Spark Plug: Testing and Inspection

1. Oil Fouled
- Identified by wet black deposits on the insulator shell bore electrodes.

- Caused by excessive oil entering combustion chamber through worn rings and pistons, excessive clearance between valve guides and stems, or worn or loose bearings. Correct oil problem. Replace the spark plug.





2. Gap Bridged
- Identified by deposit build-up closing gap between electrodes.

- Caused by oil or carbon fouling. Clean the spark plug.





3. Pre-Ignition
- Identified by melted electrodes and possibly blistered insulator. Metallic deposits on insulator indicate engine damage.





4. Normal
- Identified by light tan or gray deposits on the firing tip.





5. Carbon Fouled
- Identified by black, dry fluffy carbon deposits on insulator tips, exposed shell surfaces and electrodes.

- Caused by too cold a plug, dirty air cleaner, damaged fuel pump, too rich a fuel mixture or excessive idling. Clean the spark plug.





6. Fused Spot Deposit
- Identified by melted or spotty deposits resembling bubbles or blisters.

- Caused by sudden acceleration. Clean the spark plug.





7. Overheating
- Identified by a white or light gray insulator with small black or gray/brown spots and with bluish-burnt appearance of electrodes.

- Caused by engine overheating, wrong types of fuel, loose spark plugs, too hot a plug, low fuel pump pressure or incorrect ignition timing. Replace the spark plug.