Five free practice questions on general diesel diagnosis: content area A of the ASE A9 Light Vehicle Diesel Engines test, 9 of its 50 scored questions (18%). 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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A technical service bulletin for a hard-start concern on a pickup's diesel lists the affected trucks by vehicle identification number (VIN) range, and the truck in the shop is the model year the bulletin names. Why is its full VIN checked against the bulletin's range?
- The VIN range shows the calibration loaded now
- The registration's model year is often wrong
- The VIN shows whether the repair was already done
- The bulletin may cover only part of a model year
Show the answer
D. Correct. Changes are made at points during production, so a bulletin often applies only to trucks built within a certain VIN or build-date range, and a truck of the right model year can fall outside it. Checking the full VIN, along with the service history for whether the bulletin was already done, decides whether it applies.
Why not the others
A. The VIN identifies the vehicle as built, not the software loaded later; the current calibration is read from the ECM with a scan tool. The VIN range matters because a bulletin can cover only part of a model year.
B. The model year is coded into the VIN itself, so the registration is not the issue. The full VIN is checked because a bulletin can apply to only part of a model year's production.
C. The VIN does not change when a bulletin repair is performed; whether it was done comes from the truck's service or warranty records. The VIN check decides whether the bulletin applies at all.
ID A9-A-004 · Task A9-A-2 (ASE task list)
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Diesel fuel wets the top of a pickup's V8 diesel after it runs. Its fuel filter housing sits on top of the engine between the cylinder heads. Any of these could be the source of the fuel EXCEPT:
- a cracked fitting on the injector return line.
- a loose high-pressure line nut at an injector.
- a dribbling injector nozzle tip.
- a pinched seal under the filter housing cap.
Show the answer
C. Correct. The nozzle tip sits inside the combustion chamber, so fuel dribbling from it goes into the cylinder, where it causes smoke and carbon rather than a wet engine top. The other three leak fuel outside the engine, where it can run onto the top of it.
Why not the others
A. Injector return (leak-off) fuel flows through small fittings on top of the engine, and a cracked one leaks fuel there whenever the engine runs, so it could be the source.
B. A loose high-pressure nut seeps or sprays fuel onto the engine, so it could be the source. Such a leak is located with the engine off and the system depressurized by the manufacturer's procedure, never by feeling for it with a hand, because fuel at that pressure can be injected through the skin.
D. With the filter housing on top of the engine, a pinched cap seal lets fuel seep out onto the engine, so it could be the source.
ID A9-A-010 · Task A9-A-6 (ASE task list)
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An oil analysis report on a pickup's diesel shows soot well above the lab's limit, while silicon, glycol and wear metals are all normal. What does the high soot point to?
- Dirt getting past the air filter element
- Incomplete combustion reaching the oil
- Coolant leaking into the crankcase oil
- Engine bearings wearing faster than normal
Show the answer
B. Correct. Soot is a product of incomplete combustion carried into the crankcase with blowby, so a level above the lab's limit points at more soot reaching the oil than it can carry: over-fueling injectors, restricted intake air or excess EGR, made worse by an oil change interval too long for the engine's duty. Those are checked and the interval reviewed.
Why not the others
A. Dirt getting past the air filter shows up as high silicon, often with rising wear metals; silicon is normal here. The soot comes from combustion.
C. Coolant shows up as glycol, usually with sodium or potassium; glycol is normal here. High soot comes from combustion.
D. Bearing wear shows as rising wear metals such as lead, copper or tin, which are normal here. Soot is a combustion product.
ID A9-A-013 · Task A9-A-15 (ASE task list)
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A pickup's diesel has a diesel oxidation catalyst, a DPF and an SCR catalyst, and its manufacturer requires ultra-low-sulfur diesel. Why can repeated fills of higher-sulfur diesel fuel harm this aftertreatment?
