ASE A8 general engine diagnosis practice questions.

Five free practice questions on general engine diagnosis: content area A of the ASE A8 Engine Performance test, 12 of its 50 scored questions (24%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.

01The questions

1. Recall · General Diagnosis

   The service information for a port-injected engine with an electronic throttle warns technicians to keep their fingers out of the throttle body whenever the ignition is on, even with the engine off. What is the reason for this precaution?

   1. The PCM can move the throttle plate on its own
   2. Manifold vacuum can pull a finger into the bore
   3. The injectors spray fuel into the throttle bore
   4. The 5-volt TP sensor circuit can shock the skin
   Show the answer

   A. Correct. An electronic throttle plate is opened and closed by a motor the PCM controls, and the PCM can drive it whenever the ignition is on, even with the engine stopped. A finger in the bore can be pinched, which is why service precautions are read before work on the system.

   Why not the others

   B. With the engine off there is no manifold vacuum, so nothing pulls on a finger. The hazard with the ignition on is the throttle motor, which the PCM can drive at any time.

   C. On a port-injected engine the injectors spray into the intake ports near the valves, not into the throttle body. The warning is about the throttle motor, which can move the plate with the ignition on.

   D. A 5-volt sensor circuit cannot drive a shock through skin. The warning exists because the PCM can move the throttle plate with its motor whenever the ignition is on.

2. Diagnosis · General Diagnosis

   A light, rhythmic ticking that follows engine speed is loudest on a cold engine and fades as the engine warms up. It is loudest near the right exhaust manifold, black soot streaks mark one manifold-to-head joint, and a stethoscope on the valve cover finds the valve train quiet. Which of these could be the cause?

   1. A collapsed hydraulic lifter
   2. A failed manifold gasket
   3. Normal fuel injector clicking
   4. A loose exhaust heat shield
   Show the answer

   B. Correct. Each exhaust pulse that escapes through a gap at the manifold makes a tick, and the soot at the joint shows where it gets out. As the manifold heats, it expands and partly closes the gap, so the tick fades when warm; the joint is inspected for a failed gasket, a broken stud or a warped flange.

   Why not the others

   A. A lifter tick comes from inside the valve cover, where the stethoscope found the valve train quiet. The soot at the manifold joint and a tick that fades as the manifold warms point to an exhaust leak.

   C. Injectors click on the intake side at every temperature, and the sound does not fade as the engine warms. A tick at the exhaust manifold with soot at the joint is an exhaust leak.

   D. A loose heat shield buzzes or rattles with vibration at certain speeds and leaves no soot at a manifold joint. A rhythmic tick at a sooty joint that fades as the manifold warms points to a leaking gasket.

3. Recall · General Diagnosis

   For a relative compression test, a lab scope with a current clamp on a battery cable records starter current while the engine cranks with fuel and ignition disabled. One of the regular current peaks is clearly lower than the others. What does this show?

   1. The battery has one weak cell
   2. That cylinder has higher compression than the rest
   3. That cylinder has lower compression than the rest
   4. That cylinder's spark plug is fouled
   Show the answer

   C. Correct. The starter draws the most current as each piston pushes up against compression, so each cylinder makes one current peak. A cylinder that resists less draws less, so a low peak marks the weak cylinder, which a cranking compression or leakage test then confirms.

   Why not the others

   A. A weak cell would lower battery voltage, and so the current, through the whole crank, not on one cylinder's stroke out of every cycle. The repeating low peak belongs to one cylinder's compression.

   B. That reads the trace backward: higher compression makes the starter work harder and draw more current. A lower peak means that cylinder offers less resistance, which is lower compression.

   D. Ignition is disabled during the test, so no cylinder fires and plug condition has no effect on the current. The peaks show how hard each piston is to push up on compression.

4. Diagnosis · General Diagnosis

   A pickup boils over and pushes coolant out of the reservoir in hot, slow traffic. With a pressure tester on the system, it holds 16 psi for the manufacturer's test time with no leaks. On the cap adapter, the radiator cap, which is stamped 16 psi, opens at 7 psi. What should the technician do next?

