Firing on All Cylinders?

A running compression test is a great tool for diagnosing density misfires.

A running compression test or dynamic compression test is an often overlooked engine diagnostic tool, but it can pinpoint the cause of a misfire when the usual tests are normal. When you perform a static compression or cylinder leak-down test, you’re checking the sealing capability of individual cylinders. A running compression test, however, indicates how efficiently each cylinder pulls in air, retains it for the correct time and then releases it into the exhaust.

How to perform the test

1. Create a chart (see samples) to record your test readings.
2. Begin with a normal (“static”) compression test on a warm engine to eliminate problems such as worn rings, burnt or bent valves or holes in pistons. If you have a diagnostic trouble code, you might know which cylinder is misfiring, but test the compression of all cylinders in that bank for a comparison. Record the readings.
3. Install all the spark plugs except the one for the suspected cylinder (ground that plug wire to prevent module damage, or when equipped with a coil-on-plug ignition, simply disconnect the coil harness plug).
4. Disconnect the injector for that cylinder on a port fuel injection system.
5. Install your compression gauge in the suspected cylinder. You can remove the Schrader valve, but most techs recommend leaving it in the gauge and “burping” the gauge every 5 to 6 puffs.
6. Start the engine and record the results of an idle reading (1,200 rpm).
7. From idle, snap the throttle to 2,500 rpm. Release quickly. Record the results.

Don’t use the gas pedal. Manually open, then close, throttle as fast as possible, forcing the engine to take a gulp of air.

Sample test readings

Compare measurements between cylinders. Running compression at idle should be 50 to 75 psi (about half cranking compression). Snap throttle compression should be about 80 percent of cranking compression.

Example 1: A low snap test in cylinder No. 1 is much less than 80 percent cranking compression. Look for air intake problems: severe carbon deposits on intake valves, worn cam lobes, valve guides and springs, rockers or push rod problems, or “shutter valve” incorrectly positioned in a variable runner intake system.

Example 2: A low idle and snap test in cylinder No. 2 indicates that the cylinder is not holding compression efficiently. Look for slightly bent or burned valves, excessive carbon buildup on valves or seats, worn valve guides and springs, scored cylinder wall or a leaking head gasket.

A significantly higher snap measurement (over 80 percent of cranking compression), means the air is not leaving the cylinder efficiently. Check the exhaust side of that cylinder for a worn cam lobe, bent push rod or collapsed lifter. If the snap readings are all high, look for a clogged catalytic converter or muffler.

As you can see, a running compression test is a great test to add to your diagnostic arsenal. Try it on your next vehicle with an unexplained misfire.