Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairMedium
A technician is diagnosing an engine with a persistent P0302 misfire code (Cylinder 2 Misfire Detected). After confirming spark and fuel delivery to cylinder #2, the technician performs a relative compression test using a scanner. The relative compression for cylinder #2 is significantly lower than the other cylinders. Which of the following spark plug conditions would MOST likely be observed on cylinder #2's spark plug due to this low compression?
- AWet, oily deposits.
- BAsh fouled deposits.
- CNormal, light tan color.
- DCarbon (soot) fouled deposits.
Show answer & explanationAnswer & explanation
Correct answer: D. Carbon (soot) fouled deposits.
Low compression in a cylinder means incomplete combustion. When combustion is poor or incomplete, the air-fuel mixture doesn't burn efficiently, leading to unburnt fuel and carbon deposits accumulating on the spark plug. This results in carbon (soot) fouled deposits.
Why the other options are wrong
- A. Wet, oily deposits indicate excessive oil consumption, which can cause low compression, but the *direct* result of low compression on combustion is carbon fouling.
- B. Ash fouled deposits usually indicate oil or fuel additives, not directly caused by low compression.
- C. Normal, light tan color indicates healthy combustion, which is contrary to a low compression misfire.
Low Compression Spark Plug
A spark plug from a cylinder with low compression often exhibits carbon (soot) fouling due to incomplete combustion of the air-fuel mixture.
- Low compression affects combustion efficiency.
- Incomplete combustion leaves unburnt fuel/carbon.
- Results in black, sooty deposits on the spark plug.
Memory trick: Low compression, no good burn, leads to black carbon.