Tool Briefing: Number 6 is AWOL

Diagnosing engine power loss with vacuum and compression testing, valve timing testing and more.

Leave the compression gauge installed in the problem cylinder and install the remaining spark plugs. Some techs prefer to remove the gauge’s Schrader valve for this test, but it’s not necessary. With the engine running at idle, briefly press the gauge’s release valve to ‘burp’ the gauge, then let it close again. The reading should stabilize at about half the cranking compression. Quickly snap the throttle wide open and then release it. The reading should increase to about 80 percent of cranking compression. Repeat this test on the other cylinders and record the readings.

Low running compression, particularly in the snap-throttle test, indicates an intake restriction. High running compression indicates an exhaust restriction. The readings for all cylinders should be about the same. If a cam lobe has slipped, running compression will be high or low on that cylinder, and you’ll know whether it’s the intake or exhaust lobe.

All of these compression tests can also be performed with a pressure transducer and oscilloscope. Instead of comparing lists of numbers, you can see the results graphically, store them and even print them.

Leak Down Testing

A leak down test is a static test that will show where a cylinder is leaking. The tester uses a gauge to measure input air pressure and a second gauge to measure the pressure inside the cylinder. The difference between the gauge readings indicates the percentage of air pressure leaking out of the cylinder. If supply pressure is 100 psi and the cylinder can hold 85 psi, that indicates 15 percent leakage. Some techs use only 50 psi for this test because lower pressure reduces the risk of pushing the piston down, and it’s just as accurate. However most techs use 100 psi, probably because it's closer to the engine’s cranking compression. It makes the math easier too.

The piston should be at TDC and on the upstroke so the rings have the best opportunity to seal where they normally do on the piston ring land. Ten percent leak down for each cylinder is acceptable, and it may be hard to tell where the air is leaking out.


If leak down is greater, listen at the tailpipe, at the throttle opening, and at the oil filler to determine whether air is leaking past the exhaust valve, intake valve or rings.

When looking for engine mechanical problems serious enough to cause a driveability issue, the tools and techniques haven’t changed much over the years. But such problems have become rare enough that the diagnosis is becoming a lost art. It helps to keep in mind what’s happening inside that engine, and keep notes of all your test data too. Sometimes experience is the best tool you can have.


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