Engine Systems: Diagnosis, Service, and Repair
What this domain tests
This domain tests diagnosis and repair of petrol and diesel engine mechanical and management faults. You must be able to move from a symptom or DTC to a measurement, then to a repair, without replacing parts on suspicion.
Core rules
- Record freeze-frame data and the DTC before clearing anything; clearing codes destroys the evidence.
- A compression test is done on a warm engine with the throttle wide open, all spark plugs removed and the fuel or ignition disabled. Compare cylinders; a wet test helps separate rings from valves.
- A cylinder leak-down test confirms where compression is lost: listen at the oil filler (rings), intake (inlet valve), exhaust (exhaust valve) or look for bubbles in the coolant (head gasket).
- Use fuel-trim data to judge mixture: positive long-term trim suggests a lean condition or unmetered air; negative trim suggests rich running.
- Set valve clearance cold where the specification says cold, and follow the manufacturer's torque sequence and angle for stretch bolts.
- On common-rail diesel, do not crack high-pressure lines to bleed them; pressure can exceed 1,000 bar.
Worked example
A 1.6 L petrol engine has a steady P0302 misfire at idle. The coil is swapped to cylinder 1 and the misfire stays on cylinder 2, so the coil is not the cause. The injector is swapped and the misfire still stays, so the injector is not proven either. A dry compression test gives 9 bar on cylinder 2 against 12 bar on the others; adding oil raises it to 12 bar. Leak-down at TDC shows air from the oil filler. The reasoning points to worn rings or a damaged bore, not an ignition fault.
Traps
Do not clear codes before recording freeze frame. A compression test with the throttle closed gives false low readings. Do not add too much oil in a wet test; it can hydraulic-lock. Do not assume a timing chain is stretched without checking cam/crank correlation.