BLEVEN’S TECH TIPS FOR NOVEMBER 2004
The purpose of a cylinder leak down test is to determine the sealing ability of a cylinder when it is at the top dead center position of the compression stroke. This is not to infer that this particular test is the only way to test the quality of an engine, however, it is one good way to do it.
The leakdown tester is an air pressure gauge that is connected to an air supply line. The tester has a pressure regulator within it which regulates the pressure to 80 p.s.i.. The tester has a metered port that feeds to a second gauge, which is connected directly to the top of the engine cylinder via the removed spark plug hole. Hence the readings are always 80 p.s.i. over the reading of the second gauge.
The procedure itself is as follows:
Run the engine to warm it up to insure that the metal parts are expanded to their normal running tolerances.
Remove spark plugs as needed, usually the top ones that are easiest to reach.
Thread the tester’s adaptor into the first cylinder to be tested.
While holding your finger over the tester’s outlet hole, manually turn the engine over in the normal rotational direction until you feel the pressure under your finger.
If the engine has a mag with an impulse coupler, the coupler will click which means you are just past top dead center, and you then need to backup very slightly.
AND THIS IS THE PART THAT NEEDS CARE WHEN YOU WILL HOLD THE PROP FROM TURNING AND ATTACH THE QUICK COUPLER TO THE ADAPTER!
When the air is going into the cylinder, you slowly move the prop into the direction of pressure until it balances.
The reading on the second gauge is now recorded.
To insure accurate readings, it is good practice to retake the first cylinder, to insure that the temperature of the cooling engine did not indicate lower readings of the latter cylinders
This testing procedure does require some “experience” to insure accuracy.
AND REMEMBER, THERE IS NO SUBSTITUTE FOR THE OPINION OF A SEASONED MECHANIC IN REGARDS TO THE TRUE CONDITION OF AN ENGINE.