- Sulfur poisons catalysts and adds to DPF loading
- Sulfur lowers the fuel's cetane and causes misfire
- Sulfur makes the fuel gel at a higher temperature
- Sulfur makes the SCR system use far more DEF
Show the answer
A. Correct. Sulfur in the fuel burns to sulfur oxides that coat and poison the catalyst surfaces, and sulfate particles add to what the DPF must hold, so catalyst efficiency falls and the filter loads faster. Using the fuel the manufacturer specifies avoids it.
Why not the others
B. Sulfur content and cetane number are separate fuel properties, and the concern here is the aftertreatment. Sulfur poisons the catalysts and makes the DPF load faster.
C. Gelling depends on the fuel's cloud point, not its sulfur content. Sulfur harms the aftertreatment by poisoning the catalysts and adding to DPF loading.
D. DEF dosing follows the NOx the SCR catalyst must convert, not the fuel's sulfur. The damage from sulfur is catalyst poisoning and faster DPF loading.
ID A9-A-016 · Task A9-A-11 (ASE task list)
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A customer says her pickup's diesel is often hard to start on cold mornings. She turns the key straight to START without waiting, though a wait-to-start lamp lights on the dash at key-on. What should she be told?
- Pump the accelerator pedal while it cranks
- Wait for the lamp to go out before cranking
- Crank without stopping until the engine fires
- Turn on the headlamps first to warm the battery
Show the answer
B. Correct. The wait-to-start lamp stays on while the glow plugs or intake heater preheat the cylinders or intake air; cranking before it goes out sends fuel into air not yet hot enough to ignite reliably. If starts are still hard after waiting, the preheat system is tested.
Why not the others
A. Pumping the pedal is a carbureted-engine habit; on an electronic diesel the ECM sets starting fuel itself, so it does not help. The fix is to let preheat finish by waiting for the lamp to go out.
C. Long cranking before preheat finishes overheats the starter and drains the batteries, and the cold cylinders still may not light the fuel. Waiting for the lamp to go out lets preheat do its job.
D. Switching on the headlamps draws the battery down before cranking and does nothing about the cause. The hard start comes from cranking before preheat has finished, so she waits for the lamp to go out.
ID A9-A-027 · Task A9-A-16 (ASE task list)
02Answer key
| Question | Answer |
|---|---|
| Q1 | D |
| Q2 | C |
| Q3 | B |
| Q4 | A |
| Q5 | B |
03What area A covers
This content area covers finding the cause before any repair on a pickup or van diesel: verifying the complaint, checking bulletins and service history by VIN, scan tool data, freeze frame and tampered parts, leaks, noise, vibration and smoke, fuel quality, crankcase pressure, power and running complaints, no-start and hard-start problems, coolant and oil condition, and battery and charging faults that affect the engine.
The ASE test specification calls this content area “General Diagnosis”. It carries 9 of the 50 scored questions, 18% of the test.
- Verify the complaint and road test the vehicle
- Look up bulletins and service information by VIN
- Read DTCs, freeze frame and live scan data
- Spot leaks, tampered parts and harness damage
- Isolate noise, vibration, smoke and odor
- Check fuel quality and run a crankcase pressure test
- Diagnose starting, power, misfire and rough running complaints
- Trace engine faults to coolant, oil, battery or charging
04What the questions turn on
Read the freeze frame first
The freeze frame records conditions at the moment a DTC set, not at shutdown. An inactive code often marks an intermittent fault, so the circuit is tested under the conditions it shows. A cold-start complaint is verified after an overnight cold soak.
The derate may be the ECM's
Reduced power with a sensor circuit DTC is often the ECM protecting the engine, not proof that boost is low, so the circuit is checked first. A pedal sensor reading well above its released value makes the ECM add fuel and hold the idle up.
Pressure on both sides
Pressure read on each side of a part shows where the loss is. A large drop across the fuel filter starves the high-pressure pump. Crankcase pressure that falls under its limit once the turbocharger drain is isolated points to air passing the turbo seals.
Cranking voltage and speed
If battery voltage falls below the ECM's operating minimum while cranking, the ECM drops out and cannot inject. Slow cranking from discharged batteries also keeps an engine-driven high-pressure pump from building the rail pressure the ECM waits for, so batteries are tested first.
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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