Intake Manifold Vacuum Test

The indicated compression pressure are considered within specification if the lowest reading cylinder is within 75 percent of the highest reading.

CAUTION: If engine oil is sprayed into the combustion chamber, after carrying out the measurement run the engine at 2000 RPM for about 15 minutes, in order to burn the oil and prevent damage to the catalytic converter.

If the measurement on one or more cylinders is much lower than the specified value, spray some engine oil into the combustion chamber and repeat the compression measurement.
If the reading greatly improves then the piston rings are faulty.
If the reading stays the same then the cause is either bad valve seats or valve stem seals.
If the measurements for two cylinders next to each other are both too low then it is very likely that the cylinder head gasket between them is burnt through. This can also be recognized by traces of engine oil in the coolant and/or coolant in the engine oil.