There's a good chance that low engine compression in one (or more) cylinders is causing your 3.0L equipped Mitsubishi to misfire (or idle rough). Lack of compression in all 6 cylinders will keep the engine from starting (usually caused by a broken timing belt).
So, if you've been wanting to test the engine's compression yourself but weren't quite sure how to do it or interpret its results, this is the tutorial for you.
Contents of this tutorial:
Important Tips And Suggestions
TIP 1: If your 3.0L equipped Mitsubishi starts and runs, it's standard procedure to do the compression test with a slightly warmed up engine. The key words here are ‘slightly warmed up engine’ since the engine should not be hot.
To slightly warm up the engine, start her up (from a completely cold condition) and let her run for no more than 10 to 12 minutes.
TIP 2: The upper intake manifold plenum must be removed to access the spark plug for cylinders #2, #4 and #6. This tutorial does not include ‘remove and replace’ instructions of the upper intake manifold plenum so you'll need a repair manual or a google search. For more info on this, go to section: About Removing the Intake Manifold Plenum.
TIP 3: You'll be working around a cranking engine as you perform the engine compression test so take all necessary safety precautions. Your safety is your responsibility... so use common sense and think safety all of the time.
TIP 4: Never remove the spark plugs with a hot engine. This is important because the 3.0L V6 engine has aluminum cylinder heads. Removing the spark plugs from a hot engine can damage the spark plug hole threads in the aluminum cylinder heads.
TEST 1: Dry Compression Test
The engine compression test is divided into two parts. The first part is testing the compression of all 6 cylinders ‘dry’.
The second test done is a ’wet’ compression test. In this second test, you add a few drops of engine motor oil to the cylinders that registered low or 0 compression (I'll explain this in more detail in TEST 2).
IMPORTANT: The intake manifold plenum must be removed to install the compression tester on cylinder #2, #4, and #6. For more info, see the section: About Removing the Intake Manifold Plenum.
OK, to get started this is what you'll need to do:
Disable the ignition system by disconnecting the ignition coils from their electrical connectors. This will prevent the coils from sparking during the test.
Disable the fuel system by removing the fuel pump fuse. Disabling the fuel system will prevent fuel from being injected into each cylinder when the test is performed.
Remove the spark plugs from a slightly warmed up engine (if it starts and runs). Remember, the engine can not be hot.
When removing the spark plugs, be careful not to drop any of them on the floor, or you run the risk of having the spark plugs porcelain insulator crack and then you'll have a misfire on your hands.
If the engine does not start, don't worry about it being warmed up.
Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder. Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
Have your helper crank the engine till the needle on the compression gauge stops climbing.
Now, record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper. Release the pressure on the gauge and repeat this step one more time.
Repeat this test step on the remaining 5 cylinders.
Let's take a look at what your test results mean:
CASE 1: If Your Car Does Not Start and the results of the engine compression test are 0 PSI on all 4 cylinders then you have one of the following conditions:
- Timing belt.
- Blown head gasket.
- Blown engine.
Any compression value below 100 PSI (even if it does not 0 PSI) means internal mechanical engine trouble.
CASE 2: If Your Car Does Start, the very first thing you'll notice is that the compression values you wrote down for each cylinder are slightly different from one another. This is normal.
What is NOT normal is if the values vary too much. The cool thing is that we can find out if the variations in the values, you wrote down, indicate a problem (with that cylinder) or not.
The rule of thumb is that they can not vary more than 15% from each other and if they do then you're gonna' have a genuine misfire condition on your hands or possibly a no start condition (if more than one cylinder is affected).
How do you figure this out? -This is how you do it:
- Grab a calculator and multiply the highest compression reading that you recorded by .15. So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .15 gives you 26 (25.5 rounded off).
- Now, the next step is to subtract 26 from 170... which gives us 144 PSI.
- So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this and that engine cylinder will misfire.
To make better sense of the above calculation, let's say that my 3.0L Montero produced the following compression test results:
- Cylinder #1 170 PSI.
- Cylinder #2 175 PSI.
- Cylinder #3 170 PSI.
- Cylinder #4 170 PSI.
- Cylinder #5 165 PSI .
- Cylinder #6 120 PSI .
The next step is to do the math: 175 x .15= 26, 175-26= 149. So, now I know that cylinder #6 is the one causing the misfire!!