Five free practice questions on fuel systems and electronic controls: content area F of the ASE A9 Light Vehicle Diesel Engines test, 13 of its 50 scored questions (26%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
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On many light-duty diesels, the electric lift pump is mounted at or near the fuel tank and pushes fuel forward to the engine-mounted filter and high-pressure pump. What is an advantage of pushing fuel this way rather than pulling it from the tank?
- Rail pressure can build before the engine cranks
- A leak drips fuel rather than drawing in air
- The low-pressure filter can be left out entirely
- The return line to the tank is no longer needed
Show the answer
B. Correct. Downstream of a pushing pump the line is above atmospheric pressure, so a loose fitting or cracked line shows itself as a fuel leak, while a line under suction can pull in air without leaking any fuel. Pushing also keeps the high-pressure pump's inlet fed under positive pressure.
Why not the others
A. Rail pressure comes only from the engine-driven high-pressure pump, which does not turn until the engine cranks. The lift pump supplies low-pressure fuel; its advantage here is keeping the supply line above atmospheric pressure.
C. The fuel still needs filtering and water separation before it reaches the high-pressure pump, wherever the lift pump sits. The advantage of pushing is that the supply line runs under pressure, so leaks drip instead of admitting air.
D. Excess and leak-off fuel still has to go back to the tank, so the return line stays. Pushing from the tank keeps the supply line pressurized, so a leak shows as fuel rather than as air in the system.
ID A9-F-008 · Task A9-F-2 (ASE task list)
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On some engines, each common rail injector is fed by a short connector (transfer) tube that passes through the cylinder head and presses into the side of the injector, held by a retaining nut on the outside of the head. What does torquing that retaining nut to specification do?
- Holds the injector down on its sealing washer
- Seats the tube's tip in the injector's inlet
- Sets how far the injector tip protrudes
- Seals the line's other end at the rail
Show the answer
B. Correct. The nut forces the tube's tip against the injector's inlet so the metal-to-metal joint holds rail pressure. A loose or misaligned tube leaks inside the head where nothing shows outside, so rail pressure can fall and the engine can crank long or lack power.
Why not the others
A. The injector's own hold-down clamp or bolt holds it on its sealing washer. The connector tube's nut presses the tube into the injector's inlet to seal the high-pressure joint.
C. Tip protrusion is set by the injector bore and its sealing washer, not by the feed tube. The retaining nut seats the tube's tip against the injector inlet.
D. The high-pressure line's nut at the rail seals that end. The connector tube's retaining nut seats the tube's tip into the injector's inlet inside the head.
ID A9-F-015 · Task A9-F-5 (ASE task list)
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What is the job of the pressure relief (limiting) valve fitted to many common rail fuel rails?
- It dumps fuel to the return on overpressure
- It sets the rail pressure the ECM wants at idle
- It traps rail pressure after the engine stops
- It lets air out of the rail during priming
Show the answer
A. Correct. The relief valve is a mechanical safety that stays closed in normal running and opens only if rail pressure exceeds a set maximum, such as when pump control fails, sending fuel to the return so the rail, lines and injectors are not overstressed. On some systems it then holds a lower limp-home pressure until the engine is shut off.
Why not the others
B. The ECM sets rail pressure by controlling the pump's fuel control valve, and on some systems a separate pressure control valve. The relief valve stays closed in normal operation and opens only on overpressure.
C. Holding pressure after shutdown is not its job; rail pressure bleeds away through the injectors' leak-off after the engine stops. The relief valve opens to the return only when pressure exceeds its maximum.
D. Air is cleared by priming the low-pressure side and running the engine, not through the relief valve. That valve is an overpressure safety that dumps fuel to the return.
ID A9-F-023 · Task A9-F-7 (ASE task list)
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In an electronic unit injector (EUI), a camshaft lobe pushes the injector's plunger down on every engine cycle. What does energizing the injector's solenoid do?
- Pushes the plunger down to raise pressure
- Closes the spill valve so pressure can build
- Lifts the nozzle needle off its seat directly
- Admits oil from a pressurized rail to the plunger
Show the answer
B. Correct. While the solenoid is off, the spill valve is open and the moving plunger just pushes fuel back to the supply. When the ECM closes the valve, fuel is trapped, pressure rises until the nozzle opens, and injection lasts until the valve reopens; the start of the command sets timing and its length sets quantity.
