In this tutorial, I'm going to show you how to test the engine compression on the 3.9L V6 engine of the Dodge Dakota and Dodge Durango.
I'm also going to explain how to interpret your test results so that you can find out if your engine has an engine compression problem (or not).
Contents of this tutorial at a glance:
You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (1993-2003 3.9L V6 Dodge Dakota) (at: autotecnico-online.com).
APPLIES TO: This tutorial applies to the following vehicles:
- 3.9L V6 Dodge Dakota: 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003.
- 3.9L V6 Dodge Durango: 1998, 1999.
Symptoms Of Low Or No Engine Cylinder Compression
Engine compression problems will cause 1 of 2 problems on your 3.9L Dodge Dakota: Either the engine is not going to start or the engine will start and run but it will suffer a misfire or rough idle condition.
On the 1996-2003 3.9L Dodge Durango (that is OBD II equipped) you're gonna see one or more of the following misfire 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.
Besides a cylinder misfire condition (or a rough idle condition), your 3.9L V6 Dodge Dakota (Durango) is gonna' suffer from bad gas mileage. You're also gonna' notice a heavier exhaust smell coming out of its tailpipe. Also, since the engine is not running optimally, it's not gonna' be as peppy as it was once.
Important Tips And Suggestions
TIP 1: If the engine in your 3.9L Dodge Dakota (Durango) has been running for any length of time, let it cool down completely before you remove the spark plugs. 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
We're gonna' start off by testing the compression of all 6 cylinders.
Once you get your compression test results, I'll help you interpret them in the section: Interpreting The Compression Test Results.
The illustration above will help you to identify the cylinders that you'll be testing on your 3.9L Dodge Dakota.
If you don't have a compression tester you can run down to your local auto parts store and buy or borrow one from them. If you're interested in buying one, then take a look at my recommendations here: Which Compression Tester Should I Buy?
NOTE: Disable the ignition system before doing a compression test. You can easily disable the ignition system by disconnecting the ignition coil from its electrical connector.
OK, 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. 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 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 six 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 belt.
Your next steps should be to check the condition of the timing belt. I would also recommend that you check for a blown 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 the next step is do some math and see if the low compression value is lower by more than 15% of the highest compression value you got. To find a detailed explanation of this calculation go to: Interpreting The Compression Test Results.
CASE 3: All six 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.