How to perform a compression test

How to perform a compression test

When diagnosing an engine to see what's wrong with it, or checking an engine to see if it's in good shape, often the first step a mechanic will take is to perform a compression test on it. A compression test is relatively quick and simple to do, and doesn't require any tools except for a socket wrench, a jumper cable, and the actual compression gauge itself, which is generally fairly cheap (they run from $20 to $40). It provides a pretty good picture of how well the engine is performing, and if there's something wrong with it, exactly what *is* wrong. Of course it can't diagnose every possible engine fault, but for a test that only takes a few minutes of time and a little gauge for equipment, it provides a remarkable amount of information about the state an engine is in.

What to look for in a compression gauge

A compression gauge with threads on it, which screws into the hole, is preferable (rather than the kind which you simply hold in the hole with your hand). Make sure it matches the size of your spark plug tubes, and that its pressure range goes up to the normal compression level for your model of car.

To perform a compression test:

1. Remove all the spark plugs from the car. (You don't just take out the spark plug in the cylinder you're testing; The test is actually performed with all the plugs out of the car.)

2. Remove the wire which leads from the center of the distributor cap. That's the cap where all the spark plug wires go; The spark plug wires form a circle around the rim of the distributor cap, and then there's another wire in the center of the cap which goes to the ignition coil. Remove that one; Don't unplug the end of the wire that goes to the coil, just take off the end that plugs into the distributor. Ground this wire by connecting the tip of it to one end of a jumper cable, and the other end of the jumper cable on the engine block.

3. If your car has EFI (electronic fuel injection), pull the fuse for the EFI system to disable it while the test is being done.

4. Install the compression gauge in the spark plug hole of the cylinder you'll be testing.

5. Hold the throttle open as far as it will go. The throttle cable is the cable which gets pulled by the gas pedal; Hold open the throttle arm as if someone were pushing the gas pedal to the floor. Keep the throttle fully open while the test is being performed.

6. Have somebody crank the engine several times. As the engine is cranking, watch the compression gauge for a reading. Write down the compression level.

7. Repeat for each cylinder of the engine.

Now you have approximate compression readings for each of your cylinders. Compare them against the "normal" reading for your car in the repair manual. (You do have a repair manual for your model of car, don't you?) Although your readings should be at or close to the readings in the manual, it's actually more important that your readings for each cylinder be close to each other than that they match the manual exactly. If there's a significant variance in the readings (more than 10 or 20 psi), it usually indicates a problem somewhere in the cylinder block, head, or valves. Also look at how quickly the compression rises; It's normal for compression to be low at first, but it should quickly shoot up to the normal level. If the compression increases slowly with each crank, this usually indicates worn piston rings, which are a real pain to change because it requires rebuilding the whole engine.

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