22 General Engine Diagnosis Practice Questions & Answers

1. 1. A customer reports that a light-duty diesel pickup "loses power on long grades." No DTCs are stored and the engine idles normally in the shop. What should the technician do first?

   • A.Replace the fuel and air filters, then release the truck for a retest
   • B.Run a cylinder contribution test with the engine idling in the bay
   • C.Clear the ECM memory so that any new fault will store a fresh code
   • D.Road test or dyno test the truck to duplicate the reported condition✓ Answer

   Every diagnosis starts by verifying the concern under the conditions the customer describes; a complaint that only appears under sustained load cannot be confirmed at idle in the bay. Replacing parts or clearing codes before the condition is duplicated destroys the evidence and may hide the fault.

   Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: verify the customer's concern and determine if further diagnosis is needed; road/dyno testReport a problem with this question

2. 2. Before looking up specifications and technical bulletins for a diesel engine repair, the technician records the VIN together with the engine model, calibration and serial numbers. Why does this matter?

   • A.Emission warranty coverage is calculated from the calibration number
   • B.The serial number sets the injection timing baseline used by the ECM
   • C.Suppliers require these numbers before a core charge can be waived
   • D.Some procedures and bulletins apply only to certain build ranges✓ Answer

   Diesel engines are built in overlapping calibration and serial-number ranges, and manufacturers publish specifications, precautions and bulletins that apply only inside a stated range. Identifying the engine before pulling service information is what keeps the technician from judging a measurement against a specification that does not belong to the engine in front of him.

   Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: identify engine model, calibration and serial numbers to obtain the correct service information and bulletinsReport a problem with this question

3. 3. A light-duty diesel blows heavy black smoke under hard acceleration and feels down on power. Which of these could be the cause?

   • A.A plugged air filter element restricting airflow into the cylinders✓ Answer
   • B.A glow plug circuit that does not energize before a cold start
   • C.Coolant entering a cylinder past a failed cylinder head gasket
   • D.A crankcase filled above the full mark on the engine oil dipstick

   Black smoke is unburned carbon: too much fuel for the air actually available in the cylinder. A restricted air filter cuts the air side of that ratio, so the injected fuel cannot burn completely and power falls off. Oil in the crankcase gives blue smoke, and a glow plug fault or coolant intrusion gives white smoke.

   Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: inspect and interpret exhaust smoke colour and odourReport a problem with this question

4. 4. Steady blue-grey smoke leaves the tailpipe at idle and light load, and the engine is using a measurable amount of oil. What does this most directly indicate?

   • A.Engine oil is entering the combustion chamber and burning with the fuel✓ Answer
   • B.More fuel is being injected than the available air can burn completely
   • C.Injection timing is retarded enough that combustion continues in the exhaust
   • D.Coolant is being drawn into the cylinder and leaving as white vapor

   Blue smoke is burning lubricating oil, so the finding points to an oil path into the combustion chamber such as worn rings or liners, valve stem seals, a turbocharger seal, or an overfilled crankcase. Smoke colour narrows the search to a family of causes; it is a starting point and still has to be confirmed by testing.

   Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: inspect and interpret exhaust smoke colour and odourReport a problem with this question

5. 5. A diesel pushes white smoke and runs rough for about a minute after a cold start, then clears up and runs normally. Technician A says an inoperative glow plug or intake air heater circuit can cause this. Technician B says white smoke proves that coolant is entering the cylinders through the head gasket. Who is right?

   • A.Neither Technician A nor Technician B
   • B.Both Technician A and Technician B
   • C.Technician A only✓ Answer
   • D.Technician B only

   A diesel ignites by compression heat alone, and on a cold engine that heat is marginal, so a failed glow plug or inlet air heater leaves fuel unburned and it leaves the tailpipe as white smoke with a rough, misfiring idle that clears as the engine warms. Coolant intrusion also makes white smoke, but it persists after warm-up and usually carries a sweet odour, so colour by itself proves nothing.

   Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: interpret exhaust smoke and odour; cold-start driveability diagnosisReport a problem with this question

6. 6. Crankcase pressure measured at the oil fill tube with a manometer at rated speed and normal operating temperature is well above the manufacturer's published limit. Technician A says worn piston rings or cylinder liners can produce this reading. Technician B says a crankcase ventilation filter plugged with oil mist can produce the same reading. Who is right?

