On highway trips, a pickup's diesel loses power after 20 to 30 minutes and sometimes stalls; after sitting a few minutes it restarts and runs normally. When the fuel cap is removed after an episode, air rushes into the tank, and with the cap loosened the problem does not occur. Which of these is the cause?

A restricted vent on the fuel tank

A fuel filter loaded with debris

Water collected in the fuel separator

A lift pump that weakens when hot

With the engine fully warm and the headlamps, blower and rear defroster on, a pickup's diesel steps its idle up by about 45 rpm while the scan tool shows battery voltage below normal; the manufacturer lists this as its low-battery-voltage idle increase. Why does the ECM raise the idle?

To spin the alternator faster for output

To warm the coolant faster for the heater

To cool the alternator with more fan air

To raise the voltage the batteries rest at

A manufacturer's procedure for removing glow plugs from an aluminum cylinder head says to bring the engine to normal operating temperature first, then work each plug out without exceeding a stated maximum torque. Why does the warm engine help?

Aluminum expands more with heat than steel

Heat shrinks the plug's swollen tip

Heat burns off the carbon at the plug's tip

Heat softens the sealant on the threads

Before the cylinders of a light-duty diesel block are honed, a thick steel plate is bolted to the deck with head bolts tightened to the manufacturer's head-bolt torque. What is the plate for?

To load the bores as the head bolts will

To clamp the block to the honing machine

To guide the hone square to the deck

To shield the deck from the honing stones

A light-duty diesel's three-wire engine oil pressure sensor gets a 5-volt reference and a ground from the ECM. The manufacturer lists its signal as 0.5 volt at 0 psi, rising to 4.5 volts at full-scale pressure. Why does 0 psi give 0.5 volt rather than 0 volts?

So a signal near 0 volts can be flagged as a fault

Because the sensor needs 0.5 volt to power itself

So the ECM can read oil pressure below atmospheric

To make up for voltage lost along the ground wire

On a light-duty diesel pickup whose exhaust passes through a diesel oxidation catalyst (DOC) and then a diesel particulate filter (DPF), soot in the filter slowly burns off during long highway drives with no extra fuel added for regeneration. What oxidizes the soot at those normal exhaust temperatures?

Nitrogen dioxide formed in the DOC

Carbon monoxide formed in the DOC

Ammonia from diesel exhaust fluid

Fuel injected after the main injection

Many light-duty diesels keep their glow plugs energized for a time after a cold engine has started. What is this after-glow for?

Smoother running with less white smoke

Faster cab heat from warmer coolant

Soot burn-off in the particulate filter

On many common rail pickups, the ECM sets rail pressure mainly with a fuel control (metering) valve on the high-pressure pump. How does that valve change rail pressure?

It meters the fuel the pump takes in

It dumps fuel from the rail back to the return

It changes the speed the pump is driven at

It varies how long each injector stays open

In a piezo common rail injector, what moves the valve that starts and ends each injection?

A stack of crystals that changes length

A camshaft lobe pushing on a plunger

A solenoid coil that pulls in an armature

Engine oil at injection control pressure

After a replacement ECM was installed and programmed with the vehicle's calibration, a common rail pickup idles slightly rough with light smoke and no DTCs. The scan tool shows the same default value stored as the injector correction code for every cylinder. What should be done next?

Enter each injector's own code into the ECM

Replace the injectors with the worst balance

Program the same calibration into the ECM again

Run a parked regeneration of the DPF

10 Free ASE A9 Light Vehicle Diesel Engines Questions

Ten ASE A9 Light Vehicle Diesel Engines questions in the ASE format: four options, one best answer. They are free to use, print and share, and there is nothing to sign up for on this page.

01How to use these

The questions come from the ToolboxPrep A9 bank and are spread across the test's content areas in the proportion the ASE test specification gives them: A (General Diagnosis), B (Cylinder Head and Valve Train Diagnosis and Repair), C (Engine Block Diagnosis and Repair), D (Lubrication and Cooling Systems Diagnosis and Repair), E (Air Induction and Exhaust Systems Diagnosis and Repair), F (Fuel System Diagnosis and Repair). Answer each one before opening the explanation. Every explanation covers all four options, because knowing why the wrong answers are wrong is most of the skill.

Pace. The A9 test gives you 75 minutes for 60 questions (50 scored, 10 unscored), about 1 min 15 s a question. Try these at that pace.
Print. The printed version shows the questions and the answer key without the explanations, so it works as a handout.

