There are two kinds of compression tests you can run on your engine. The "Static" test and the "Dynamic" or "Running" test. If you know how to do these tests right, and how to interpret the results, they can tell you a great deal about the health of your engine.
Static compression can tell you about the health of your piston rings, cylinder walls, and valves.
Dynamic compression reveals problems in the intake and exhaust systems of the engine.
To perform these tests, you will need some basic tools.
Other helpful tools:
Combine the compression tests with other tests to pinpoint the source of a problem.
Static compression Test
To perform a static compression test:
Static Compression Test
If the wet
reading is significantly higher than the dry reading, then the
problem lies in the rings or cylinder walls. The problem is weak or
worn piston rings, or a scored cylinder wall, or both. It does not,
however, rule out leaking or burnt valves.
A zero pressure reading on a cylinder indicates something is broken or badly cracked, like a piston, cylinder wall, or combustion chamber. In can also indicate a severely burned-out head gasket or a valve partially burned away.
Two adjacent cylinders with low readings usually indicate a bad head gasket, rather than mechanical damage to other parts. Usually. Not always.
Dynamic or Running Compression Test
The tools needed
for the dynamic test are the same as for the static test, but you MUST
have a screw-in type compression gauge, preferably one with an external
pressure release valve.
Interpreting the Dynamic (Running) Compression Test
At idle, the
dynamic compression readings should be VERY APPROXIMATELY half of the
"Dry" static compression readings, usually 50-75 psi. See the "What's
happening" section below.
much less than 80% of static compression on one cylinder indicates
restriction in the intake side of the engine, somewhere between the
combustion chamber and the intake manifold's plenum. Look for a flat
camshaft lobe, collapsed hydraulic lifter, heavy deposits on the back of
the intake valve, problems with the intake rocker arm. Some manifolds,
like Vortec's, have dividers or shutters that could be out of place.
much higher than 80% indicates a restriction in the exhaust side of the
engine. Look for a flat camshaft lobe, a collapsed lifter, or a crushed
with this test is that at idle, the throttle plates are closed, and the
engine is not pulling all the air it can. The "Idle" readings should
compare to the static "Dry" readings, adjusted by how much vacuum the
engine is pulling.