Manufacturer’s Professional Repair Information for the “Car Savvy Do it Yourselfer” Over 90 years of service.

BASIC SPARK PLUG TROUBLESHOOTING CHART

NOTE: This is GENERAL information. This article is not intended to be specific to any unique situation or individual vehicle configuration. The purpose of this Troubleshooting information is to provide a list of common causes to problem symptoms. For model-specific Troubleshooting, refer to SUBJECT, DIAGNOSTIC, or TESTING articles available in the section(s) you are accessing.
CONDITION & POSSIBLE CAUSE CORRECTION
Normal Spark Plug Condition
Light Tan or Gray deposits No Action
Electrode not burned or fouled No Action
Gap tolerance not changed No Action
Cold Fouling or Carbon Deposits
Overrich air/fuel mixture Adjust air/fuel mixture, see ENGINE PERFORMANCE section
Faulty choke Replace choke assembly, see ENGINE PERFORMANCE section
Clogged air filter Clean and/or replace air filter
Incorrect idle speed or dirty carburetor Reset idle speed and/ or clean carburetor
Faulty ignition wires Replace ignition wiring
Prolonged operation at idle Shut engine off during long idle
Sticking valves or worn valve guide seals Check valve train
Wet Fouling or Oil Deposits
Worn rings and pistons Install new rings and pistons
Excessive cylinder wear Rebore or replace block
Excessive valve guide clearance Worn or loose bearing
Gap Bridged
Deposits in combustion chamber becoming fused to electrode Clean combustion chamber of deposits
Blistered Electrode
Engine overheating Check cooling system
Wrong type of fuel Replace with correct fuel
Loose spark plugs Retighten spark plugs
Over-advanced ignition timing Reset ignition timing see ENGINE PERFORMANCE
Pre-Ignition or Melted Electrodes
Incorrect type of fuel Replace with correct fuel
Incorrect ignition timing Reset ignition timing see ENGINE PERFORMANCE
Burned valves Replace valves
Engine Overheating Check cooling system
Wrong type of spark plug, too hot Replace with correct spark plug, see ENGINE PERFORMANCE
Chipped Insulators
Severe detonation Check for over-advanced timing or combustion
Improper gapping procedure Re-gap spark plugs
Rust Colored Deposits
Additives in unleaded fuel Try different fuel brand
Water In Combustion Chamber
Blown head gasket or cracked head Repair or replace head or head gasket
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