02The questions

Diagnosis · General Diagnosis

On highway trips, a pickup's diesel loses power after 20 to 30 minutes and sometimes stalls; after sitting a few minutes it restarts and runs normally. When the fuel cap is removed after an episode, air rushes into the tank, and with the cap loosened the problem does not occur. Which of these is the cause?
1. A restricted vent on the fuel tank
2. A fuel filter loaded with debris
3. Water collected in the fuel separator
4. A lift pump that weakens when hot
Show the answer

A. Correct. As fuel is drawn out, a blocked vent lets vacuum build in the tank until the pump can no longer pull enough fuel, so the engine starves; the rush of air at the cap and the cure with the cap loosened both show the tank under vacuum. The tank's vent valve and line are cleared or replaced.

Why not the others

B. A loaded filter restricts flow all the time and would not recover after a few minutes' rest or be cured by loosening the fuel cap. The inrush of air at the cap shows vacuum in the tank.

C. Water in the separator lights the water-in-fuel lamp and causes rough running, but loosening the cap would not cure it. Air rushing into the tank shows a vent restriction.

D. A pump that weakens when hot would still fail with the cap loose, and its weakness would not make air rush into the tank. The tank vacuum points at a restricted vent.

ID A9-A-001 · Task A9-A-13 (ASE task list)
Recall · General Diagnosis

With the engine fully warm and the headlamps, blower and rear defroster on, a pickup's diesel steps its idle up by about 45 rpm while the scan tool shows battery voltage below normal; the manufacturer lists this as its low-battery-voltage idle increase. Why does the ECM raise the idle?
1. To warm the coolant faster for the heater
2. To cool the alternator with more fan air
3. To raise the voltage the batteries rest at
4. To spin the alternator faster for output
Show the answer

D. Correct. An alternator's available output rises with rotor speed, and at base idle it may not carry heavy loads and recharge the batteries at the same time; raising the idle turns it faster so it can, and the ECM returns to base idle once voltage recovers. A truck that keeps calling for this has its batteries and charging system tested.

Why not the others

A. Some diesels do raise idle to warm a cold engine for the heater, but this engine is fully warm and the manufacturer names this increase as its low-battery-voltage strategy. It is there to raise charging output.

B. The trigger is low battery voltage, not alternator temperature. The extra speed is for output: an alternator turned faster can supply the heavy loads and recharge the batteries.

C. A battery's resting voltage is set by its state of charge, not by idle speed. Raising the idle spins the alternator faster so it can put more current back into the batteries.

ID A9-A-015 · Task A9-A-18 (ASE task list)
Recall · Cylinder Head and Valve Train Diagnosis and Repair

A manufacturer's procedure for removing glow plugs from an aluminum cylinder head says to bring the engine to normal operating temperature first, then work each plug out without exceeding a stated maximum torque. Why does the warm engine help?
1. Heat shrinks the plug's swollen tip
2. Heat burns off the carbon at the plug's tip
3. Heat softens the sealant on the threads
4. Aluminum expands more with heat than steel
Show the answer

D. Correct. Aluminum grows about twice as much as steel for the same temperature rise, so the threaded bore in a warm head loosens its grip on the steel plug. With the torque limit, that lowers the chance of snapping a plug whose tip is held by carbon in its bore.

Why not the others

A. Metal expands when heated, so heat cannot shrink a swollen tip; a carbon-bound tip is the reason for the torque limit. The benefit comes from the aluminum head expanding more than the steel plug.

B. A shut-down engine at operating temperature is far too cool to burn off carbon, which builds up on the tip even with combustion heat around it; that carbon is the reason for the torque limit. The warm engine helps because the aluminum bore expands more than the steel plug and loosens its grip.

C. A glow plug seals at its seat, not with sealant on its threads, so there is nothing for heat to soften. The warm engine helps because the aluminum bore expands more than the steel plug.

ID A9-B-004 · Task A9-B-2 (ASE task list)
Recall · Engine Block Diagnosis and Repair

Before the cylinders of a light-duty diesel block are honed, a thick steel plate is bolted to the deck with head bolts tightened to the manufacturer's head-bolt torque. What is the plate for?
1. To clamp the block to the honing machine
2. To load the bores as the head bolts will
3. To guide the hone square to the deck
4. To shield the deck from the honing stones
Show the answer

B. Correct. Head-bolt clamping load pulls on the bosses around each cylinder and distorts the bores slightly. Honing with this torque plate bolted down reproduces that distortion, so the bores come out round and straight with the head installed instead of only on the bare block.

Why not the others

A. The block is held on the honing machine by the machine's own fixture. The plate is tightened with head bolts to head-bolt torque so the bores carry the distortion the cylinder head's clamping will put into them while they are honed.

C. The hone's alignment comes from the machine and its stroke; the plate's openings clear the cylinders so the stones never touch it. Its job is to put the head-bolt distortion into the bores while they are honed.

