Five free practice questions on general powertrain diagnosis: content area A of the ASE L1 Advanced Engine Performance Specialist test, 6 of its 50 scored questions (12%). 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 turbocharged engine with its MAF sensor ahead of the turbocharger was fitted with an aftermarket blow-off valve that vents to the atmosphere instead of back into the intake. Why does the engine now stumble and run rich each time the throttle snaps closed under boost?
- Lost boost makes the PCM add fuel to restore power
- Unmetered air enters through the open valve
- Trapped boost surges back through the MAF sensor
- Air the MAF has already counted is vented outside
Show the answer
D. Correct. The PCM fuels for the air the MAF has counted, but when the valve dumps that boosted air outside, the engine never receives it and the mixture goes rich for a moment. A recirculating valve returns the air ahead of the turbocharger, so nothing measured is lost.
Why not the others
A. The PCM does not add fuel to make up for lost boost; it fuels for the airflow the MAF reports. The rich stumble is fuel for air that was measured and then vented.
B. While it is venting, the valve is pushing boosted air out, not drawing air in, and unmetered air would make the mixture lean, not rich. The rich stumble comes from measured air being lost.
C. A blow-off valve opens precisely to release boost trapped against the closed throttle, so it prevents surging back through the compressor. The problem is that the air it releases had already been measured and fueled.
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On a coil-on-plug V6, four coils on different cylinders have failed within a few months. Any of these could cause the repeated failures on several cylinders EXCEPT:
- a leaking spark plug tube seal at one cylinder.
- spark plugs worn far past their gap in every cylinder.
- water forced into the plug wells by engine washing.
- oil from leaking valve cover gaskets in the plug wells.
Show the answer
A. Correct. A leaking tube seal lets oil into one plug well and can ruin that one cylinder's coil boot again and again, but it cannot reach the coils on other cylinders. Failures spread over several cylinders point to a cause they share.
Why not the others
B. This could cause it. Wide gaps need higher firing voltage on every cylinder, which stresses each coil's secondary winding and boot insulation, so several coils can break down.
C. This could cause it. Water driven into the plug wells by pressure washing reaches every well at once, letting the spark track down the boots and corroding the terminals, so coils on several cylinders can fail.
D. This could cause it. Oil seeping into the plug wells on both banks softens and tracks the coil boots on many cylinders, so several coils can fail.
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A pickup owner says fuel economy has dropped about 15 %. On a level road in Neutral, the truck now coasts from 50 mph down to 30 mph in noticeably less time than an identical truck. Any of these could cause both findings EXCEPT:
- an ECT sensor reading colder than actual.
- a brake caliper that does not fully release.
- tires inflated well below the placard pressure.
- a wheel bearing adjusted with far too much preload.
Show the answer
A. Correct. In Neutral the engine is disconnected from the wheels, so nothing the engine does, including extra fuel from a cold-reading ECT sensor, can change how quickly the truck coasts down. The faster coast-down points to drag in the brakes, wheels or tires.
Why not the others
B. This could cause both. A dragging caliper holds a pad against the rotor, slowing the truck on a coast-down and making the engine work harder all the time, which costs fuel.
C. This could cause both. Underinflated tires flex more and raise rolling resistance, so the truck slows faster when coasting and burns more fuel to hold speed.
D. This could cause both. Excess preload makes the bearing drag, adding friction that shortens the coast-down and takes power the engine must make up with fuel.
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On a turbocharged engine with its MAF sensor ahead of the turbocharger, the front-mounted air-to-air intercooler's outer fins are packed with leaves and road debris, while coolant temperature stays normal under load. Why can this make the PCM retard spark timing under boost?
- Hotter charge air is less dense, so the mixture goes lean
- Hotter charge air makes the engine knock sooner
- Boost escapes through the blocked outer fins
- The MAF sensor now reports more air than enters
Show the answer
B. Correct. The intercooler can no longer shed heat to the outside air, so compressed air enters the cylinders hotter, and hotter charge air detonates more easily. The knock sensors pick up the knock and the PCM pulls spark timing, costing power until the fins are cleaned.
