Do a Compression Test

Compression tests are usually done to test the engine in racing cars and other vehicles with high-performance engines. The test can be used to diagnose problems in the engine or to measure and improve engine performance. Some basic knowledge about automobile parts and maintenance is helpful if you want to learn how to do a compression test.

Steps

1. Bring the engine to its normal running temperature. This can be done in the following ways.
   • If you haven't driven your car recently then the engine is most likely cold. Start your vehicle as usual and keep the engine running for a few minutes. This should warm up the engine to its normal operating temperature; however you should be careful not to overheat the engine by keeping it started for too long. Usually keeping the engine started for 20 minutes is sufficient.
   • Turn off the car engine and allow the engine to cool if you have recently taken your car for a drive. If the engine is hot, it should be given approximately 1 hour to rest before you do a compression test on it.
   • If you can't start the car engine then just continue with the test anyway. Even though you won't be able to test the engine's performance accurately you will still be able to diagnose that the engine is faulty due to its internal compression if you get a low reading.
2. Switch off the engine completely before beginning the test.
3. Remove the fuel pump relay. This will disable the fuel system so fuel will not be injected into the cylinders.
4. Disconnect the electrical connection from the ignition coil pack. This will disable the ignition system since the ignition coil pack will be unable to create and distribute the spark from the spark plug.
5. Disconnect the wires for each spark plug then remove the spark plugs. Handle the spark plugs cautiously since the ceramic insulator of a spark plug can break or malfunction if not handled delicately.
6. Put the engine compression gauge into the first cylinder's spark plug hole (this is the hole that is nearest to the drive belt). Don't use any additional tools to tighten the compression gauge, you should only use hand force.
7. Ask a helper to crank the engine over continuously. The needle on the pressure gauge will rise and once the needle has risen to its maximum reading your helper should stop the engine. This maximum reading is the maximum compression value for the first cylinder that you have just tested.
8. Repeat the engine compression test for the other cylinders until you have tested all of the cylinders on your engine.

Video

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Tips

• Readings for compression tests usually vary between 175 to 250 PSI (Pounds per Square Inch). A higher reading generally indicates better engine performance; however you should check your vehicle manual to find out what the ideal reading should be for your vehicle.

Things You'll Need

• Compression gauge

Sources and Citations

• http://troubleshootmyvehicle.com/main_index/ford/4.0L_misfire/ford_engine_compression_1.php
• http://www.c-speedracing.com/howto/comptest/comptest.php

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