How To Test The Engine Compression (2006-2010 3.9L V6 Engine)

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.

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 some recommendations to you:

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. 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. 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. 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. 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.

Interpreting The Compression Test Results

There's a good chance that the compression values of all six cylinders (from TEST 1) were not similar.

Up to a certain point this is normal since all 6 cylinders do not wear out at the exact same rate. Also, quite a few different problems inherent to the 3.9L V6 engine design will cause certain cylinders to wear out at a faster rate than others.

In this section, we're going to crunch the numbers, using the engine compression values that you got from TEST 1, to find out if the cylinders with the lower engine compression values are causing a problem or not.

The rule of thumb is that the lowest compression value can not vary more than 15% of the highest value (that you wrote down in TEST 1). If any value is lower by more than 15%, then that engine cylinder is going to misfire. This misfire will cause your Chevy Malibu's 3.9L V6 engine to idle rough.

How do you figure this out? You can find out by using my online low compression calculator here: Online Low Engine Compression Calculator or by calculating this 15% difference manually.

To understand how to figure out this 15% thing manually, we need to do some math and to explain it, I'll use the following compression test results:

• Cylinder #1 175 PSI.
• Cylinder #2 165 PSI.
• Cylinder #3 160 PSI.
• Cylinder #4 120 PSI.
• Cylinder #5 160 PSI.
• Cylinder #6 170 PSI.

The next step is to do the following math:

• Multiply 0.15 (15%) by the highest value: 175 x 0.15. The result is 26.25. I'll round it out to 26.
• Next, I subtract 26 from 175: 175 - 26 = 144.
• So now I know that the lowest possible compression value is: 144 PSI.

This means that cylinder #4, which has a compression value of 120 PSI, is the one causing the misfire because it's below the 144 PSI minimum.

Once we've found the 'dead' cylinder, the next step is to find out what's causing the low compression value. For this step, go to: TEST 2: Wet Engine Compression Test.

TEST 2: Wet Engine Compression Test

If in TEST 1 you got some cylinders that have low engine compression (especially if their compression value varies greater than 15% of the highest value) then your next step is to do a wet engine compression test on those specific cylinders.

The purpose of a wet compression test is to find out what is causing the low compression value in those specific cylinders.

There are two culprits that usually cause low engine compression. One is severely worn out piston rings and the other are severely worn or damaged cylinder head valves.

So a wet engine compression test done on the cylinders with low engine compression will let us know if their cylinder head valves or engine piston rings are severely worn or damaged, all without having to remove and disassemble the engine.

To do a wet compression test all we have to do is add about 1 or 2 tablespoons of engine oil to the affected cylinder and then retest its compression.

These are the test steps:

1. 1

   Add 1 or 2 tablespoon of engine oil in the cylinder you need to retest. I suggest using a small and long funnel so that the oil will reach the inside of the cylinder.

2. 2

   Install the compression gauge on the cylinder you just added oil to.

3. 3

   Have your helper crank the engine till the needle stops climbing on the compression gauge.

4. 4

   You'll see one of two results:
   
   1.) The needle will climb higher than the previous compression number you recorded for this specific cylinder, or...
   
   2.) The needle will not move at all or stay at the same number you recorded earlier.
   
   What ever value your compression tester reads, write it down again.

5. 5

   Repeat steps 1 thru' 4 on any other cylinder you need to check.

Let's examine your test results:

CASE 1: The compression value of the cylinder you added oil to shot up. This test result tells you that the low or near 0 PSI compression value of this specific cylinder is due to worn out piston rings.

CASE 2: The compression value of the cylinder you added oil to DID NOT increase. This test result confirms that the cylinder head valves of this specific cylinder are worn-out or damaged.

More 3.9L V6 Test Tutorials

You can find a complete list of 3.9L V6 diagnostic test tutorials here: GM 3.9L Index Of Articles.

Here's a list of articles you'll find there:

• How To Test A Blown Head Gasket (2006-2010 3.9L V6 Engine).
• Ignition System Wiring Diagram (2006-2009 3.9L Chevrolet Impala).
• Ignition System Wiring Diagram (2006-2007 3.9L Chevrolet Malibu).
• How Often Should I Replace The Spark Plugs (2006-2010 3.9L V6 Engine).