Engine compression problems can cause a no-start problem or a rough idle/misfire problem.
The cool thing is that testing and interpreting an engine compression test is not that hard.
In this tutorial I'll explain the test and how to interpret your specific test results so that you can find out if an engine compression problem is behind the engine performance problem you're trying to diagnose.
Contents of this tutorial:
You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (1991-2003 3.9L Dodge Ram Pickup y Van) (at: autotecnico-online.com).
Tools You'll Need:
- Compression Gauge Tester
- A Helper
- Pen and Paper
Symptoms Of Engine Compression Problems
It's been my experience that engine compression problems usually cause one of two things.
Either the engine is not going to start or the engine will start and run but run with a misfire problem.
When an engine compression problem causes a misfire condition, you're going to see one or more of the following trouble codes (if your 3.9L V6 Dodge Ram pickup or van is OBD II equipped):
- 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.
You're also going to see one or more of the following symptoms:
- Rough idle condition that does not set a misfire trouble code.
- Misfire felt only when accelerating the vehicle.
- Misfire felt when the engine is idling but goes away when engine is accelerated.
- Bad gas mileage.
- Heavy exhaust smell coming out of the tailpipe when the engine is running.
Important Tips And Suggestions
TIP 1: If the engine has been running for any length of time, let it cool down completely before you 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: To disconnect the spark plug wires from the spark plugs, use a spark plug wire puller.
Using a spark plug wire 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).
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.
2) Where to buy: You can buy an engine compression tester just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!
TEST 1: Dry Compression Test
To find out if an engine compression problem is behind a no-start or misfire problem, it's important to test all 6 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:
- 1
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. - 2
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. - 3
Have your helper crank the engine till the needle on the compression gauge stops climbing.
- 4
Record the value at which the needle stopped and the number of the engine cylinder on a piece of paper.
- 5
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 test result confirms that your 3.9L V6 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 the compression values vary too much, they'll cause a rough idle or misfire problem.
To find out, the next step is to go to: Interpreting Your Compression Test Results.
