TEST 2: Wet Compression Test
If in TEST 1 you found that one (or more) cylinders had low (or no) compression, the next step is to find out if that compression test result is due to worn cylinder head valves or worn piston rings.
We can do this by simply adding about 1 or 2 tablespoons of motor oil to the affected cylinders and retest them. The compression result that we're looking for (in the re-test after the oil has been added) is if the compression value goes up or not.
If the compression value goes up, then we can conclude that the low compression (in that cylinder) is due to worn piston rings. If the compression value does not go up, then we can conclude the problem is in the cylinder head valves.
At the bottom, I'll show you how to interpret your test results.
OK, this is what you'll need to do:
Add a tablespoon of engine oil in the cylinder you need to retest. I suggest using a small and long funnel so that the oil will reach the inside of the cylinder.
Once you've added the oil, install the compression gauge, and as before... just hand tighten it.
Now, have your helper crank the engine till the needle stops climbing on the compression gauge.
As before, your job is to keep an eye on the gauge, and you'll see one of two results:
1.) The needle will climb higher than the previous compression number you recorded for this specific cylinder, or...
2.) The needle will not move at all or stay at the same number you recorded earlier.
What ever value your compression tester reads, write it down again.
If you have another cylinder that needs to be tested... repeat steps 1 thru' 4 on it now.
Let's take a look at what your compression test results mean:
The Wet compression test will only give you one of two results (on the specific cylinder you added engine oil and re-tested):
CASE 1: Your compression tester registered a higher value This compression test result confirms that the low compression value registered in this cylinder in the dry test is due to worn piston rings.
CASE 2: Your compression tester DID NOT register a higher value (than the previous recorded value), This result tells you that the low compression value registered in this cylinder in the dry test is due to worn/damaged cylinder head valves.
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!
If this info saved the day, buy me a beer!