How To Test Engine Compression (3.0L Mitsubishi)

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.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (3.0L Mitsubishi) (at: autotecnico-online.com).

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.

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 some recommendations to you:

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:

1. 1

   Disable the ignition system by disconnecting the ignition coils from their electrical connectors. This will prevent the coils from sparking during the test.

2. 2

   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.

3. 3

   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.

4. 4

   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.

5. 5

   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: You got 0 PSI on all cylinders. This test result indicates the engine has internal damage. The most likely causes will be:

• Broken 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: One or more cylinders have a compression value that is lower than the rest. This is could be normal -up to a certain point.

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. To interpret these test results, go to: How To Interpret The Compression Test Results.

How To Interpret The Compression Test Results

The rule of thumb is that the compression between cylinders 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? You can find out by using my online low compression calculator here: Online Low Engine Compression Calculator or manually this way:

1. Grab a calculator and multiply the highest compression reading that you recorded by 0.15. So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X 0.15 gives you 26 (25.5 rounded off).
2. Now, the next step is to subtract 26 from 170, which gives us 144 PSI.
3. 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 0.15= 26, 175-26= 149. So, now I know that cylinder #6 is the one causing the misfire!!

If you do have one or more cylinders with low engine compression (that's causing a problem), your next step is to do a wet compression test. Go to: TEST 2: Wet Compression Test.

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.

OK, this is what you'll need to do:

1. 1

   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.

2. 2

   Once you've added the oil, install the compression gauge.
   
   As before just hand tighten the compression gauge.

3. 3

   Have your helper crank the engine till the needle stops climbing on the compression gauge.

4. 4

   You'll see one of two results:
   
   1.) The needle will climb higher than the previous compression number you recorded for this specific cylinder.
   
   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.

5. 5

   Repeat steps 1 thru' 4 on any other cylinders that need to be tested.

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 (from autofix.com.au at web.archive.org).

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:

1. 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).
2. 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.
3. 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.