How to check spark plug problems

Spark plug maintenance: How to check spark plugs

By Steve Temple,

In times past, one of the easiest maintenance items on a vehicle was checking and changing the spark plugs. That was before engine compartments became shrouded in emissions tubing and computerized sensors. If you’re lucky enough to have an automobile where the spark plugs are relatively easy to reach, take advantage of it. That’s because they offer telltale indications of what’s going on in the combustion chamber and the internal health of your engine.

Before you begin ratcheting out your spark plugs in search of suspicious symptoms, a few words of caution: First, be sure to check all of the plugs. There could be a serious problem brewing in just one cylinder that you wouldn’t want to overlook. Second, if your plugs indicate a problem related to the plug’s heat range or the plug is simply worn out, you can fix these problems with a new, and correct, set of plugs. If the diagnosis is more serious, though, and your plugs are oil-foiled because of a worn piston ring, new plugs won’t make the worn ring go away. Get the vehicle to your mechanic for the overhaul, and then replace the plugs.

The following is a comprehensive list of conditions that you may find useful when you are checking spark plugs:


When the engine is running the way it should, normal-reading plugs will look pretty much the same way they did when they were new and first screwed into the engine block.

Normal, but with red coating

The red coating is a result of the additives in lower-quality unleaded fuel and will be visible on the plug’s ceramic insulation. The red coating is not an indication of any engine problems.

Fuel fouled

Fuel-fouled plugs may have a shiny coating on the tip and side electrode, which may indicate a too-rich fuel mixture, ignition problems or a plug heat range that’s set too low. First, check to make sure your spark plugs have a heat range that is compatible with your engine (especially if you’ve made performance modifications). This information is available in your vehicle owner’s manual. To resolve the too-rich fuel mixture, have the fuel injection (or carburetor) adjusted to correct the air/fuel mix.

Detonation damage

This plug condition indicates that your engine timing is off and you probably need a tune-up. Another possibility is that the gasoline you’re using does not have a high enough octane rating. Again, check the owner’s manual to verify the manufacturer’s recommendation on octane level.

Worn plug

This is an easy fix–replace the plug. Most plug manufacturers indicate the recommended service life on the packaging.

Carbon fouled

If your plug tip and side electrode are blackened, they have been running with too much fuel (or possibly too cool from a stuck-open thermostat). Other sources of the problem may include bad wiring or leaking injectors; in some cases the vehicle has been driven at too slow a speed for extended periods of time. The combustion process is not being allowed to have its natural burning-off, or cleaning, effect.


The plug will reveal that the side electrode has been burned away from running too hot. The plug is firing too soon, not enough fuel is present in the air/fuel mixture or there isn’t enough fuel in the combustion chamber for a sound combustion event. Check your fuel injection and timing. Take quick action, because a plug in this condition is just short of falling completely apart.

Oil ash fouled

Engine oil is getting to your plugs from worn piston rings or valve guides/seals. Get to your mechanic now.

Mechanical damage

A mechanically damaged plug will look as if it’s been beaten to death by its piston, an indication that it extended too far down into the combustion chamber.

The two most common spark plug problems are hot fouling and cold fouling. The “too hot” category includes the preignition and detonation damage. Some performance improvements may be the cause for this type of plug damage. If your vehicle has performance upgrades such as a high-output coil, ignition, exhaust or cams, these can alter the engine’s recommended plug heat range, so you should consider using a spark plug with a heat range lower than the manufacturer’s recommendations.

A few of the symptoms indicated by your spark plugs have simple fixes–others require the hands and expertise of a qualified mechanic. Eitherway, the main advantage to checking out your spark plugs is for a quick diagnostic tool that gives you a fairly good idea of how well your engine is performing.