(by Tommy Sandham)
Remove all four spark plugs.
Make sure the car is in neutral.
Fit the compression tester to number one cylinder (the one nearest the radiator)
Make sure your hands are clear of the fan blades, then crank the engine over on the starter motor.
The compression gauge should rise fairly quickly then settle on a reading. As a rough guide you are looking for between 140 - 170 pounds per square inch depending on engine variant. Refer to the above table for actual compression readings.
Make a note on a piece of paper of the reading you get.
Test each cylinder in turn and note down the results.
Interpreting the results:
Four readings, all within 10 pounds square / inch of each other (1.38 Kg sq/m) -- a sound engine. Ideally the readings should match the figures in the table.
Three readings within 10 pounds square / inch plus (1.38 Kg sq/m one lower reading - a faulty cylinder (see below).
Two good cylinders, plus two low pressure cylinders next to each other indicates a blown head gasket or two faulty cylinders.
Some people say that if all four cylinders are within 20 pounds of each other, all is well. I work to within 10 pounds. A fault shows up quite dramatically on the compression tester.
Having got your set of readings, you now need to do further tests to determine if you have worn or broken piston rings OR leaking valves.
Add a pipette of oil to the low cylinder. (If you do not have a pipette, use a piece of plastic tube with about two inches of engine oil in it). This oil will seal broken rings temporarily.
Repeat the test. If the pressure rises by say 20 pounds square inch, then you have broken or badly worn piston rings. If the pressure does NOT rise, then you have leaking valves. A faulty inlet valve often gives itself away by spitting back through the carburettor when running.
Lastly, some people say do the test with the throttle wide open. Others with the throttle closed. I always do it throttle closed as you need fewer hands! Either way be consistent.
I have used the compression tester on dozens of engines over the years, and the results have always been accurate.
Page Updated:- 03 October, 2006