To perform a diesel compression
test, use the following procedures:
Remove all injectors or glow plugs. Refer to the manufacturer’s manual for instructions.
Install the compression gauge in the recommended opening. A heat shield must be used to seal the gauge when it is installed in place of the injector.
Disconnect the fuel
shut-off solenoid to disable the fuel
Crank the engine
and note the highest reading on the gauge.
Some manufacturers warn against performing a wet compression test on diesel
engines. If too much oil is squirted into the cylinder, hydraulic lock and part damage may result, because oil does not compress in the small cylinder volume.
A wet compression test should be used when cylinder pressure reads below the manufacturer's specifications. It helps you to determine what engine parts
are causing the problem. Pour approximately 1 tablespoon of 30-weight motor oil
into the cylinder through the injector opening, then retest the compression pressure.
If the compression reading goes up with oil in the cylinder, the piston rings and cylinders may be worn and leaking pressure. The oil will temporarily coat and seal bad compression rings to increase pressure; however, if the compression reading stays about the same, then engine valves or head
gaskets may be leaking. The engine oil seals
the rings, but does not seal a burned valve or a blown head gasket
. In this way, a wet compression test helps diagnose low-compression problems.
Do not put too much oil into the cylinder
during a wet compression test or a false reading may result. With excessive oil in the cylinder, compression readings go up even if the compression rings and cylinders are in good condition.