D. The stones work inside the bore and do not reach the deck surface. The plate is torqued with head bolts so the bores carry the same distortion they will have under the cylinder head while they are honed.

ID A9-C-002 · Task A9-C-3 (ASE task list)
Recall · Lubrication and Cooling Systems Diagnosis and Repair

A light-duty diesel's three-wire engine oil pressure sensor gets a 5-volt reference and a ground from the ECM. The manufacturer lists its signal as 0.5 volt at 0 psi, rising to 4.5 volts at full-scale pressure. Why does 0 psi give 0.5 volt rather than 0 volts?
1. Because the sensor needs 0.5 volt to power itself
2. So the ECM can read oil pressure below atmospheric
3. To make up for voltage lost along the ground wire
4. So a signal near 0 volts can be flagged as a fault
Show the answer

D. Correct. Keeping the working range between 0.5 and 4.5 volts leaves a band at each end that a healthy sensor never produces. A signal pulled near 0 volts by a short to ground or a lost reference, or near 5 volts by a short to the reference, sets a circuit DTC instead of being shown as a believable oil pressure.

Why not the others

A. The sensor is powered by the 5-volt reference and its ground, not by its signal output. The 0.5-volt starting point is a design choice that lets the ECM tell a true 0 psi reading from a shorted or open signal circuit.

B. Oil gallery pressure is not meant to be read below atmospheric; 0.5 volt is simply where the manufacturer's scale starts at 0 psi. The band below it is kept free so a shorted or open signal circuit can be recognized as a fault.

C. The ECM measures the signal against its own sensor ground, and a good ground wire carries only a few milliamps with almost no voltage drop. The 0.5-volt floor exists to separate real readings from circuit faults.

ID A9-D-004 · Task A9-D-1 (ASE task list)
Recall · Air Induction and Exhaust Systems Diagnosis and Repair

On a light-duty diesel pickup whose exhaust passes through a diesel oxidation catalyst (DOC) and then a diesel particulate filter (DPF), soot in the filter slowly burns off during long highway drives with no extra fuel added for regeneration. What oxidizes the soot at those normal exhaust temperatures?
1. Nitrogen dioxide formed in the DOC
2. Carbon monoxide formed in the DOC
3. Ammonia from diesel exhaust fluid
4. Fuel injected after the main injection
Show the answer

A. Correct. The DOC's catalyst turns part of the nitric oxide in the exhaust into nitrogen dioxide, which oxidizes soot at the temperatures reached in steady highway driving, well below what oxygen alone needs. This is passive regeneration; when driving does not keep pace with soot loading, the ECM schedules an active regeneration.

Why not the others

B. The DOC removes carbon monoxide by oxidizing it to carbon dioxide; it does not produce CO to attack soot. What the DOC makes that burns soot at normal temperatures is nitrogen dioxide.

C. Ammonia from diesel exhaust fluid reacts with NOx in a selective catalytic reduction (SCR) catalyst; it plays no part in burning soot. Passive soot burning relies on nitrogen dioxide from the DOC.

D. Post-injected fuel burning in the DOC is how the ECM raises exhaust temperature for active regeneration, and the stem says no fuel is added. Passive soot burning relies on nitrogen dioxide from the DOC.

ID A9-E-001 · Task A9-E-8 (ASE task list)
Recall · Air Induction and Exhaust Systems Diagnosis and Repair

Many light-duty diesels keep their glow plugs energized for a time after a cold engine has started. What is this after-glow for?
1. Warmer fuel so it does not gel
2. Faster cab heat from warmer coolant
3. Smoother running with less white smoke
4. Soot burn-off in the particulate filter
Show the answer

C. Correct. In the first minutes after a cold start the cylinder walls are still cold, so some fuel burns late or not at all, giving white smoke, knock and rough running. Keeping the plugs hot steadies ignition until the engine warms.

Why not the others

A. Glow plugs heat the combustion chambers, not the fuel supply; gelling is handled by fuel heaters and the right seasonal fuel. After-glow steadies combustion in the cold engine.

B. Glow plugs put very little heat into the coolant; cab heat comes from the engine warming up. After-glow is about combustion quality in the cold cylinders.

D. Glow plugs sit in the combustion chambers and play no part in DPF regeneration, which relies on exhaust heat from the DOC. After-glow is for smooth, cleaner running while cold.