Why not the others
A. The MAF ahead of the turbocharger measures the mass of air, and that same mass reaches the cylinders whatever its temperature, so the fuel still matches it and the mixture does not go lean. The hotter charge itself is what knocks and makes the PCM retard spark.
C. Debris on the outside of the fins blocks cooling air; it does not open a path for boost to leak from the sealed charge-air passages. The hotter charge air causes the knock retard.
D. The MAF sits ahead of the turbocharger and is not affected by what happens in the intercooler. Spark is retarded because the hotter charge knocks.
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A customer says a sedan's engine idles at about 1,100 rpm for roughly 40 seconds after every cold start, then settles to 650 rpm. No DTCs are stored, fuel trims stay within plus or minus 3 %, ECT climbs steadily from the outside temperature, and during the fast idle the scan tool shows spark timing a few degrees after top dead center. The service information describes a cold-start mode for this engine. Which of these is the cause?
- The converter warm-up mode at work
- An intake leak at the manifold gasket
- An ECT sensor reading colder than actual
- The PCM relearning idle airflow
Show the answer
A. Correct. Raising idle airflow while retarding spark sends more heat into the exhaust, so the catalytic converter reaches its working temperature sooner after a cold start. A timed fast idle with retarded spark, normal trims and a normally rising ECT is the warm-up mode working as designed, so the customer is told it is normal operation.
Why not the others
B. An intake leak lets in unmetered air at every start and at warm idle too, and on most engines it pushes fuel trim positive. Here the trims stay within 3 % and the idle settles to 650 rpm, so there is no leak to find.
C. An ECT sensor that reads cold would keep the PCM in its cold-engine settings longer and would not track the outside temperature at startup. The ECT reading starts at the outside temperature and climbs steadily, so the sensor is reading correctly.
D. A PCM relearning idle airflow, as some engines must after a battery disconnect or throttle cleaning, does it once, not after every cold start. A timed fast idle with spark after top dead center at each cold start matches the warm-up mode the service information describes.
02Answer key
| Question | Answer |
|---|---|
| Q1 | D |
| Q2 | A |
| Q3 | A |
| Q4 | B |
| Q5 | A |
03What area A covers
This content area covers driveability and emission problems that start outside the engine controls: telling mechanical faults from electrical ones, variable valve timing and lift, cooling system and engine mechanical faults, transmission, final drive and tire size changes, and intake and exhaust problems or modifications. It also covers the shared routine of verifying the concern, researching service information, finding the root cause and verifying the repair.
The ASE test specification calls this content area “General Powertrain Diagnosis”. It carries 6 of the 50 scored questions, 12% of the test.
- Confirm the concern: fault or normal operation
- Inspect for missing, modified or tampered mechanical parts
- Research service information and system operation
- Separate mechanical from electrical faults, VVT and VVL included
- Trace driveability faults to cooling or engine mechanical problems
- Check transmission, final drive and tire size effects
- Diagnose intake and exhaust problems and modifications
- Find root causes and verify the repair
04What the questions turn on
Fault or normal operation
Some complaints are the system working as designed. A timed fast idle with retarded spark after each cold start can be a converter warm-up mode. A PCM limiting torque because the transmission fluid is too hot is protecting it; the heat source is found next.
Held off on purpose
In the composite vehicle questions, a part that reads off may be held off by the computer on purpose. Coolant below the enable temperature keeps TCC and EGR commands at zero, and a fault in one valve lift sensor sends both banks to low lift.
Repeat failures share a cause
When a part keeps failing, look for what is killing it. Sludge that plugged one cam oil control valve plugs the next unless it is cleaned out. Coils failing on several cylinders point to a cause they share, such as oil in the plug wells.
Add the trims together
When a repair is judged on short-term plus long-term fuel trim, add the two. Each can pass alone while the total fails, and a lean total showing mostly at idle is the pattern of an air leak, so the repair is not verified.
05The other content areas
The L1 test has six content areas. Each has its own page of free questions:
The ASE L1 Advanced Engine Performance Specialist 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.
ToolboxPrep is an independent study site. It is not affiliated with or endorsed by ASE (the National Institute for Automotive Service Excellence). Our questions are original practice questions written to the published ASE test specifications; they are not actual test questions.