Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
A technician is diagnosing an intermittent misfire on a vehicle with a Coil-On-Plug (COP) ignition system. The misfire only occurs after the engine has reached operating temperature and has been running for an extended period. When the engine cools down, the misfire disappears. Which of the following components is MOST likely failing due to thermal breakdown?
- AThe fuel injector.
- BThe crankshaft position sensor (CKP).
- CThe ignition coil.
- DThe spark plug.
Show answer & explanationAnswer & explanation
Correct answer: C. The ignition coil.
Ignition coils are susceptible to thermal breakdown. As they heat up, internal windings can expand, causing insulation to fail, leading to internal shorts or opens that only manifest when hot. This results in an intermittent misfire that resolves when the coil cools down.
Why the other options are wrong
- A. A fuel injector failing when hot could cause a misfire, but the term 'thermal breakdown' is more directly associated with electrical components like ignition coils or sensors.
- B. A CKP sensor failing when hot can cause a no-start or complete engine shutdown, but typically not an intermittent misfire on a single cylinder that resolves when cool.
- D. While spark plugs can fail, thermal breakdown causing an intermittent misfire that disappears when cool is less common for a spark plug itself.
Ignition Coil Thermal Breakdown
Ignition coils can fail due to thermal breakdown, where heat causes internal insulation or winding issues, leading to intermittent misfires that appear when hot and disappear when cool.
- Misfire occurs when hot, disappears when cold.
- Internal insulation breakdown.
- Winding expansion/contraction.
- Common failure mode for ignition coils.
Memory trick: Hot coils 'Break Down' and misfire.