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Reading an Engines Spark Plugs

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.

1. Normal
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
4. Detonation
    
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.
7. Pre-Ignition
    
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
    

Article Type: 
Maintenance