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Compression Testing 101: FAQ / How To

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Compression Test: FAQ

What is a compression test used for?
A compression test is used to determine the health of your engine by measuring cylinder pressure. It is very inexpensive and simple to do. A compression tester costs roughly $20 and can be picked up at any local auto parts store. The test only takes about 20 minutes to complete.

What do the results tell you?
The results will tell you how warn (or possibly damaged) your engine is. Possible causes for low cylinder pressure are bent valves, blown head gasket, excessively worn piston rings, damaged piston, broken ring lands, etc... The list of possibilities goes on. Please keep in mind that a compression test is not a diagnostic tool.

What is a leak down test used for?
A leak down test is performed to identify where you are losing compression and exactly how much. It's a diagnostic tool. If a compression test yields poor results, it is best to follow one up with a leak down test immediately so you are able to find and fix the problem.

What is a “wet” compression test?
A wet compression test is used to indicate if low cylinder pressures are caused by worn piston rings only. The test is done exactly the same as the “dry” test, however a small amount of engine oil (about 1-2 teaspoons) is added into the cylinder to create temporarily seal (to prevent possible blow-by). If compression rises significantly afterwords, then worn piston rings are the culprit.

What are the factory specs for a compression test?
On the B18B1, the cylinder pressure must not exceed 199psi or be any lower than 135psi.
On the B18C1/B18C5, the cylinder pressure must not exceed 270psi or be any lower than 135psi.
On all three engines, there must not be any variation greater than 28psi between any cylinders.

What’s required to do a compression test?
Compression Tester
5/8” spark plug socket
6” extension and a ratchet
Motor oil
Torque wrench

Continue on to the next page for the how to procedure. Compression Test: How to Procedure

1) Go drive the car around the block a couple times to warm the engine up to normal operating temperatures. This will allow the piston rings to fully expand and ensure that the battery charged enough to crank over the engine multiple times. If your engine is down for some reason (will not run) you can skip this step and perform the test cold. Keep in mind that the results will be a little lower.

2) Remove the spark plugs wires and set them aside. Avoid placing them on the exhaust heat shield. Instead, lay them over towards the intake manifold so they do not get damaged. (TIP: You may also want to unplug the power connector to the distributor to prevent any possible ignition system damage.)



3) Remove all four spark plugs from the engine. You’ll need a 5/8” sparkplug socket, 6” extension, and a ratchet.



4) Disable the fuel injection system by removing the 15amp fuse found in the panel over by the battery. It is necessary that you do this otherwise the injectors will be spraying fuel inside the cylinders during the test. That will wash oil off the cylinder walls, lower compression test results, and dump unburnt fuel down the exhaust system next time you start the engine up.





5) Assemble your compression tester with the correct adapter (14mm). Be sure to put some motor oil on the threads and O-rings to ensure they won’t get damaged during the test, and then insert the compression tester inside the cylinder one.





6) With your foot placed on the gas pedal at WOT (very important!), put the key in the ignition and allow the engine to continuously crank over approximately seven times or until the gauge needle stops moving.

Record your results and move on to the next cylinder. Compresstion Test: My Results

Here are my results:









Cylinder 1: 180psi
Cylinder 2: 178psi
Cylinder 3: 172psi
Cylinder 4: 180psi

For a B18B1 with approximately 82,000 miles on it, she’s running as healthy as a horse. There’s not much variation between the cylinders (8psi to be exact) and the over all compression numbers are well within spec.

Have a question regarding this article? Please run a search because compression tests have been covered extensively before. For your convenience, here are some search results / links for you to sift through:

C-Speedracing.com's Compression Test Article
C-Speedracing.com's Leak Down Test Article
"How to do a compression check?" common topic
"compression test" search results
"compression" search results
"compression results" search results
"compression numbers" search results
"leak down" search results

If none of those search results answered your question(s), please do not private message me directly. Instead, make a thread in your specific generation forum.

Thank you.
 
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