4/6/2009
How To Test Compression Drop in Hot or Cold Engine
Here's a handy procedure that you or an installer customer can use to determine if there are broken valve springs or loose valve guides in the cylinder head. This procedure could be a course of action taken in response to a customer complaint of engine miss, either hot or cold. To find out what cylinder is affected, complete the steps below.
First, check the compression using standard procedures (remove all spark plugs, open throttle valve, etc.). Write down the compression readings. Now, install all but one of the spark plugs. In the remaining cylinder, install a compression gauge with a pressure release.
Start the engine and let it idle. Release the pressure at the gauge and then let the compression build up, again with the engine still at idle. Repeat this procedure for all of the cylinders and write the new compression numbers below the standard procedure readings.
The amount of compression at idle is not an important factor in itself, what is important is the difference between the two readings. The difference between the compression at cranking speed and the compression at idle should be equal for all of the cylinders. If the engine miss complaint is with a cold engine, do this test cold.
For a warm engine, do the test warm. Hopefully this simple test will save you some diagnostic time when searching for the dreaded engine miss.
Some or all of this information was provided by the Automotive
Parts Remanufacturers Association (APRA). For more information on
technical bulletins available through APRA call 703-968-2772 or visit www.AutoBulletins.com.
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