TEST 2: Wet Compression Test
Low cylinder compression is due to one of two things: worn/damaged cylinder head valves or worn piston compression rings.
The cool thing is that you and I can pinpoint the problem without tearing into the engine just yet. How? By doing a ‘wet’ engine compression test.
A ‘wet’ engine compression test simply involves adding a small amount of motor oil to the affected cylinder and then testing its compression once again. If the low compression problem is in the piston rings then the oil will help seal the piston rings and cause the compression value to shoot up. If the low compression problem is due to worn cylinder head valves then the oil won't be able to seal in the compression and the compression value (on your tester) will remain the same value you recorded/wrote down in the ‘dry’ compression test.
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 then repeat steps 1 thru' 4 on it now.
Let's take a look at what your compression test results mean:
CASE 1: Your compression value shot up 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 value DID NOT shoot up (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.
About Removing the Intake Manifold Plenum
The thing that complicates doing a compression test, on the majority of the 3.0L Mitsubishis covered by this tutorial, is the fact that the upper intake manifold plenum must be removed.
Since I don't include any instructions on this procedure, I want to recommend the following great tutorial on how to do it: CHANGING SPARK PLUGS – MITSUBISHI 3.0 V6 24VALVE (at autofix.com.au).
For the DIY'er that doesn't have a lot of wrenching experience, this might be a job that's best left to an experienced automotive mechanic/technician. Since paying for his expertise may end up saving you from a tremendous headache.
If you do tackle the compression test job, here are some pointers:
- As you're removing bolts, nuts, etc., place them in a container and away from the engine. This is important since you'll want to avoid having anything fall into the open intake manifold runners (that will be exposed when the plenum is removed).
- You WILL NOT be able to place rags in the open intake runners while doing the compression test. Why? Because if you do place rags in the open intake runners (to avoid stuff falling into them) then these rags will get sucked into the cylinder as the engine cranks.
- Use new gaskets. Do not re-use the old intake manifold plenum gasket. Also, when installing the new one, do not coat it in any type of sealer (like RTV Silicon). This is not necessary and could back fire on you since the sealer can cause a vacuum leak.
Yes, testing engine compression on the 3.0L equipped Mitsubishi isn't for the ‘faint of heart’. So take all necessary safety precautions and avoid having anything fall into the open intake manifold runners. Or consider letting a professional mechanic do it.
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!