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Used Spark Plugs Tell Stories

Oct 16, 2023

There is a wealth of information to be discovered in viewing a used spark plug. The condition of the tip – along with other evaluations like a compression test – can certainly guide you in determining if an operating engine needs an overhaul or is good to go for a parade or a tour with just minor mechanical work.

In that regard, a good indication that an engine is generally at peak operating performance is spark plug tips that are grayish-tan to white in color when removed.

However, don't be dismayed if ceramic insulator tips (center and ground electrodes on a plug) are pinkish-red in color. That pink coloration is created by deposits from additives in unleaded fuel when they’re burned off during combustion.

A tried-and-true warning sign that an engine needs adjustment is when a plug's insulator is tinted the color of charcoal and the firing tip is damp with gas. Also, smell the plug for the odor of gas. Such evidence indicates the cylinder from which the plug was removed is not burning all the fuel it consumes. At fault could be a sticking choke, overly rich fuel-air mixture, ignition problems, or a plug with a heat range that's too cold.

A spark plug insulator that is cracked, chipped, or broken and with a ground electrode exhibiting damage could indicate overly advanced timing as well as the previous use of low-octane fuel.

Center and ground electrodes that are eroded, have rounded edges, and are excessively worn away indicate spark plugs that have been in use too long. Another indicator that plugs have exceeded their life is found in an engine that is difficult to start and misfires during acceleration.

Plugs with firing ends covered with a black and sooty coating could indicate a weak ignition system (a faulty magneto or generator) or an overly rich fuel mixture (from a misadjusted carburetor). Also at fault could be an improperly adjusted or malfunctioning choke.

Melted center and ground electrodes and damaged ceramic insulator tips indicate that the incorrect spark plug was in use. Worse yet, such evidence points to the possibility of cross firing of ignition cables, overadvanced timing, too lean a fuel mixture, accumulation of combustion chamber deposits, or hot spots in the combustion chamber due to poor heat dissipation. You will also want to check for improper spark plug installation or head gasket protrusion into the combustion chamber.

When electrodes or insulator tip plugs are coated with tan-color deposits, you will want to check for worn piston rings or valve guides. Engine misfiring can also contribute to this kind of deposits.

A black oily substance coating the center and ground electrodes warns of the presence of oil in the combustion chamber. You will want to check the engine for worn rings, worn valve guides, and worn valve seals.

When the center and ground electrodes are bent out of position (either down or to one side of the plug) or the ceramic tip is broken and missing from the firing tip, there's a good chance that the improper thread length was used when the plugs were last changed or that there's a foreign object inside the cylinder.

A brownish-yellow glazed coating on a ceramic insulator tip results from fuel additives containing lead, which become conductive over the firing tip.