   • A.Technician B only
   • B.Both Technician A and Technician B✓ Answer
   • C.Neither Technician A nor Technician B
   • D.Technician A only

   Crankcase pressure rises either because more combustion gas is getting past the rings or because normal blow-by cannot get out. Worn rings and liners, a cracked piston or a leaking turbocharger seal add gas, while a plugged ventilation filter traps it and pushes oil past seals, so the ventilation side must be ruled out before the base engine is condemned.

   Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: perform crankcase pressure (blow-by) test and determine needed actionReport a problem with this question

7. 7. A used-oil analysis on a diesel engine reports glycol present along with elevated sodium and potassium. What does this finding indicate?

   • A.The oil has been left in service far beyond its recommended interval
   • B.Dirt is bypassing the air filter and reaching the cylinder walls
   • C.Fuel is diluting the oil during repeated post-injection regenerations
   • D.Coolant is leaking internally into the engine's lubrication system✓ Answer

   Glycol is the base of engine coolant and sodium and potassium come from its inhibitor package, so finding them in the oil means coolant is crossing into the lubrication system internally. Silicon with abrasive wear metals would mean dirt ingestion, and fuel dilution shows up as a drop in viscosity and flash point instead.

   Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: check engine oil condition and interpret oil analysis resultsReport a problem with this question

8. 8. A used-oil analysis reports elevated silicon along with abrasive wear metals. What is the technician's best next step?

   • A.Replace the crankcase ventilation filter and recheck crankcase pressure
   • B.Shorten the oil change interval and keep monitoring the engine
   • C.Inspect the air filter housing, seals and ducting for a leak path✓ Answer
   • D.Pressure test the cooling system to locate an internal coolant leak

   Silicon in the oil is airborne dust, and dust reaches the oil only after it has gone through the cylinders, so the fault lies in the air induction sealing rather than in the oil. Finding and closing the leak path stops the abrasion; changing the oil more often only dilutes the evidence while the wear continues.

   Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: check engine oil condition and interpret oil analysis resultsReport a problem with this question

9. 9. On a cold morning a diesel with two batteries cranks slowly and will not start; no DTCs are stored. Voltage drop across a battery cable connection measures 0.6 V while cranking and the service data allows 0.2 V maximum. Technician A says the high-resistance connection by itself can cause the no-start, because cranking speed determines how much heat compression develops. Technician B says cranking speed cannot cause a no-start as long as the starter still turns the engine over. Who is right?

   • A.Neither Technician A nor Technician B
   • B.Both Technician A and Technician B
   • C.Technician B only
   • D.Technician A only✓ Answer

   A diesel is a compression-ignition engine, so the charge has to reach ignition temperature from compression heat alone; slow cranking gives blow-by and heat transfer more time per stroke and peak temperature never gets there. A drop of 0.6 V against a 0.2 V limit is excessive resistance in the circuit, and correcting it is the repair.

   Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: diagnose no-crank, crank-but-fails-to-start and hard-starting conditionsReport a problem with this question

10. 10. A misfire DTC is stored for cylinder 4. A scan-tool cylinder cut-out test produces no rpm change when cylinder 4 is disabled, and a cranking compression test reads far below the service specification on that cylinder while the others are within spec. What is the most likely cause?

    • A.A restricted fuel filter starving the fuel supply during acceleration
    • B.A crankshaft position sensor signal that confuses the misfire monitor
    • C.Low compression in cylinder 4 caused by a mechanical fault in that cylinder✓ Answer
    • D.An open injector solenoid circuit at the cylinder 4 harness connector

    Compression ignition needs compression, so a cylinder that measures far below specification cannot fire no matter what the injector does, and the cut-out test confirms the cylinder was already contributing nothing. An injector circuit fault would normally set its own circuit code, and a restricted filter or a sensor fault would affect every cylinder rather than one.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: diagnose engine misfire, rough running and low power complaintsReport a problem with this question

11. 11. A DTC for the intake air temperature sensor circuit is stored on a diesel with a driveability complaint. Technician A says the stored code proves the sensor itself has failed and should be replaced. Technician B says the stored code proves the wiring to that sensor is open. Who is right?

    • A.Both Technician A and Technician B
    • B.Technician B only
    • C.Technician A only
    • D.Neither Technician A nor Technician B✓ Answer

    A generic circuit DTC only reports that the ECM saw a value outside the range or rationality window its software allows; it names the circuit and the failure criteria, not the failed part. The sensor, the wiring, a connector terminal, the ECM, or even a real operating condition can produce it, so the code directs pinpoint testing rather than replacing anything.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: retrieve and record DTCs, freeze frame and operating data; SAE J1979 diagnostic trouble code definitionsReport a problem with this question

12. 12. An intermittent low-power complaint sets a DTC about once a week. Why should the technician record freeze-frame data before clearing the code?

