ASE A1 Engine RepairGeneral Engine DiagnosisHard
A technician is diagnosing an engine that exhibits a very rough idle, misfires, and a strong smell of unburnt fuel from the exhaust. A cylinder contribution test shows Cylinder 3 has significantly lower power output. A spark plug inspection reveals a completely fouled (black, wet, oily) plug in Cylinder 3. Which of the following is the most likely cause?
- AA faulty ignition coil for Cylinder 3
- BA damaged piston ring in Cylinder 3
- CA restricted fuel injector in Cylinder 3
- DA leaking intake manifold gasket near Cylinder 3
Show answer & explanationAnswer & explanation
Correct answer: B. A damaged piston ring in Cylinder 3
A fouled spark plug that is wet and oily, combined with low power output and a strong smell of unburnt fuel, strongly indicates oil entering the combustion chamber due to a damaged piston ring.
Why the other options are wrong
- A. A faulty ignition coil would cause a misfire and unburnt fuel, but the plug would likely be wet with raw fuel, not oil, unless severe and prolonged.
- C. A restricted fuel injector would cause a lean condition, not a wet, oily fouled plug.
- D. A leaking intake manifold gasket would typically cause a lean condition, rough idle, and potentially misfires, but would not cause an oily fouled spark plug.
Oily Fouled Spark Plug
An oily fouled spark plug indicates engine oil is entering the combustion chamber, usually due to worn piston rings, worn valve guides/seals, or a compromised head gasket.
- Indicates oil in combustion chamber
- Causes misfires and reduced power
- Commonly due to worn piston rings or valve seals
Memory trick: Wet and Oily plug? Think rings are failing, no joke!