TEST 2: Wet Compression Test
Low cylinder compression is usually due to 1 of 2 things. Either the cylinder head valves (of the affected cylinder) are severely worn/damaged or the piston rings are worn/damaged.
You and I can find out by adding a small amount of motor oil to the low compression cylinder. This is what is known as a ‘Wet’ compression test.
If the problem is bad piston rings, the motor oil that we're gonna' add will help bring the compression value up when we check that cylinder's compression again.
If the problem is bad cylinder head valves, the motor oil WILL NOT help raise the compression value of the cylinder. It's as simple as that!
OK, this is what you'll need to do:
Add a tablespoon (or two) 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:
CASE 1: The compression gauge registered a higher compression value after adding motor oil. This tells you that the low compression problem is due to worn piston compression rings.
Here's why: The reason the compression value shot up is due to the fact that the motor oil you just added helped the piston rings to create a tighter seal. This type of test result only happens when the problem is due to worn piston rings.
CASE 2: The compression gauge DID NOT register a higher compression value after adding motor oil (in other words, it stayed the same). 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 in any auto parts store in any neighborhood, in any city, but you'll be paying at least twice as much. Go to the above compression tester links, browse and compare, you'll see a big price difference!
If this info really saved the day, buy me a beer!