Compression test results...

How To Read Results?

If you have a compression test performed on a used vehicle you're looking to purchase, it might be interesting to know how to read the results. First of all, the test is made with a compression gauge, which is an instrument that measures pressure produced individually by each cylinder. If you have a 4-cylinder car, you'll get four results. For example you may get the following results:

#1 cylinder: 175 psi
#2 cylinder: 170 psi
#3 cylinder: 175 psi
#4 cylinder: 160 psi

Pressure is measured in "psi" (pound per square inch); in the metric system it is measured in kilopascals (kPa). Usually, manufacturers do not give out detailed specifications on compression numbers but they do offer the following rule: a minimum of 100 psi per cylinder, and a maximum difference of 25% between each cylinder. You should start to worry if one of your cylinders is getting close to 100 psi. Most new engines in good condition compress at around 175 psi. If we look at the example above, we find that the #4 cylinder at 160 psi is O.K. But let's compare it with the cylinder having the highest pressure level at 175 psi.

With a simple rule of three:
175 psi = 100%
160 psi = ?
(160 x 100) / 175 = 91.4% for the #4 cylinder. So 100% ? 91.4% = 8.6% which is well below the 25% limit, thus a good result.

What If...

Suppose that #4 cylinder were at 120 psi, the difference ratio would be 31.4% which would indicate excessive wear in the cylinder. When a cylinder has a low compression level, it is possible to find the leak by performing a "liquid test." You do the compression test again but this time you add a little quantity of motor oil (one teaspoon) in the cylinder in order to temporarily increase its tightness. If during this new test the pressure rises by 5% or more it means that the piston rings are causing the trouble. But if the pressure is the same then probably the valves are the source of trouble. Note that this "liquid test" cannot be performed on flat engines such as Volkswagen, Porsche and Subaru. Moreover, if two adjacent cylinders have a low compression level it means you have a broken head gasket. In any case you'll have to open up the engine. And the worst-case scenario would be to have to rebuild the whole motor.

Doing The Test Right

For best results the engine should be operating at normal temperature, or until the radiator fan starts. Disconnect the starter and the fuel supply to prevent any danger of explosion, and remove all the sparkplugs. Keep the throttle valve open to the max throughout the operation. Connect the pressure gauge to the sparkplug hole, and then turn on the starter until the gauge pointer gets up to the maximum. Repeat the operation four or five times while writing down the result each time.

Remember that while checking compression levels you should also watch for unusual sounds in the engine, for any kind of oil leaks as well as smoke in the exhaust system. If you're going to perform this test yourself be sure to follow the recommendations in the car maker's repair manual.

Considering it's only been a week since it was ran it shouldn't have too much of an effect on your readings. Did you remove all the plugs and do the test or did you remove just hte one you were testing and then reinstall it when you were done with that cylinder and move to the next? Did you do the test with the throttle blade on the throttle body open? It's possiable that the battery was wearing down if you didn't have all the plugs in it. An easy fix it to hook up a batter charger while doing the test.

American_Muscle said:

Well the last time it really RAN was like 1 month or 2 ago.

I just turned it on and drove it for a mile maybe.

I took out all the plugs at once.
A battery charger?It's a brand spankin new red top.
Would that really be the problem?

What about the starter?

Click to expand...

If you had all the plugs out it should free wheel enough that you shouldn't be having a starter or battery problem. If you starter works good enough to start the car when it's hot I don't see that being an issue. You should be able to tell if the engine wasn't spinning as fast as it had been when you started. If it still sounds the same that isn't the problem. Go take the car and drive it so it's good and worm. Put it under a good load, bring it back let it cool off enought so you can work on it, pull all the plugs and do the test again. I think you will see much better results. That way everything will be in good order. Your batter will be on a good full charge from driving it and the rings will be seated better. Again if it is a starter problem you should be able to hear the engine slowing down while cranking.

has to be a warm engine to get accurate results. take all the plugs out and make sure the throttle plate is wide open.

Patrick, I concur. The method and readings matter not since it was done on a cold mill, and that supercedes all other considerations.

I would not even read much into the deltas since the motor was cold.

The starter being tired and/or not getting the juice it needs can affect readings - A lot of guys like to have a charger on the car while cranking.

Good luck bud.

American_Muscle said:

Ummm thanks....but I already knew all that.

Click to expand...

Then you're already aware that your test results are invalid and that the correct proceedure needs to be followed if you're going to get any usable data from it.

I see that Hissin has already beaten me to it.