Testing the engine compression on the 4.0L Jeep Cherokee and Wrangler is not difficult (especially since the spark plugs are easy to remove).
In this tutorial I'm gonna' explain how to do an engine compression test and how to interpret its test results.
You'll be able to find out if an engine compression problem is behind a misfire or an 'engine no-start' problem.
Contents of this tutorial at a glance:
You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (1993-2001 4.0L Jeep Cherokee) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 4.0L Jeep Cherokee: 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001.
- 4.0L Jeep Wrangler: 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001.
Symptoms Of Low Or No Engine Cylinder Compression
If the engine starts and runs, in your 4.0L Jeep Cherokee or Wrangler, a compression problem will cause a misfire or a rough idle condition.
If your Jeep is OBD II equipped, you'll see one or more of the following misfire trouble codes lighting up the check engine light:
- P0300: Random Cylinder Misfire.
- P0301: Cylinder #1 Misfire.
- P0302: Cylinder #2 Misfire.
- P0303: Cylinder #3 Misfire.
- P0304: Cylinder #4 Misfire.
- P0305: Cylinder #5 Misfire.
- P0306: Cylinder #6 Misfire.
Besides a cylinder misfire condition (or a rough idle condition), you'll see one or more of the following symptoms:
- Bad gas mileage.
- You're also gonna' notice a heavier exhaust smell coming out of its tailpipe.
- Blue smoke coming out of the tail-pipe.
- The engine is not as peppy as it was once.
If the compression problem is affecting all 6 cylinders (as in 0 PSI compression in all of them), then the engine is not gonna' start.
Important Tips And Suggestions
TIP 1: If the engine in your 4.0L Jeep Cherokee (Wrangler) has been running for any length of time, let it cool down completely before the spark plugs are removed.
Removing the spark plugs from a hot engine can result in damage to the spark plug hole threads.
One method that I have used to cool the engine down in a matter of 15-20 minutes is placing a box fan on top of it to cool it down.
TIP 2: The engine has to be cranked to test the compression, for this reason take all necessary safety precautions while working around the engine when it's being cranked.
TIP 3: Have your helper wait outside of the vehicle till you're done setting up the test.
This way you'll avoid having your helper accidentally crank the engine while you're installing the compression tester.
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
TEST 1: Finding The Dead Cylinders
To get started, we'll test the compression of all six cylinders.
If you don't have an engine compression tester, you can run down to your local auto parts store and buy one (or rent one) from them.
If you'd like to save a few bucks on its purchase, check out my recommendations here: Which Compression Tester Should I Buy?
These are the test steps:
Disable the ignition system by disconnecting the ignition coil from its electrical connector.
This will prevent the ignition coil from sparking during the test.
Remove the spark plugs. The engine must be cold before removing them.
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 plug's porcelain insulator crack 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.
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.
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 steps 3 through 5 on the remaining cylinders.
Let's examine your test results:
CASE 1: All 6 cylinders had 0 PSI. This test result tells you that the engine has an internal mechanical problem.
The most common cause of this condition is a broken timing chain or a blown head gasket.
Your next step should be to check the condition of the timing chain and head gasket.
CASE 2: One or more cylinders had a low compression value compared to the others. This could be normal or it could be causing a problem.
To find out if the compression values are normal or not, go to: Interpreting The Compression Test Results.
CASE 3: All 6 compression values were similar and above 120 PSI. This lets you know that a compression problem is not behind the no start or misfire problem you're trying to troubleshoot.