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Dry or wet test

Question : Can a dry or wet compression test determine valve seals leakage and worn out piston rings? If so, can it cause spark plugs to be oily or sooty?

Answer : The wet and dry test is usually done to determine piston and piston ring wear. If the valve seats are worn and there is leakage, the cylinder will not be able to hold pressure. So if you do both the wet and dry test, and the cylinder pressure is still low, you will need to check whether the seats are worn or there is a leak in the cylinder head gasket. This is done using another test, which involves putting the piston on TDC on the compression stroke and applying air pressure through the spark plug hole. Then by listening in the inlet manifold, one can tell if there is a leak in the inlet seat, etc. There are many other problems that can be detected by applying pressure this way.

Spark plugs being oily, usually indicates "blow-by" and could be wear in the pistons and rings or even worn valve guides or seals. Unfortunately it is not easy to identify valve seal leakage unless the head is taken off or if one has a scope to look at the valve through the spark plug hole. Leaking valve seals usually show as heavy deposits on the back of the valve. Sooty plugs are usually because of too rich a mixture.


Dry or wet test

Drop in compression

Wet and dry compression

Grind the valves

Compression test



Spark plugs
Timing belt


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