Wednesday, May 12, 2010

Compression Testing

What is a compression test?

A compression test is done to tell you whether your engine has good compression. An engine is essentially a self-powered air pump, so it needs good compression to run efficiently, cleanly and to start easily.
As a rule, most engines should have 140 to 160 lbs. of cranking compression with no more than 10% difference between any of the cylinders.

Low compression in one cylinder usually indicates a bad exhaust valve. Low compression in two adjacent cylinders typically means you have a bad head gasket. Low compression in all cylinders would tell you the rings and cylinders are worn and the engine needs to be overhauled.

How to do a compression test?

Step 1: Remove the air cleaner to have access to the spark plugs as if you were going to change them.
















Step 2: Remove all of the spark plugs.

















Step 3: Disable the fuel system and block open the throttle.

















Step 4: Install the tester in the first cylinder's spark plug hole.
















Step 5: Crank the motor, and read the gauge.

















Part One: Dry Test

When doing a dry test, there are 2 things we are looking for:

1. Does the cylinder have OVER 100 PSI... this is the minimum compression needed to fire most gasoline today. The manufacturers minimum specification for compression can be found in the repair manual.

2. Comparison to the other cylinders. The difference between the lowest cylinder and the highest cylinder should be less then 15%. For most engines that's a difference of under 30 PSI.

Now complete this step for all cylinders, and write down the number you get as you go.If your compression numbers are over 120 PSI, and the difference between the high and low numbers is under 30, your likely OK.

Part two: Wet test

You will need to run the test again but this time, add a tablespoon of oil to each cylinder before the test. This is called a wet compression test. Run the test on all cylinders in the same order as before, and compare the numbers. Did the very lowest cylinders come up? If so, this indicates poor piston ring seal and a worn engine. If not, a very low cylinder is probably caused by a valve train issue. ( burnt or sticking valve)

Step 6: Re-install the plugs and wires.

References
http://autos.yahoo.com/maintain/repairqa/engine/ques056_1.html
http://www.diy-auto-repair.com/compressiontest.html


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