The engine compression test is usually done to find out the internal health of the engine.
But it'll also help you to find out if a misfire is being caused by a cylinder or cylinders with low compression.
In this tutorial I'm gonna' explain how to test the compression of the 5.2L V8 engine in the 1993-1998 Jeep Grand Cherokee. I'm also explaining how to interpret your test results.
Contents of this tutorial:
Symptoms Of Engine Compression Problems
Generally, engine compression problems can be lumped into one of two categories. Either the compression problem causes the engine to not start.
Or the engine compression problem causes a misfire condition. To be a bit more specific, the engine still starts and runs, but runs with a misfire.
If the engine is misfiring, due to a compression problem and your 5.2L Jeep Grand Cherokee is OBD II equipped, you'll see one or more of the following trouble codes:
- 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.
- P0307: Cylinder #7 Misfire.
- P0308: Cylinder #8 Misfire.
You'll also see:
- Rough idle condition that does not set a misfire trouble code.
- Bad gas mileage.
- Heavy exhaust smell coming out of the tailpipe when the engine is running.
Important Tips And Suggestions
TIP 1: Do not remove the spark plugs from a hot engine.
If your Jeep Grand Cherokee's 5.2L V8 engine has been running for any length of time, you need to let it cool down for at least an hour before attempting to remove the spark plugs.
Attempting to remove the spark plugs from a hot engine can damage the threads in the spark plug holes. This is a nightmare you want to avoid.
TIP 2: Use a spark plug wire puller to disconnect the spark plug wires from the spark plugs.
Using a spark plug puller will help you avoid damaging the spark plug wires when disconnecting them from the spark plugs.
To see an example of this tool, check out this article: How To Use A Spark Plug Wire Puller And Where To Buy One (at: easyautodiagnostics.com).
TEST 1: Dry Compression Test
To get the most accurate result from your engine compression test, you need to test all 8 cylinders.
If you don't have an engine compression tester, you can run down to your local auto parts store and buy or rent it there. If you'd like to save a few bucks on its purchase, check out my recommendations here: Which Compression Tester Should I Buy?.
Let's get testing:
Remove the spark plugs.
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.
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 the value at which the needle stopped and the number of the engine cylinder on a piece of paper.
Repeat steps 2-4 on the remaining cylinders.
Let's take a look at what your compression test results mean:
CASE 1: You got 0 PSI in the majority of the cylinders. This tells you that your Jeep Grand Cherokee's engine has serious internal problems. This is usually due to:
- Busted timing chain.
- Engine threw a rod.
Any compression value below 100 PSI (even if it's not 0 PSI) means internal mechanical engine trouble.
CASE 2: All cylinders have compression but their values are not the same. It's normal for each cylinder's compression value to vary slightly from one another. But if they vary too much, you'll have a bona-fide misfire or rough idle condition on your hands.
To find out, the next step is to go to: Interpreting The Engine Compression Test Results.