Compression Test?

A compression test is a good indicator of how well crucial engine parts are doing their job of sealing the cylinders. Pistons, rings, head gaskets, cylinder walls, heads and valves all need to do their part at making a tight seal so that power is not lost when the air/fuel mixture burns. This is so that maximum energy will be transferred to the pistons and rotating assembly. A compression test can help you pinpoint a problem or tell you that your engine is mechanically sound and that the problem is elsewhere (fuel or ignition).

To do the test, you first need a compression gauge. A good one will come with several fittings (like spark plugs) that screw into the spark plug hole. The gauge is an air pressure gauge and will have a check valve to trap pressure that is generated by the cylinder under test. There will be a push button release valve to bleed of pressure after checking each cylinder.

Here's how to do the test:
1) Make sure your battery is fully charged
2) Start the engine and warm it up first
3) Remove all of the spark plugs
4) Disable the ignition system
5) Open the throttle wide open during the entire test.
6) Attach the compression gauge to #1 cylinder
7) Crank the engine over through 5 complete cycles (compression strokes)
8) Record your reading
9) Release pressure and go to next cylinder
10) Repeat test for all cylinders

100 to 130 psi is about normal for each cylinder. The lowest cylinder should be at least 80% of the highest. If you get a low reading (say 50-60 psi) on 2 adjacent cylinders you probably have a blown head gasket. A zero reading on any one cylinder could be a stuck open valve or a burnt piston (hole in it). A low reading across all cylinders could be late valve timing (slipped chain) or just worn rings.

Hope this is enough to answer all your questions and to get you started. Brian

Write back if you need anything more.