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

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 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).