Reading Your Plugs

Normal (unleaded fuel)

The Look The ceramic will have a grayish-tan to white color. The electrode will look new with sharp edges and a slight coloration. There will be no evidence of oil, wetness, or glazing.

The Problem None—the engine runs great

The Fix None

Normal (leaded fuel)

The Look Similar to unleaded fuel, the ceramic will be discolored but with a more chocolate appearance. The electrode will also have a like-new look, but with a slight yellowish coloration.

The Problem None—the engine runs great

The Fix None

Rich Air-To-Fuel Ratio

The Look The electrode, ceramic insulator, and inside metal surroundings contain black, sooty deposits.

The Problem Engine misfires or performs poorly. Spark plug may get choked out and not fire at all.

The Fix A leaner adjustment needs to be made to the carburetor. Clean or replace the spark plug. Check heat range of the plug; a hotter plug may be needed.

Carbon Fouling

The Look Similar to a rich air/fuel ratio plug, there is a sooty black carbon deposit on the electrode, ceramic insulator, and inside the metal barrel.

The Problem Idling for long periods of time, misfiring plugs, poorly performing engine.

The Fix Ignition system not outputting properly. Check plug wires, coil, and ground to coil. Air fuel mixture could be too rich. Check heat range of the plug; a hotter plug may be needed.

This plug is damaged as a result of detonation. Early signs of detonation will show brownish specs on the ceramic insulators, while more advanced problems will have cracked or chipped insulators. Detonation was a little more severe in this plug’s case—the entire ceramic insulator and electrode were blown away.

Detonation

The Look In the beginning, brown specs are noticeable on the ceramic insulator. At a more advanced state, you will see cracks and/or chips on the ceramic insulator.

The Problem Poor engine performance and may misfire or skip.

The Fix Timing is advanced too far. Fuel’s octane is too low, or fuel- delivery timing is off.

Oil-Fouled

The Look There is oil on the entire inside of the plug.

The Problem Poor performance, skips, hard starting, and sometimes a bluish smoke coming from the exhaust is present.

The Fix Oil is getting into the combustion chamber. In a new engine, the rings may not have seated; in an older engine, ring and valve seals may be worn. Rebuild the engine.

Here is the end result of an engine that is letting oil get into the combustion chamber. Not only is the spark plug oil-fouled but also the entire combustion chamber.

Pre-Ignition

The Look The ceramic insulator will have brown specs and both the center and side electrode may show signs of melting.

The Problem Engine runs irregularly and has a continuous decline in performance.

The Fix Engine’s timing is advanced too much. Carbon buildup is inside the engine’s combustion chamber. Air/fuel mixture is set too lean. Check heat range of the plug; a cooler plug may be needed.

Glazing

The Look The ceramic insulator has a shiny- yellow-varnish– looking glaze.

The Problem Poor engine performance; may misfire or skip.

The Fix Glazing occurs when the engine has been idling for long periods of time. A powdery deposit forms on the ceramic insulator, then the engine rpm suddenly increases to full or near full throttle. This heats the powdery buildup and melts it into a glaze. The powdery deposit appears due to the aforementioned engine problems as well as additives in fuels and oils. Avoid idling for long periods of time and then going to full throttle.

Worn-out

The Look The center and side electrodes have rounded edges.

The Problem The engine no longer performs well.

The Fix Replace spark plugs.

Deposit Fouling

The Look A red, brown, or white powdery coating on the ceramic insulator.

The Problem Poor engine performance and may misfire or skip while the engine is under load.

The Fix These deposits can come from additives in fuel and oil. Clean the plug and change the fuel to prevent deposits from occurring.

Overheating

The Look Electrode will have a worn and melted- away appearance. Ceramic insulator may be burned and discolored.

The Problem Poor running engine that continues to worsen.

The Fix Radiator, fan, or other cooling-system parts may not work properly. Check heat range of the plug; a cooler plug may be needed.