How To Perform A Compression Test

Checking an engine's compression can be a great way to help quickly diagnose possible engine problems and gauge the health of an engine. Low compression can indicate headgasket and valve problems as well as worn piston rings.

Performing a compression test is an easy, straightforward procedure. It's something every home mechanic should know how to do. The main tools needed are a compression tester and a spark plug wrench. Follow along as RepairNation.com shows you what's involved.

What you need: Compression gauge Spark plug wrench Oil squirt gun (optional) Pen and paper (optional)

Steps:
(Compression tests should always be done when engine is warm)

1. Remove All Spark Plugs
2. Insert gauge Screw compression tester into spark plug hole, making sure it is snug and seals well. Make sure it is away from the fan and any moving parts.
3. Crank engine Crank engine for about 5 seconds
4. Check reading Check reading on the compression gauge and record the number on a piece of paper.
5. Repeat Repeat the above steps for each cylinder.
6. Compare numbers Compare your compression readings with those of your vehicle's repair manual and note any differences.

1. Low compression reading
Compression readings lower than specified by the manufacturer indicates excess air is leaking from the combustion chamber for some reason. Possible causes include:

If you get a low reading in one or more cylinders and you want to pinpoint the problem, squirt a tablespoon of motor oil into the spark plug hole and perform another compression test. The oil helps the piston rings seal. If the reading jumps up to normal, it indicates you probably have worn piston rings for that cylinder. If the reading stays the same, you probably have leaks occurring with the valves or possibly the headgasket.

3. No compression
No, or extremely low compression could be caused by:

4. Uneven readings
Good engine performance requires that compression be balanced among cylinders. Some variation of readings is acceptable. Most auto manufacturers allow a maximum variance of 20% between the highest and lowest compression readings among all cylinders. Many engine rebuilders, however, feel that a 10% maximum variation is a better limit for best engine performance. To apply this, let's say that after performing your compression test, the majority of your cylinders had around 160 psi. compression One cylinder was higher at 165 and the lowest cylinder reading was 150 psi., a difference of 15 psi. Well, 165 x .10 = 16.5 psi. - that's our maximum variation. Your engine would be within the 10% variation specifications.

5. High readings
High readings indicate carbon buildup. Carbon buildup is the accumulation of carbon deposits on piston, valve and combustion chamber surfaces. It can be caused by excessive idling or a poorly tuned motor. It can cause pre-ignition. Pre-ignition is a condition caused when glowing red-hot material in the combustion chamber ignites the air/fuel mixture before the spark plug fires. If allowed to persist, pre-ignition can lead to serious engine damage including broken pistons and even bent connecting rods.

Compression Tester:
Compression testers are available at auto parts stores and most discount stores for around $20. If you buy one, consider getting one that has a screw-on type stem. The advantage of this type is that one person can easily perform the test alone.