Engine Repair and Engine Performance
Core concepts
A candidate must be able to explain the four-stroke cycle (intake, compression, power, exhaust), valve timing, and how camshaft profile and timing affect volumetric efficiency. Know the difference between mechanical and vacuum advance, the role of the mass airflow (MAF) and manifold absolute pressure (MAP) sensors, and how the powertrain control module (PCM) uses oxygen sensor feedback to trim fuel. Understand compression testing (wet versus dry), cylinder leakage testing, and how to interpret a vacuum gauge. Diagnostic trouble codes (DTCs) and freeze-frame data should be read before parts are replaced.
Traps to expect
Questions often confuse cause and effect: a misfire code does not mean the coil is bad. Watch for items about spark timing versus ignition order, and about lean versus rich conditions. A lean condition raises combustion temperature and can set a P0171; a rich condition can foul plugs. Another trap is assuming low compression always means rings: a wet test that raises compression points to rings, while no change points to valves or head gasket. Also remember that a restricted exhaust can mimic a fuel or ignition fault.
Worked example
A vehicle idles rough and a dry compression test reads 90 psi on cylinder 3, with others at 150 psi. Adding oil raises cylinder 3 to 145 psi. Rule of thumb: a rise of about 20 psi or more after adding oil indicates worn rings; no rise indicates a valve or head gasket leak. Here the rings are the likely cause, so a leak-down test should confirm before teardown.
Study strategy
Practice reading a vacuum gauge pattern, tracing a no-start through spark, fuel, and compression checks in order, and explaining how a scan tool's live data supports a diagnosis rather than replacing it.