ID A9-E-024 · Task A9-E-7 (ASE task list)
Recall · Fuel System Diagnosis and Repair

On many common rail pickups, the ECM sets rail pressure mainly with a fuel control (metering) valve on the high-pressure pump. How does that valve change rail pressure?
1. It dumps fuel from the rail back to the return
2. It meters the fuel the pump takes in
3. It changes the speed the pump is driven at
4. It varies how long each injector stays open
Show the answer

B. Correct. The valve sits on the pump's inlet side and lets in only as much fuel as the ECM wants pressurized; the pump compresses whatever it receives, so more fuel in means higher rail pressure and less means lower. Metering the inlet avoids pressurizing fuel that would only be dumped, which saves engine power and keeps the fuel cooler.

Why not the others

A. Releasing fuel from the rail is the job of a pressure control or relief valve on the rail, not of the pump's metering valve. The metering valve controls how much fuel enters the pump to be pressurized.

C. The high-pressure pump is driven by the engine at a fixed ratio, so its speed follows engine speed. Rail pressure is set by how much fuel the metering valve lets into the pump.

D. Injector open time sets how much fuel each injection delivers, and the ECM controls it at the injectors. The metering valve sets rail pressure by controlling the fuel entering the pump.

ID A9-F-002 · Task A9-F-7 (ASE task list)
Recall · Fuel System Diagnosis and Repair

In a piezo common rail injector, what moves the valve that starts and ends each injection?
1. A camshaft lobe pushing on a plunger
2. A solenoid coil that pulls in an armature
3. Engine oil at injection control pressure
4. A stack of crystals that changes length
Show the answer

D. Correct. A piezo actuator is a stack of thin ceramic crystal layers that lengthens slightly when the ECM charges it with voltage and shrinks when it is discharged; that movement, amplified hydraulically, opens and closes the control valve. It switches faster than a solenoid, allowing closely spaced injections in each cycle.

Why not the others

A. A cam-driven plunger belongs to an electronic unit injector. In a common rail system the pressure comes from the rail, and a piezo injector's valve is moved by its crystal stack.

B. A solenoid coil pulling an armature is how the other common type of common rail injector works. A piezo injector uses a crystal stack that changes length when charged.

C. Oil at injection control pressure drives the intensifier of a hydraulic electronic unit injector, a different system. A piezo common rail injector is moved by a crystal stack.

ID A9-F-022 · Task A9-F-7 (ASE task list)
Diagnosis · Fuel System Diagnosis and Repair

After a replacement ECM was installed and programmed with the vehicle's calibration, a common rail pickup idles slightly rough with light smoke and no DTCs. The scan tool shows the same default value stored as the injector correction code for every cylinder. What should be done next?
1. Replace the injectors with the worst balance
2. Enter each injector's own code into the ECM
3. Program the same calibration into the ECM again
4. Run a parked regeneration of the DPF
Show the answer

B. Correct. Each injector's correction code, printed on the injector, describes how that injector differs from nominal, and the ECM fuels each cylinder from the code held in its own memory; a new ECM starts with default values, so every cylinder is fueled as if its injector were average. The codes are read from the injectors, or from the old ECM if it still communicates, and entered by cylinder.

Why not the others

A. Uneven balance is expected when every cylinder is fueled with a default correction; replacing injectors would not fix that. The missing codes are entered first.

C. The calibration loaded correctly; the injector codes are separate data that loading a calibration does not supply. They have to be entered for each cylinder.

D. A regeneration cleans the particulate filter but does nothing about fueling. The light smoke and rough idle come from every injector being run on a default correction.

ID A9-F-033 · Task A9-F-12 (ASE task list)

03Answer key

Question	Answer	Content area
Q1	A	General Diagnosis
Q2	D	General Diagnosis
Q3	D	Cylinder Head and Valve Train Diagnosis and Repair
Q4	B	Engine Block Diagnosis and Repair
Q5	D	Lubrication and Cooling Systems Diagnosis and Repair
Q6	A	Air Induction and Exhaust Systems Diagnosis and Repair
Q7	C	Air Induction and Exhaust Systems Diagnosis and Repair
Q8	B	Fuel System Diagnosis and Repair
Q9	D	Fuel System Diagnosis and Repair
Q10	B	Fuel System Diagnosis and Repair

04Questions by content area

Working on one part of the A9 test? Each content area has its own page with five more free questions.

05More free questions

The A9 study guide covers each content area and how to study for it, and the ASE A9 Light Vehicle Diesel Engines practice test page describes the full A9 question bank. The same free format is available for the other 15 tests.

How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.

ToolboxPrep is an independent study site, not affiliated with or endorsed by ASE (the National Institute for Automotive Service Excellence). Our questions are original, written from ASE’s published test specifications and task lists and other public sources. They are not actual ASE test questions and not recalled or “brain dump” questions. ASE is a trademark of its owner, and we never use the ASE logo. Standards and service information are cited, not reproduced.