Engine systems diagnosis and repair
Engine systems diagnosis and repair
This domain tests how you confirm and correct faults in petrol and diesel engines: mechanical condition, timing, air intake, compression, ignition or combustion, and exhaust aftertreatment. The exam expects a diagnostic sequence, not parts swapping.
Core rules and concepts
- Start with the customer complaint and a visual inspection: fluid levels, leaks, disconnected hoses, wiring, and recent work.
- Retrieve OBD-II codes and freeze-frame data before clearing. A code identifies a circuit or system, not automatically the failed component.
- For a misfire, separate ignition, fuel, and mechanical causes. Use a spark tester, injector pulse test, and cylinder compression or leak-down test.
- On diesel engines, check low-pressure fuel supply before high-pressure common-rail components; air ingress and clogged filters mimic pump failure.
- Verify timing marks and valve clearance on engines with adjustable tappets. Incorrect timing can bend valves on interference engines.
Worked example
A petrol engine has a steady misfire and P0302 for cylinder 2. Fuel trims are normal on other cylinders. Swap the cylinder 2 ignition coil with cylinder 1; the misfire moves to cylinder 1. That confirms the coil because the fault followed the part. If the misfire stays on cylinder 2, test injector pulse, spark, and compression before condemning the coil.
Traps that cost marks
Treating P0302 as “replace cylinder 2 coil” without a swap test; checking compression with the throttle closed on a petrol engine; ignoring a vacuum leak that affects one bank; and using an impact wrench on a timing pulley. After repair, clear codes, road-test, and confirm monitors.