Why not the others
A. The camshaft lobe, through a rocker, pushes the plunger down; the solenoid has nowhere near that force. It closes the spill valve so the plunger's stroke can build pressure.
C. The needle is lifted hydraulically, once fuel pressure beats the nozzle spring. The solenoid's job is to close the spill valve so that pressure can build.
D. Oil from a high-pressure rail drives a hydraulic electronic unit injector, a different system. In an EUI the cam drives the plunger and the solenoid closes the spill valve.
ID A9-F-029 · Task A9-F-10 (ASE task list)
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A technician plans to run the scan tool's relative compression test on a diesel pickup. Technician A says the test is run with the engine idling so that each cylinder fires. Technician B says the test reports each cylinder's compression pressure in psi. Who is right?
- A only
- B only
- Both A and B
- Neither A nor B
Show the answer
D. Correct. Neither is right. The ECM disables injection while the engine cranks and measures how much each cylinder's compression stroke slows the crankshaft, so Technician A is wrong about idling. The result compares cylinders with one another, often as a percentage, not as psi, so Technician B is wrong; a low cylinder is then confirmed with a compression gauge rated for diesel pressures.
Why not the others
A. Technician A is wrong: the engine is cranked with injection disabled, so no cylinder fires and only compression slows the crankshaft. Technician B is also wrong: the test compares cylinders with each other and does not give pressure in psi.
B. Technician B is wrong: the result is relative, comparing each cylinder with the others, not a pressure in psi. Technician A is also wrong, because the engine is cranked with injection disabled, not idled.
C. Neither technician is right. The test cranks the engine with injection disabled (not idling, as Technician A says) and compares cylinders by crankshaft speed (not in psi, as Technician B says).
ID A9-F-035 · Task A9-F-13 (ASE task list)
02Answer key
| Question | Answer |
|---|---|
| Q1 | B |
| Q2 | B |
| Q3 | A |
| Q4 | B |
| Q5 | D |
03What area F covers
This content area covers how fuel gets from the tank to the cylinders of a pickup or van diesel and the electronics that control it: tanks, lines and selector valves, lift pumps, filters and heaters, priming and bleeding, supply pressure control, high-pressure lines and tubes, and common rail, HEUI, electronic unit injector and rotary pump systems, pedal and PTO controls, connectors, programming, meter tests, and faults from other vehicle systems.
The ASE test specification calls this content area “Fuel System Diagnosis and Repair”. It carries 13 of the 50 scored questions, 26% of the test.
- Inspect tanks, lines, selector valves and lift pumps
- Service filters, heaters and water separators; prime and bleed
- Test supply pressure control, high-pressure lines and tubes
- Test common rail, HEUI, EUI and rotary pump systems
- Diagnose pedal sensors and PTO elevated-idle controls
- Repair connectors, program the ECM and enter injector codes
- Measure voltage, drop, current and resistance with a DMM
- Trace power loss to cruise, ABS or transmission faults
04What the questions turn on
Leakage opens the control valve
The fuel control valve meters what the high-pressure pump takes in. When rail pressure matches desired but the valve sits far more open than normal, fuel is leaving the rail, such as injectors leaking to the return. A return-flow test shows which ones.
Suction pulls in air
A line under suction can draw in air without leaking any fuel, while a pushed line drips at a leak. With clear tubes, solid fuel at the filter inlet but bubbles at its outlet places the air leak at the filter assembly itself.
A new ECM needs the codes
Each injector's correction code tells the ECM how that injector differs from nominal. A replacement ECM starts with default values, so every cylinder is fueled as average until each injector's own code is entered. A new DPF likewise needs its learned load values reset.
Ohm's law finds the resistance
Voltage lost outside a load divided by its current gives the circuit's resistance: 2.4 volts lost at 7.5 amps is 0.32 ohm. A part that reads in range at its own terminals but high through the harness puts the extra resistance in the wiring.
05The other content areas
The A9 test has six content areas. Each has its own page of free questions:
The ASE A9 Light Vehicle Diesel Engines practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.
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