Doing a compression test on your 1.8L or 2.4L Mitsubishi (Chrysler, Dodge) vehicle is a pretty easy test to do and in this article I'll show you how to get it done.
More importantly, I'm also gonna' show you how to interpret the results of the test.
Why do an engine compression test? Well, it may help you to find out if the engine is shot and if this is the reason why it won't start or the reason you have a hard to diagnose misfire condition.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (2.4L Mitsubishi) (at: autotecnico-online.com).
Important Tips And Suggestions
TIP 1: If your vehicle starts and runs then I suggest that you perform the compression test with a slightly warmed up engine. My recommendation is to let a completely cold engine run for about 15 minutes and no more.
TIP 2: If your vehicle DOES NOT start and run then you don't have to worry about testing the compression of the cylinders with a warmed up engine.
TIP 3: The compression test is done with the engine cranking. So you need to take all necessary safety precautions. Be careful, use common sense and think safety all of the time.
TIP 4: You'll need to remove the spark plugs to perform the compression test. You should never remove spark plugs from a hot engine. Removing spark plugs from a hot engine can and will damage the spark plug threads in the cylinder head. This is a nightmare you do not want to experience.
TEST 1: Dry Compression Test
The illustration above will help you to make sense of the results of the engine compression test, since you need to know the number of the engine cylinder you're testing.
OK, this is what you'll need to do:
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 then don't worry about it being warmed up.
Disable the fuel system. You can easily do this by simply disconnecting the four fuel injectors from their electrical connectors.
Disabling the fuel system will prevent fuel from being injected into the engine cylinders when the engine is being cranked by your helper.
Disable the ignition system. If your vehicle has the Coil-on-Plug Ignition System, you can skip this step and go to step 4 since you've already disabled the Ignition System when you removed the ignition coils from the engine to get to the Spark Plugs.
If your vehicle has a distributor type ignition system, then you can disable the ignition system by disconnecting all of the distributor's electrical connectors. Do not move on to the next step without first doing this.
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.
OK, now for you signed up for! 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.
Now, repeat steps 5 and 6 on the remaining three cylinders. Once you're done, the next step is to interpret the results of your compression values.
Interpreting The Results Of The Engine Compression Test
Depending on whether your car starts and runs or not, your test results will have two different intepretations and here they are:
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:
- 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: If your vehicle does not 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 Mitsubishi Lancer produced the following compression test results:
- Cylinder #1 175 PSI.
- Cylinder #2 170 PSI.
- Cylinder #3 165 PSI .
- Cylinder #4 120 PSI.
The next step is to do the math: 175 x .15= 26, 175-26= 149. So, now I know that Cylinder #4 is the one causing the misfire!!