    • A.It identifies which component in the circuit has failed internally
    • B.It resets the readiness monitors so the code cannot set falsely
    • C.It captures the operating conditions present when the code set✓ Answer
    • D.It records every sensor value for the entire drive cycle in order

    Freeze frame is a snapshot of load, speed, temperature and fuel data taken at the instant the code set, and clearing the code erases it. On an intermittent fault that snapshot is what tells the technician which conditions to reproduce on the road test, and it is the same set of conditions used later to verify the repair.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: retrieve and record DTCs, freeze frame and operating dataReport a problem with this question

13. 13. This question contains the words LEAST LIKELY. Read the question carefully. Steady blue smoke with high oil consumption is LEAST LIKELY to be caused by which of these?

    • A.A crankcase filled above the full mark on the oil dipstick
    • B.A turbocharger center-housing seal leaking oil into the exhaust
    • C.An intake air heater element that does not energize on a cold start✓ Answer
    • D.Valve stem seals hardened enough to pass oil down the guides

    Blue smoke means oil is being burned, so the causes are all oil paths into the combustion chamber or the exhaust stream. An intake air heater that will not energize affects starting heat, not oil control, and its symptom is white smoke and rough running on a cold start that clears once the engine warms.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: inspect and interpret exhaust smoke colour and odourReport a problem with this question

14. 14. A diesel has a heavy knock that appears to come from one cylinder. Technician A says a knock that disappears when that cylinder is cut out proves the injector solenoid circuit is at fault. Technician B says the cut-out test isolates the noise to that cylinder without identifying which part has failed. Who is right?

    • A.Both Technician A and Technician B
    • B.Technician A only
    • C.Technician B only✓ Answer
    • D.Neither Technician A nor Technician B

    Cutting a cylinder out removes its combustion load, so a noise that quiets is a noise driven by cylinder pressure in that cylinder, which narrows the search to a rod bearing, a piston, a valve train part or the injector itself. The test locates the noise, not the part, and an injector circuit fault would generally set its own circuit code.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: diagnose engine noises and determine needed actionReport a problem with this question

15. 15. A vibration is felt only above 45 mph, varies with road speed rather than engine speed, and does not change when the transmission is shifted to a different gear at the same road speed. What does this indicate?

    • A.An engine misfire that appears only under sustained highway load
    • B.A source in the driveline or wheels rather than in the engine itself✓ Answer
    • C.A vibration damper failing on the front of the engine crankshaft
    • D.A turbocharger rotating assembly out of balance under high boost

    Changing gear at a fixed road speed changes engine speed while wheel and driveshaft speed stay the same, so a vibration that ignores the gear change is tied to something turning at road speed. Anything driven by the crankshaft would shift frequency with the gear change, which is what separates an engine source from a driveline source.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: diagnose engine vibration concerns and determine needed actionReport a problem with this question

16. 16. A diesel pickup starts normally when it is driven daily but cranks slowly after sitting for three days. Charging system output is within specification and both batteries pass a load test. Technician A says both batteries should simply be replaced as a matched pair. Technician B says a key-off parasitic draw test should be run and compared with the manufacturer's maximum. Who is right?

    • A.Neither Technician A nor Technician B
    • B.Both Technician A and Technician B
    • C.Technician A only
    • D.Technician B only✓ Answer

    Batteries that pass a load test and a charging system that meets specification point away from the battery and alternator and toward something discharging the batteries while the vehicle sits. Measuring key-off current draw against the published maximum tells the technician whether a circuit is staying awake, and replacing good batteries would leave that drain in place.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: diagnose engine complaints caused by undercharge, overcharge, no-charge and key-off parasitic drawReport a problem with this question

17. 17. During the visual inspection of a diesel with a driveability complaint, the technician finds a non-original programmer module spliced into the engine harness. What should be done before the diagnosis continues?

    • A.Return the engine and its harness to original configuration, then retest✓ Answer
    • B.Continue the electrical tests, since the module only adds fuel delivery
    • C.Reprogram the ECM to match the calibration the added module expects
    • D.Ignore the module, because aftermarket parts do not set trouble codes

    A device that intercepts or rewrites sensor and injector signals makes both the scan data and the DTCs untrustworthy, because the values the ECM reports are no longer the values the engine produced. Restoring the original configuration and duplicating the concern again is what tells the technician whether a genuine fault exists at all.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: inspect for damaged, missing, modified or tampered componentsReport a problem with this question

18. 18. A diesel has a sharp ticking noise that is loudest at cold start and quiets as the engine warms up. A black soot streak is found around one exhaust manifold port on the cylinder head. What is the most likely cause?

