Engine Mechanical: Inspection, Diagnosis, and Repair
Engine Mechanical: Inspection, Diagnosis, and Repair
This domain tests mechanical condition assessment: compression, leak-down, cylinder balance, timing, valve clearance, head gasket integrity, and correct reassembly. The exam favors evidence over parts swapping.
Core rules
- Run compression tests with the engine at normal temperature, throttle wide open, all spark plugs removed, and ignition and injection disabled. Record each cylinder and compare to specification and to the others.
- A wet compression test adds a small amount of oil to the cylinder. If compression rises, suspect rings; if it stays low, suspect valves, head gasket, or a mechanical fault.
- Leak-down testing confirms where pressure escapes: intake, exhaust, crankcase, or cooling system. Listen at the oil filler, intake, exhaust, and radiator.
- Check timing marks, valve clearance, and head bolt torque sequence. Replace torque-to-yield bolts and gaskets as specified; do not reuse stretch bolts.
Worked example
A four-cylinder engine misfires on cylinder 2. Compression is 150 psi on cylinder 2 and 185–190 psi on the others. Adding oil raises cylinder 2 to 180 psi. The rise points to piston-ring sealing, not a burned valve. A leak-down test showing air at the oil filler supports worn rings or a damaged piston. The repair decision now depends on bore inspection, not on replacing the ignition coil.
Traps
Testing compression with the throttle closed, on a cold engine, or with fuel injection active. Also common: ignoring valve clearance on engines with mechanical adjusters, fitting head gaskets upside down, and not checking block and head flatness.