   1. Run a combustion leak test on the coolant
   2. Replace the thermostat with a new one
   3. Replace the cap and retest the system
   4. Flush the radiator and refill the coolant
   Show the answer

   C. Correct. A cap that opens at 7 psi instead of 16 psi lets the system run at less than half its rated pressure, which lowers the coolant's boiling point, so it boils over in hot traffic. The system itself holds pressure, so a new cap of the specified rating is installed and the cooling system rechecked.

   Why not the others

   A. Combustion gases in the coolant would also push it out of the reservoir, but the system holds pressure and the cap has already failed its test. Replacing the cap comes first; the leak test follows only if the boil-over continues.

   B. A thermostat sticking closed would overheat the engine, but the test results already show the fault: the cap opens at 7 psi against its 16 psi rating. The cap is replaced first.

   D. Flushing addresses a plugged or contaminated system, which nothing here shows. The cap releasing at 7 psi instead of 16 psi is the fault the tests found.

5. Diagnosis · General Diagnosis

   The wiring diagram shows fuse 18, hot in RUN, feeding the EVAP purge valve, the EVAP canister vent valve and the A/C compressor clutch relay coil; the PCM grounds each one. Circuit DTCs are stored for both EVAP valves, and the A/C compressor clutch never engages. Which of these could be the cause?

   1. An open in fuse 18 or its feed
   2. An open purge valve winding
   3. A failed A/C pressure switch
   4. An open canister vent valve ground wire
   Show the answer

   A. Correct. The three parts share fuse 18 and its feed, so one open there stops all three at once, while a fault in one part's own winding or ground wire would stop that part alone. The fuse and its feed are checked first, and if the fuse has blown, the circuit it feeds is checked for a short before a new fuse goes in.

   Why not the others

   B. An open winding would set a DTC for the purge valve alone; it cannot stop the vent valve or the A/C clutch relay. Three parts failing together point to the feed they share.

   C. A pressure switch can keep the clutch from engaging, but it has no connection to the EVAP valves' circuits. One fault that explains all three symptoms is the shared feed through fuse 18.

   D. The PCM grounds each part on its own wire, so an open in the vent valve's ground would affect only that valve. The common point the diagram shows is fuse 18.

02Answer key

03What area A covers

This content area covers the first steps on any driveability job and the base engine checks behind them: verifying the complaint and researching service information, noise, vibration and exhaust smoke, vacuum, power balance, compression and leakage tests, valve adjustment and cam timing including variable valve timing, oil and cooling system problems, and battery, starting and charging faults that affect how the engine runs.

The ASE test specification calls this content area “General Diagnosis”. It carries 12 of the 50 scored questions, 24% of the test.

• Verify the complaint, inspect and road test
• Research service information, bulletins and history
• Diagnose engine noise, vibration and exhaust smoke
• Run vacuum, power balance, compression and leakage tests
• Check valve clearance, cam timing and variable valve timing
• Trace driveability faults to oil or cooling problems
• Test battery, starting and charging systems

04What the questions turn on

Compression first, then leakage

A cranking compression test shows which cylinder is weak. A leakage test, with the piston at top dead center on compression so both valves are closed, shows where the air escapes: intake, exhaust, crankcase or coolant. That decides the repair.

Low but even compression

When compression is low and even on every cylinder and a squirt of oil does not raise it, the rings are sealing. Valve timing retarded by a jumped chain closes every intake valve late and lowers all the readings together.

Reading a vacuum gauge

A steady reading below specification points to something that affects every cylinder evenly, such as retarded valve timing or a constant intake leak. A needle that drops sharply at a regular rhythm points to one cylinder, such as a valve that is not sealing.

Where unmetered air shows up

On a MAF engine, air entering behind the sensor goes unmeasured. At idle it is a large share of the airflow, so fuel trim reads high. At cruise the same leak is small next to total airflow, so the trim returns near normal.

05The other content areas

The A8 test has five content areas. Each has its own page of free questions:

The ASE A8 Engine Performance 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.