TEST 2: Wet Compression Test
To find out if the low cylinder compression is due to worn out cylinder head valves or worn out piston rings, the next step is to do a ‘Wet’ compression test. Knowing if the problem is in the cylinder head valves or in the piston rings will help you decide what you need to replace (and how much it's gonna' cost) before you do any tear down.
To be a bit more specific, this test will tell you if you only need to have the cylinder heads worked on or if you only need to have the block worked on. Both of these options are not the ideal since just repairing one and not the other is not a good idea. Why?
Because by repairing only one, but not the other, you're only delaying the inevitable by about six months, which is: you're gonna' have to replace or repair the entire engine anyway.
OK, getting back to the test at hand, what you're gonna' do is add about 2 tablespoons of oil to the cylinder (that recorded no or low compression in the previous test) and then check its compression once again.
What will happen is that if the low compression values is due to worn piston rings, the compression value will go up from the previous one you got doing the Dry compression value.
If the compression value does not go up (from the previous one), then you'll know that the problem lies in the cylinder head valves.
OK, to get this pot of water boiling, this is what you need to do:
Add a small amount of engine oil to the cylinder that reported low compression or no compression in the ‘Dry’ compression test
- The amount should be about 1 to 2 tablespoons of oil.
Install the compression tester onto the cylinder.
- Do not use any type of tool to tightened the compression tester. Hand tight is fine.
When all is set up, have your helper crank the engine.
You'll get one of two results, either the compression value will go up (from the one you recorded before) or it will stay the same.
CASE 1: The compression value shot up. This tells you that the piston compression rings are worn out and thus the problem is in the bottom end.
The reason why the compression value shot up, with the engine oil added, is because the oil is aiding the piston rings to create a near perfect seal.
Since the oil is not letting the compression escape by the rings, this results in your compression gauge reading a higher value than with the ‘dry’ compression test.
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
The reason why the compression reading did not shoot up, is because the engine oil can not help the intake or exhaust valve seal the compressed air. Therefore, the compression value will stay the same.
Why An Engine Compression Test?
It doesn't happen very often, that one or more (but not all) engine cylinders wear out at an accelerated rate than the others but it does happen enough that you should be aware that a compression test should be an integral part of your misfire code diagnostic strategy.
So many things can cause a misfire condition or misfire codes (if the vehicle is OBD II equipped: P0300, P0301, P0302, P303, P0304, P0305, P0306) to set that you could think they are nearly impossible to troubleshoot. Well, the good news is that every possible cause can be tested to see if indeed it is the one causing the problem! And you'll find most of those testing articles here at troubleshootmyvehicle.com.
Which Compression Tester Should I Buy?
There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make two recommendations to you:
1) Which one to buy: The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.
Engine Compression Gauge Testers
2) Where to buy: You can buy an engine compression tester just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!