Technical stuff: Performing a cylinder compression test

Want to find out how solid your engine is? Perform a compression test. It's quick and easy!

Performing a compression test:

1. Obtain an engine compression tester kit (see below). Borrow one from a friend, buy one, or try renting one from Autozone (it might even be free, or they'll do it for you).
2. Open your hood and pull the wires from your spark plugs (all of them).
3. Remove one spark plug (using the proper tools) and screw in the compression tester.
4. Go try to start your car (it won't start) for a few seconds
5. Return to the engine compartment and record the reading on the gauge (____ psi).
6. Replace the spark plug and move to the next cylinder.
7. Repeat the above steps until all cylinders have been tested.

Calculating your cylinder compression:

1. Find out the standard compression ratio for your car when new (listed in your owner's manual, or online). For example, my MR2 has a compression ratio of 9.4:1. This varies by car model.
2. Estimate the atmospheric pressure of where you live. At sea-level, this is 14.7 psi. At a higher altitude, like in Provo, the atmospheric pressure is 12.46 psi.
3. Multiply your ideal compression ratio by the atmospheric pressure. For me, adjusted to my altitude this would be 9.4 x 14.43 psi = 135.64 psi. This is the ideal value that my cylinders should show when performing the compression test.
   
4. Do the math for where you live, then compare the results you found in your tests to your ideal number. Expect some variance due to your testing instrument, the tightness of the fit of the tester in the cylinder head (where the plugs screw-in to). I'd say that a variance of 15 psi between cylinders and the ideal compression is normal.

So now, what does all this mean?

Using my car as an example, I got a read of 130 psi on all four cylinders. Since my ideal compression is 135psi, I'd say that everything is looking good.

Low compression in one or all cylinders can mean a few things:
- You may not be doing the test right, or your meter is faulty. Get a second opinion with a different meter or have a friend give it a shot
- Bad news, low compression usually means that one or more of the following are faulty: bad piston seals, rings, valve seals, the piston itself, cylinder walls, head gasket. These are expensive repairs, usually requiring an engine overhaul.

If it's your car, you might want to start considering other options as it's only a matter of time before your car experiences a semi-catastrophic break down.

This is a great test to perform when purchasing a used car. It's quick and no seller worth his salt would refuse you performing the test. And it tells so much about the car's health that you can't see from a test drive or by looking at the engine. If you find a used car that fails the compression test, run like heck! Do not spend another minute with the car. And don't even think about buying it!

I just performed a compression check and all four cylinders are nice and tight at 130psi each (135psi is the ideal).