A8 Engine
Performance
A thermostatic fan clutch on a belt-driven cooling fan is designed to:
Technician A says primary winding resistance on a coil-on-plug coil is measured across its two low-voltage terminals. Technician B says secondary resistance is measured between those same two terminals. Who is right?
Cranking compression readings and wet readings are shown. Specification: no cylinder may read below 75% of the highest cylinder. These readings indicate:
| Cylinder | Cranking (psi) | Wet (psi) |
|---|---|---|
| 1 | 165 | 168 |
| 2 | 162 | 165 |
| 3 | 118 | 121 |
| 4 | 160 | 163 |
An engine has a rough idle that smooths out above 2000 rpm. Which of these is the LEAST likely cause?
Several unrelated warning lamps illuminate together and a scan tool cannot communicate with three modules, though the engine runs normally. The technician should first check:
A vehicle sets a code for a very small EVAP leak. The fuel cap seals correctly and holds pressure. What is the MOST likely cause?
Technician A says freeze frame data records operating conditions present when the code was stored. Technician B says a permanent DTC is not cleared by disconnecting the battery. Who is right?
An exhaust restriction is suspected. Which procedure correctly uses a manifold vacuum gauge to confirm it?
Any of these could cause a single-cylinder misfire EXCEPT:
At idle, short-term fuel trim is +4% and long-term is +23%. At 2500 rpm both drop to near 0%. The cause could be a:
The clean-air hose in a positive crankcase ventilation system provides:
A high-mileage engine has a rattle at cold startup that stops after two seconds, plus a stored camshaft position correlation code. The technician should inspect the:
Technician A says converter restriction is measured at the downstream oxygen sensor port. Technician B says more than 3 psi at 2000 rpm indicates a restriction. Who is right?
A coil-on-plug engine misfires under hard acceleration but idles smoothly, and secondary firing voltage rises sharply under load. What is the MOST likely cause?
An engine runs poorly with no stored diagnostic trouble codes. Which of these is the LEAST likely cause?
Which procedure correctly verifies that a vacuum-operated EGR valve is opening and flowing exhaust?
A cylinder leakage test on cylinder 4 reads 32 percent, and air is heard escaping from the oil filler opening. The cause could be:
A port-injected engine has a long crank time when hot but starts normally when cold. Fuel pressure drops to zero within ten minutes of shutdown. The cause could be a:
Technician A says starter current draw is measured while the engine starts and runs. Technician B says starter circuit voltage drop is measured with the circuit unloaded. Who is right?
In a speed-density fuel injection system, the manifold absolute pressure sensor allows the PCM to calculate:
A catalytic converter efficiency code is stored. Which of these is the LEAST likely cause?
A gasoline direct injection engine with 90,000 miles has a rough idle and misfire codes on multiple cylinders. What is the MOST likely cause?
Which procedure correctly measures voltage drop across a PCM ground circuit?
Technician A says a magnetic pickup crankshaft sensor generates its own AC voltage. Technician B says a Hall-effect camshaft sensor produces a sine wave that grows with speed. Who is right?
An engine stalls when the air conditioning compressor engages at idle, but idles steadily with the air conditioning off. The cause could be a:
Reading Your Results: A8 Engine Performance, Test 1
A score by itself only tells you whether you passed. What follows is how to read the questions you missed on this test. A8 sits on top of every other subject, and the thing it tests hardest is whether you can tell a mechanical fault from a fuel fault from an electrical one when all three produce the same complaint.
If you missed the questions where the condition names the cause
A driveability complaint that appears only under one condition is telling you something, and several questions on this test are decided entirely by that condition. A rough idle that smooths out above 2000 rpm points at a fixed volume of unmetered air, because that leak is a large share of total airflow at idle and a trivial share at speed. A long crank when hot but a normal cold start with rail pressure falling to zero after shutdown is a system that cannot hold pressure, and heat is what turns residual fuel to vapor. A stall when the air conditioning compressor engages is an idle control system that cannot compensate for an added load it should expect. Read the condition before the options, because on this test it usually eliminates two of them outright.
If you missed the questions built on a measurement
Three questions here hand you readings and expect you to reason from them rather than just compare them to a limit.
If you missed the questions about what a test can and cannot prove
Several procedure and Technician A/B questions on this test are really about the limits of a measurement. Freeze frame records the conditions present when a code set, which tells you when the fault happens rather than which part failed. Starter current draw is measured while cranking with the engine disabled, since an engine that fires gives you a reading over a partial crank. Coil primary resistance is measured across the two low-voltage terminals and proves the winding is intact, not that the coil can deliver under load. A voltage drop on a PCM ground is taken with the circuit live, because a ground that measures fine with no current flowing can still be starving the module. Knowing what a test cannot tell you is worth as much on this exam as knowing what it can.
If you missed the EXCEPT and LEAST likely questions
Four appear on this test, and missing more than one is a reading habit rather than a knowledge gap. Work out what the three real causes share before you look for the odd one. On the single-cylinder misfire question, three answers are faults that exist in one cylinder only, and one would affect every cylinder equally. On the converter efficiency question, three answers all either damage the converter or make the downstream sensor read as though it is not working, and one does neither. Naming the shared trait first turns a four-way guess into a single check.