How To Test Compression Drop in Hot or Cold Engine - Engine Builder Magazine
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How To Test Compression Drop in Hot or Cold Engine


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.

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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

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

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