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Judging the air-fuel ratio

Question : How can I know air-fuel ratio is at optimum by looking at plugs?

Answer : Normally, the plug insulator should have a tan to grey colour with no blistering or sooting. Blistering means that the plug is running too hot and that could be because the plug is of the wrong heat range or the mixture is too lean. Sooty plugs could mean that the mixture is too rich.

Spark plugs covered with oil

Question : My 1996 Wira 1.8, 4G93, 16 valves, was overhauled two years ago. Now, the spark plugs are covered with oil. Is it due to a worn valve seal, or worn piston rings? Only the first and third cylinders are affected.

Answer : It could be either one or both, but more likely it is the rings. Before you pull everything apart, do a compression test and if the readings are good, then take the head off and check the valve guides and seals. If the guides are worn, it can also lead to oily plugs. Look at the back of the valves when you remove them. If there are oily deposits, then it is likely that the guides and possibly the seals are gone.


Sooty spark plugs

Judging the air-fuel ratio

Spark plugs covered with oil

Are 'fancy' plugs good ?

Spark plug won't last

Misfiring spark plugs

Spark plugs poser

Soiled plugs, tired engine

Wrong spark plugs


Sump repairs won't last

Not too hot, not too cold

Get the spark right

Glow and new glory

Window into the ( car's ) soul

Clogged plugs

Plug cables

Up in smoke


Spark plugs
Timing belt


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