Tech Tips

Compression Testing

Written by  September 30, 2006

When your engine is not quite performing the way it did when you bought it and it just doesn’t seem to make any more power, it is time to test the cylinder for leakage.
This is not a top secret type test; however, it is a test that will give you a window into the condition of the cylinder and piston rings, and it will give you an indication of whether you need to have a top-end service scheduled. Compression testing is a required item per your service manual. A lot of people do not have the time or the knowledge to perform this test, so I am going to try to shed a little light on this subject.
There are two types of compression tests that are most widely used today:
Compression tester:
Most do-it-yourselfers are familiar with the basic compression tester. This inexpensive pressure gauge measures maximum compression pressure as the engine is turned over with the electric start or kick start. The throttle needs to be opened fully so it does not give a false reading.
Leak-down tester:
This measures pressure in a hot or cold engine, but instead of using the engine to compress the air, you supply your own with an air compressor. The testing tool sets the incoming pressure, usually at about 85 pounds per square inch, and then tells you how well the cylinder holds that compression. Most importantly, it allows you to easily determine where the compression leak is. You can move the piston up and down in the cylinder, and thereby find a defect in the cylinder wall if pressure drops at a particular point in the piston's stroke. More than likely, the problem will be worn or stuck rings or a poor valve seal, and you can determine which one by listening to hear where the pressurized air is escaping. If you hear it through the exhaust, your exhaust valves are not sealing properly. Escaping air in the carburetor indicates that it's the intake valves. If you hear the hissing of escaping air at the oil filter or crankcase breather, the problem is most likely stuck or worn rings.
The pressure differential between cylinders is almost as important as the reading itself. A significant difference between cylinders is more troubling than if all cylinders are equally low. So until this reading has been evaluated, you might want to check the basics such as spark plug color and/or damage to the plug insulator tip; this is a window into the combustion chamber and will tell you what is going on.
Compression testing equipment can be purchased relatively inexpensively at most auto parts stores or name-brand tool dealers and delivers good results. It determines if the engine needs to be torn down completely or if the cylinder heads simply need to be looked at. We sometimes fear the worst when it comes time to internal engine problems. The tool distributors have really stepped up in the way of getting testing components down to an affordable level that all of us can afford and use with ease.
I have had many different teachers over the years, and the one thing they all seem to say consistently is the engine must be warm to the touch to give the best reading in your diagnosis. Your manual will say the exact pressure that your engine must have with a + / - tolerance; I believe it is a 10 percent variance. So when testing you may want to recheck more than once. For those of you who are testing a 2-cycle engine, this is a must at servicing due to cold seizures, especially when the weather gets colder.
B-safe out there!

Dave Miller