An engine compression test is fast becoming a key test in diagnosing a hard to troubleshoot misfire condition. In this article, you'll learn how to do the engine compression test and, more importantly, how to interpret the results of the test on your Jeep 4.0L Wrangler, 4.0L Cherokee, or 4.0L Grand Cherokee.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar la Compresión del Motor (4.0L Jeep) (at: autotecnico-online.com).
To successfully accomplish the tests in this article, here are some important tips:
TIP 1: Be alert and take all necessary safety precautions, since you'll be working around a cranking engine.
TIP 2: Your Jeeps 4.0L engine has to be warmed up, but not hot, if your Jeep SUV starts and runs. To be a little more specific: If the engine has been running for a long period of time, let it cool down for about an hour. If the engine is completely cold, start it up and let it run for about 20 minutes.
If your Jeep's engine doesn't start, don't worry about doing the compression test with a warm engine.
TIP 3: Once you install the compression tester, hand tighten it only. Do not use any type of tool to tighten it.
TIP 4: If you're trying to solve a misfire condition (Codes: P0300, P0301, P0302, P0303, P0304, P0305, P0306) or a rough idle condition, I recommend that you test for spark first.
Engine Compression Test 1
As you may already know, each cylinder in the engine of your 4.0L Jeep Cherokee (or Grand Cherokee, Wrangler) needs Spark, Air, and Fuel to run. If any one of these components of the combustion process are missing in any of the six engine cylinders, the engine is gonna' run rough or suffer a misfire condition.
But what often gets overlooked, is the mechanical condition of the cylinder, when diagnosing a rough idle, misfire condition. This is where the following engine compression test comes in handy:
You'll need to disable the fuel system and you can easily accomplish this by either finding and removing the ASD Relay or disconnecting all 6 fuel injectors from their electrical connector.
Disable the ignition system. It's important that the ignition system not create and deliver spark to the spark plug wires. You can disable the ignition system by disconnecting the ignition coil (Distributor Type Ignition System).
Remove all 6 spark plugs. Be careful and don't to drop any of them, since dropping them can brake the ceramic insulator and then you'll have a misfire on your hands.
Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder (this is the spark plug hole closest to the drive belt). Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
When everything's in place, have someone crank the engine (or you can use a Remote Start Switch) while you watch the needle on the compression tester.
What you'll see, on the compression tester as the engine cranks, is the needle climbing till it reaches a point where it'll stop climbing (even tho' the engine is still cranking). Once the needle reaches this point, that specific engine cylinder has reached its maximum compression value and you can have your helper stop cranking the engine.
Write this engine compression value down, along with cylinder # and repeat this test on the remaining 5 cylinders.
After all cylinders have been tested, turn the page to find out how to interpret your compression test results.
Interpreting The Results Of The Engine Compression Test
Now, we're gonna' interpret those numbers you obtained from the tests, and here's what you'll need to do:
- Grab a calculator and multiply the highest compression reading recorded by .15. So let's say, for the sake of this example, that the highest reading was 180 PSI (Pounds per Square Inch). Multiplying 180 by .15 gives us 27.
- Now I subtract 27 from the highest reading, which in my example, the highest compression reading is 180... so subtracting 27 from this reading, I get: 153.
- So then, 153 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.
I'll give you another specific example... let's say the my 4.0L Jeep Cherokee (Wrangler or Grand Cherokee) produced the following compression readings:
- Cylinder #1 = 175 PSI
- Cylinder #2 = 95 PSI
- Cylinder #3 = 170 PSI
- Cylinder #4 = 170 PSI
- Cylinder #5 = 160 PSI
- Cylinder #6 = 165 PSI
The next step would be to apply the formula above and I get 149 PSI as the lowest possible reading (175 x .15= 26, 175-26= 149). So, now I know that cylinder #2 is the one causing the misfire!!
Continued in the next page...