An article showing how to diagnose common spark plug conditions and their causes.
Examining a vehicle's spark plugs can help determine the current running conditions of the engine. Bad spark plugs can cause detonation, pre-ignition, rough idling, poor running conditions, power loss and even engine failure. Check the engine spark plugs each time a routine tune up is done.
|2. Normal with Red Coating
Cause: A common result from fuel
additive in unleaded fuel.
|3. Fuel Fouled
Cause: Signals a fuel rich mixture.
Can also mean the wrong spark plug is being used
Cause: Over-advanced timing, low
octane fuel. Excessive carbon build up
|5. Worn Plug
Cause: Plug is used beyond its service
life and needs to replaced.
|6. Carbon Fouled
Cause: Fuel Rich mixture with extended low speed driving.
Cause: Spark timing is wrong
Lean-fuel mixture could be present
|8. Oil Ash Fouled
Cause: Piston rings excessively worn
Valve guide seals need to be replaced
Cylinder heads may need full overhaul
Engine may need complete overhaul
|9. Mechanical Damage
Cause: Wrong plug being used
Excessive low end engine damage has occurred