The compression tester, also know as a compression gauge, is one of the most basic tools used for diagnosing engine problems.
A compression test is a quick way to determine the general condition of the pistons rings and can help determine what repairs might be needed before starting to work on the engine.
There are two basic types. Both have a gauge that measures the pressure in the cylinder as the piston moves upwards.
The fastest compression gauge to use is the type that just has a rubber nipple that you push into the spark plug hole and then crank the engine over with the starter. The main disadvantage of this type of gauge is that you really need two people to do the test. One person holds the gauge in place while the other cranks the engine. The othere disadvantage to this type of gauge is that it will not work on most modern engines with the spark plugs recessed deeply into the valve cover or cylinder head.
The second type of compression gauge has a hose attached to it and a fitting on the end that actually screws into the spark plug opening and seals with an "O"-ring. This type of gauge does not need a second person and works on almost any type of engine. This really is the best gauge to have if you are planning on doing a lot of engine work.
NOTE FOR DIESEL ENGINE OWNERS:
Diesel engines have a much higher compression than a gasoline engine and require a special gauge that can read the higher pressure. If you try to use a "normal" gauge on a diesel you will probably ruin the gauge.
Using a compression tester
Using a compression tester is pretty easy but there are certain things you need to do to get accurate readings.
It is best to do the testing with the engine warmed up but sometimes this is not possible so cold is ok too.
1. Remove all of the spark plugs from the engine.
2. Disable the ignition so that you do not damage the ignition system or shock yourself.
3. If the engine is fuel injected, disable the fuel pump by removing the fuel pump relay, disconnecting the pump connector or whatever other means is easiest.
4. If the engine uses a carburetor with an electric fuel pump, disable the fuel pump. If the engine has a mechanical fuel pump, you should not have any problems leaving it attached.
5. Open the throttle all the way and block it open. This allows the engine to draw in unresricted air and give an accurate compression reading.
6. Start with the number one cylinder and connect the compression tester by screwing it into the spark plug hole. If you are using the hand held type, you will have to hold it into the spark plug hole while your helper cranks the engine.
7. Using the starter, crank the engine at least 4 or 5 revolutions or until the gauge does not raise any higher on each revolution.
8. Record the reading on a note pad.
9. Remove the tester from the cylinder and release the pressure using the release valve on the tester.
10. Repeat the test on all of the cylinders and record your results.
Anylizing the test results
The actual compression readings will vary from engine to engine. If you have the factory specifications, you can compare your readings to them. A modern higher compression engine or a racing type engine will have much higher readings compared to a 1940's tractor engine.
The most important thing is that all of the readings should be pretty close to each other. If one cylinder is a lot lower than the rest, then there is most likely a problem with the piston rings, valves etc.
One additional test you can do if you have one cylinder that is lower than the rest is to remove the tester, squirt a few squirts of engine oil into the spark plug hole and repeat the test. If the reading increases, the problem is almost certainly the piston rings.