the part reports information or does work. Sensors measure a condition and send a signal to the ECM; actuators receive a command and act. The fuel injector opens on command, so it is the actuator.

• A (correct): An injector receives a command from the ECM and does physical work by opening to spray fuel, so it is an actuator.
• B: An oxygen sensor reports exhaust oxygen content to the ECM, so it is a sensor.
• C: The MAP sensor reports manifold pressure as a signal, so it is a sensor.
• D: This part reports engine temperature to the ECM, so it is a sensor.

Answer: D

Correct answer: Vibration frequencies typical of knock. Knock produces sharp pressure waves that shake the block at characteristic frequencies. A piezoelectric knock sensor converts that vibration into a signal, and the ECM responds by retarding spark until the knock stops.

• A: Exhaust oxygen is measured by the oxygen sensor in the exhaust stream, not by a sensor on the block.
• B: Crank position comes from the crankshaft position sensor reading a toothed wheel.
• C: No engine sensor reads octane directly; the knock sensor only senses the vibration knock causes.
• D (correct): The knock sensor is usually a piezoelectric device that turns the block vibrations caused by knock into an electrical signal.

Answer: B

Correct answer: 30 g. An AFR of 12:1 means 12 g of air for every 1 g of fuel, so the mixture has

equal mass parts.

Check: the air is 360 g, and 360 divided by 30 is 12.

• A: This divides the total by 12, treating the whole mixture as if it were the air; air and fuel together make 13 equal parts.
• B (correct): At 12:1 the mixture is 12 parts air plus 1 part fuel, or 13 parts; 390 divided by 13 gives 30 g of fuel.
• C: 360 g is the air in the mixture, not the fuel.
• D: This divides 390 by 11, subtracting the fuel part from 12 instead of adding it; the mixture has 12 + 1 = 13 parts.

Answer: B

Correct answer: Reducing nitrogen oxides to nitrogen and oxygen. Lambda 1.08 is lean, so the exhaust carries leftover oxygen. The oxidizing reactions for CO and hydrocarbons have plenty of oxygen, but the NOx reduction needs an oxygen-poor exhaust, so it loses efficiency. This is why closed-loop control keeps lambda close to 1.00.

• A: This reaction needs oxygen, and a lean mixture leaves extra oxygen in the exhaust, so it still works.
• B (correct): Lambda above 1 means extra oxygen in the exhaust, and NOx reduction works best when little oxygen is present.
• C: This reaction also uses oxygen, which a lean mixture supplies in excess.
• D: A catalytic converter treats exhaust after combustion and has nothing to do with fuel octane.

Answer: A

Correct answer: It is denser, so each volume holds more oxygen and needs more fuel. Air temperature changes air density. Cold air is denser and packs more oxygen into each unit of volume, so the ECM uses the IAT reading to estimate a larger air mass and inject matching fuel.

• A (correct): Cold air is denser, so the same volume carries more air mass and oxygen, and the ECM adds fuel to match.
• B: This reverses the relationship; hot air is the thinner, less dense air.
• C: Air is about 21 percent oxygen regardless of temperature; cold changes its density, not its makeup.
• D: In speed-density systems the ECM needs IAT to estimate air mass, and it also uses IAT for spark timing.

Answer: B

Correct answer: Test spark, fuel delivery and compression for cylinder 3. A diagnostic code points to a condition, not a specific failed part. A misfire can be caused by a worn plug or bad coil, a clogged injector, or low compression, so the technician tests spark, fuel and compression in that cylinder before replacing anything.

• A: Misfires can come from ignition, fuel or compression problems, so replacing the plug without testing may not fix the cause.
• B (correct): The code describes what happened, not why; checking each function for that cylinder finds the actual cause.
• C: The ECM detected and reported the misfire; reporting a fault does not mean the controller caused it.
• D: Clearing the code does not fix anything, and unburned fuel from an ongoing misfire can overheat the catalytic converter.

Answer: C

Correct answer: To cool the compressed air so it becomes denser. Compressing intake air raises its temperature, which makes it less dense. The intercooler removes that heat, so each cylinder receives more air mass, allowing more fuel and power while reducing the chance of knock.

• A: The turbo's compressor does the compressing; the intercooler is a heat exchanger, not a compressor.
• B: The intercooler sits in the air path, not the fuel path, and the boost system works on air.
• C (correct): Compressing air heats it; cooling it packs more air mass into each cylinder and lowers knock risk.
• D: Dust is removed by the air filter, which comes before the turbocharger.

Answer: B

Correct answer: It is rich for gasoline but lean for ethanol. Rich and lean are judged against the stoichiometric value for the fuel in use. A number below the reference has extra fuel (rich) and a number above it has extra air (lean). 12 is below 14.7 but above 9, so it is rich for gasoline and lean for ethanol.

• A: 12 is above ethanol's 9:1, so for ethanol it has extra air and is lean.
• B (correct): 12 is below 14.7, so it is rich for gasoline, and above 9, so it is lean for ethanol.
• C: This reverses both comparisons; a lower AFR than the reference means rich.
• D: Stoichiometric means matching the reference for that fuel; 12:1 matches neither 14.7 nor 9.

Answer: A

Peak cylinder pressure should arrive shortly after top dead center for the strongest push. Because the flame takes time to spread and the piston moves faster at higher rpm, the ECM starts the burn more crankshaft degrees before TDC, which is spark advance.

• A (correct): The burn takes a short time, and a faster piston covers more degrees in that time, so the spark must lead TDC by more.
• B: Advancing the spark generally raises the risk of knock; the ECM retards timing to stop knock.
• C: Spark timing is set electronically by the ECM and is separate from valve timing.
• D: If the burn needed less time, the spark could fire later, not earlier; advancing points to the opposite reasoning.

Answer: A

Correct answer: It increases by about 25 percent. At a fixed pressure, fuel delivered is roughly proportional to the time the injector is open.

So about 25 percent more fuel is delivered per pulse.

• A (correct): The change is 0.8 ms, and 0.8 divided by the original 3.2 ms is 0.25, so about 25 percent more fuel.
• B: This divides the 0.8 ms change by the new 4.0 ms value; a percent increase is based on the original value.
• C: A longer opening time at the same pressure lets more fuel through, not less.
• D: With pressure constant, the amount of fuel depends on how long the injector stays open, which increased.