    • A.A valve clearance that is set tight on one cylinder when cold
    • B.An injector nozzle that ticks louder until the fuel warms up
    • C.A piston skirt slapping the bore until the piston warms and grows
    • D.An exhaust leak at the manifold joint that seals as parts expand✓ Answer

    Soot deposited around a joint is exhaust gas escaping and leaving carbon where it blows, so the streak locates the leak. A cold manifold has the largest gap and the loudest pulse; as the iron expands the joint closes up and the tick fades, which is the classic pattern for a leaking manifold gasket or a cracked manifold.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), tasks: inspect for exhaust leaks; diagnose engine noisesReport a problem with this question

19. 19. On the first very cold morning of the season a diesel starts, runs a few minutes, loses power and stalls. The fuel filter is found packed with a soft waxy material. What is the most likely cause?

    • A.Fuel blended with an excessive amount of sulfur for the season
    • B.Water in the tank that froze and blocked the fuel pickup screen
    • C.Engine oil too thick at that temperature to let the pump turn
    • D.Fuel cooled past its cloud point and dropped wax crystals out✓ Answer

    Diesel fuel carries dissolved paraffin wax that comes out of solution as the fuel cools past its cloud point, and the filter media is where those crystals collect first. The engine runs until the restriction starves it, which is why the stall comes minutes after a normal start on the first hard freeze.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: check fuel for contamination, quality and cold-weather operabilityReport a problem with this question

20. 20. A diesel surges at a steady highway cruise. Technician A says that because no DTC is stored, the concern cannot be an electronically controlled fault. Technician B says the surge can be confirmed at idle in the shop, so a road test is unnecessary. Who is right?

    • A.Technician B only
    • B.Neither Technician A nor Technician B✓ Answer
    • C.Both Technician A and Technician B
    • D.Technician A only

    Monitors only set a code when a value leaves the range or fails the rationality test written into the software, so a fault can be well outside the driver's tolerance and still store nothing. A surge that appears only under road load also cannot be duplicated at no-load idle, so the concern has to be reproduced under the conditions the customer described.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), tasks: verify the customer's concern and road test; retrieve and interpret DTCsReport a problem with this question

21. 21. A repair has been completed for a DTC that set only under load on a long highway grade. What best verifies that the repair was successful?

    • A.Compare scan data with a known-good vehicle idling in the shop
    • B.Confirm that no codes are present with the key on and engine off
    • C.Operate the vehicle under the conditions the freeze frame recorded✓ Answer
    • D.Clear the codes and return the vehicle to the customer to monitor

    A repair is only proven when the fault fails to return under the same conditions that set it, and the freeze frame recorded exactly what those conditions were. Checking for codes with the engine off never loads the engine, and handing the vehicle back to the customer moves the verification step onto the customer instead of the technician.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: clear DTCs and verify the repairReport a problem with this question

22. 22. This question contains the word EXCEPT. Read the question carefully. Heavy black exhaust smoke under load can be caused by all of these EXCEPT:

    • A.Worn valve stem seals that let oil into the combustion chambers✓ Answer
    • B.An injector delivering more fuel into one cylinder than commanded
    • C.An air inlet restriction that limits airflow reaching the cylinders
    • D.An exhaust gas recirculation valve stuck open under higher loads

    Black smoke comes from an air-fuel ratio that is too rich for the air available, so anything that cuts air or adds fuel produces it, including an inlet restriction, an EGR valve stuck open that displaces fresh air, and an over-fuelling injector. Oil passing valve stem seals burns as blue smoke and is an oil-control problem rather than an air-fuel one.

    Source: ASE A9 Test Specifications, Area A (General Diagnosis), task: inspect and interpret exhaust smoke colour and odourReport a problem with this question

Practice questions based on the ASE A9 Light Vehicle Diesel Engines task list. This site is not affiliated with or endorsed by the National Institute for Automotive Service Excellence (ASE). Injection pressures, torque specifications, clearances, glow-plug and heater resistance values, regeneration temperatures and service intervals are set by the engine manufacturer, differ by engine family and model year, and are revised by service bulletin — always work to the service information for the vehicle in front of you, never to a practice test. Emissions and aftertreatment hardware varies by model year and market. Diesel fuel systems operate at pressures that can inject fuel through skin; follow the manufacturer's depressurization and safety procedures. Confirm current test content and registration requirements with ASE before you test. About ASE certification →