|Dry or wet test
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.