Engine compression problems can cause a no-start problem or a misfire condition. In this tutorial, I'm going to explain how to test the engine compression on the 3.9L V6 GM engine and more importantly, how to interpret your test results.
GM vehicles equipped with the 3.9L V6 engine are: 2006-2010 Chevrolet Impala, 2006-2007 Chevrolet Malibu, 2006-2008 Chevrolet Uplander, 2006-2009 Pontiac G6, and 2006-2009 Pontiac Montana.
Contents of this tutorial at a glance:
You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (2001-2010 3.9L V6 GM) (at: autotecnico-online.com).
Symptoms Of Low Or No Engine Cylinder Compression
Engine compression problems usually cause one of two types of problems. In a nutshell: 1.) The engine is either going to crank but not start or 2.) The engine is going to crank and run but will run with a misfire.
Now when your 3.9L Chevrolet Malibu is having engine compression problems that are causing a misfire, you're going to see one or more of the following symptoms:
- 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 misfire problem causing a misfire trouble code to light up the check engine light on the instrument cluster, you're going to get bad gas mileage from your 3.9L V6 Chevy Malibu (Impala, Uplander, G6, or Montana).
You're also going to notice a heavier exhaust smell coming out of its tailpipe. Also, since the engine is not running optimally, it's not going to be as peppy as it was once.
If the engine compression problem is causing your engine to not start, well it's not going to start.
With the help of the test instructions in this engine compression test tutorial, you'll be able to find out if a compression problem is behind the misfire condition or no-start condition of your 3.9L Chevy Malibu.
If the engine compression problem is causing your Chevy Malibu 3.9L V6 engine to not start, it's usually because most of the cylinders have 0 PSI compression. A problem like this is usually caused when the engine has overheated and has blown head gaskets or it has thrown a rod.
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.
TEST 1: Finding The Dead Cylinders
All right, we're going to start off by testing the compression of all 6 cylinders.
If you don't have an engine compression tester, you can run down to your local auto parts store and you can borrow one from them for small cash deposit (which they will return to you once you return the tool). If you're interested in buying your own compression tester, check out my recommendations here: Which Compression Tester Should I Buy?
Once we have the engine compression values of all six cylinders, we can then interpret the results (in the next section).
NOTE: Testing the compression of cylinders 1, 3, and 5 (which are located facing the engine's firewall) can be a challenge. What will make it a little easier to remove the spark plugs is to first remove the alternator.
IMPORTANT: Do not remove the spark plugs from a hot engine or you run the risk of stripping the threads of the spark plug holes in the cylinder heads. If the engine has been running for any length of time, let it cool down completely before removing them.
OK, these are the test steps:
- 1
Disconnect the ignition coil from its electrical connector. This will disable the ignition system and will prevent spark from being fired during the test.
- 2
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. - 3
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.
- 4
Have your helper crank the engine till the needle on the compression gauge stops climbing.
Now, 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 this test step on the remaining cylinders.
Let's examine your test results:
CASE 1: One or two cylinders had a much lower compression value than the others. This might be normal or it might be a problem.
The next step is to find out if these low compression values are causing a problem. Go to: Interpreting The Compression Test Results.
CASE 2: Three or more cylinders had almost no compression or 0 PSI compression. This test result will cause your 3.9L V6 engine to 'crank but not start'.
You can find out what is causing these compression readings by doing a wet compression test. For this test go to: TEST 2: Wet Engine Compression Test.
CASE 3: The compression value of all six cylinders was similar and above 120 PSI. This is the correct test result. You can conclude that a compression problem is not behind your misfire condition or engine 'no-start' problem.
Here's why: If the engine had a compression problem causing a misfire or a no-start condition, then your test results would have indicated one or more cylinders with low or very close to 0 PSI compression. Since your test results indicate that all 6 cylinders have very similar compression values, you can rule out engine compression as a source of the misfire or engine no-start problem you're trying to diagnose.
