Five ASE A8 Engine Performance practice test questions, answers included.

1. Recall · General Diagnosis

   During a power balance test at idle, disabling a cylinder drops engine speed by 90 to 120 rpm on five of the six cylinders and by only 10 rpm on cylinder 4. What does this result show?

   1. Cylinder 4 is producing the most power
   2. Cylinder 4 is producing little power
   3. Cylinder 4 has low compression
   4. The other five cylinders are misfiring
   Show the answer

   B. Correct. Shutting off a cylinder that is doing its share of the work makes the engine slow noticeably; when shutting one off barely changes the speed, that cylinder was contributing little to begin with. The test shows which cylinder is weak, not why, so compression, spark and injector checks on cylinder 4 come next.

   Why not the others

   A. That reads the test backward. The more work a cylinder does, the more the engine slows when it is shut off, so the smallest drop marks the weakest cylinder, here cylinder 4.

   C. Low compression is only one possible reason for a weak cylinder; a spark or fuel fault in that cylinder gives the same small drop. The result shows that cylinder 4 contributes little, and further tests find the cause.

   D. Cylinders that slow the engine by 90 to 120 rpm when shut off are doing their work. A misfiring cylinder would show a small drop like cylinder 4's, so the result points to cylinder 4 alone as weak.

2. Recall · Ignition System Diagnosis and Repair

   Spark plugs for the same engine are made in more than one heat range. How does a hotter plug differ from a colder one?

   1. It carries tip heat into the head more quickly
   2. It carries tip heat into the head more slowly
   3. It fires with a higher-voltage spark
   4. Its threads reach farther into the chamber
   Show the answer

   B. Correct. Heat range describes how fast the plug carries heat from its insulator tip into the cylinder head. A hotter plug has a longer insulator nose, so the heat has a longer path out and the tip runs hotter, which burns off deposits; a plug that runs too hot can glow and start pre-ignition.

   Why not the others

   A. That describes a colder plug, whose shorter insulator nose gives heat a short path into the shell and the head. A hotter plug sheds tip heat more slowly, so its tip runs hotter.

   C. Heat range has nothing to do with the spark: the voltage comes from the coil and is whatever the gap and cylinder conditions require. Heat range is how fast the insulator tip sheds heat into the head.

   D. Reach is the length of the threaded section, set by the cylinder head and independent of heat range; plugs of different heat ranges are made with the same reach. Heat range is how fast the tip sheds heat.

3. Recall · Fuel, Air Induction and Exhaust Systems Diagnosis and Repair

   On a port fuel injection system with a return line, the fuel pressure regulator on the rail has a hose to intake manifold vacuum. What is the purpose of that vacuum connection?

   1. To raise fuel pressure at idle for a smoother idle
   2. To keep the pressure drop across the injectors constant
   3. To draw fuel vapor from the regulator into the intake
   4. To let the PCM measure engine load through the regulator
   Show the answer

   B. Correct. Injector flow depends on the difference between rail pressure and the pressure in the manifold at the injector tip. Vacuum on the diaphragm lowers rail pressure as manifold pressure falls, so that difference stays constant and pulse width alone sets how much fuel is delivered.

   Why not the others

   A. Manifold vacuum works the other way: high vacuum at idle pulls on the diaphragm and lowers rail pressure. Rail pressure is highest when vacuum is lowest, at wide-open throttle, which keeps the pressure across the injector tips the same.

   C. The vacuum side of the regulator is sealed from the fuel by its diaphragm, and fuel vapor from the tank is handled by the EVAP purge system. The hose only lets manifold pressure act on the diaphragm.

   D. The regulator is a mechanical valve with no sensor or wiring; the PCM reads load from the MAP or MAF sensor. The vacuum hose simply lets manifold pressure act on the regulator's diaphragm.

4. Recall · Emissions Control Systems Diagnosis and Repair

   On an EVAP system, what is the job of the canister vent valve?

   1. It meters stored vapor from the canister into the intake manifold
   2. It admits fresh air to the canister and closes to seal the system
   3. It reports fuel tank pressure and vacuum readings to the PCM
   4. It keeps liquid fuel out of the vapor line if the vehicle rolls over
   Show the answer

   B. Correct. Open, the vent valve connects the canister's fresh-air side to the atmosphere, so outside air can sweep vapor out of the charcoal during purge and the tank can vent through the canister while it is filled. For a leak test the PCM closes it, sealing the system so pressure or vacuum can be held and measured.

   Why not the others

   A. That is the purge valve, which sits between the canister and the intake manifold and is opened by the PCM to draw stored vapor into the engine. The vent valve works at the canister's fresh-air side.

   C. That is the fuel tank pressure (FTP) sensor. It reports what the sealed system holds during a leak test; the vent valve is the part that opens or seals the canister's fresh-air side.

   D. That is the rollover valve's job at the tank. The vent valve sits on the canister's fresh-air side and controls whether outside air can pass in or out there.

5. Recall · Computerized Engine Controls Diagnosis and Repair

   On an OBD II vehicle, what does a malfunction indicator lamp (MIL) that flashes while the engine runs indicate?

   1. A fault seen on one trip and stored as pending
   2. Misfire that can damage the catalytic converter
   3. The PCM running in a default (limp-in) mode
   4. A fault that has not returned on the last three trips
   Show the answer

   B. Correct. OBD II rules require the MIL to blink while the engine is misfiring at a rate that can damage the catalytic converter. Unburned fuel burning in the converter overheats it, so the load should be reduced and the misfire diagnosed promptly.

   Why not the others

   A. A pending DTC is recorded with the MIL still off; the lamp lights steadily only once the fault is confirmed. A flashing MIL is reserved for misfire that can damage the catalyst.

   C. When the PCM falls back on a default strategy, OBD II rules call for a steady MIL, not a flashing one. Flashing is reserved for misfire that can damage the catalyst.

   D. A fault that stops recurring lets the MIL go out after the required good trips; the lamp does not flash to show it. Flashing means catalyst-damaging misfire is happening now.

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