I presume you mean a compression test. To do this on a petrol engine you want the engine warmed up remove the spark plugs and ground the coil lead, fit the compression tester into No 1 Cyl and with full throttle crank the engine over for 3-4 revolutions, read off the compression readings on the gauge, remove and re set gauge and do the next cyl, write down your readings, ideally you want no more tan 10% variation from the highest to lowest reading, a wet test is the same except oil is added to the cylinder normally 4-5 squirts from an oil can, this will seal the rings for the test, re test comps and if the cylinder had a low reading and the re test was still low, the valves are leaking, if the comps increase the rings are faulty, there is also a leak down test that can be done, this is with the cylinder at TDC so both valves are closed, a fitting is screwed into the plug hole and low pressure air is pumped into the cylinder, you can hear the leaks in valves in the manifolds and exhaust and ring blow by will be heard at the oil filler cap.
You need more specialized equipment for a diesel, this is far more complex to test and a special comp tester is required.
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Tuesday, November 24th, 2009 AT 6:12 PM