How To Test The Engine Compression (3.1L V6 Chevrolet And Pontiac Vehicles)

Interpreting The Compression Test Results

Depending on the age, mileage, maintenance history or repair history of your 3.1L Chevrolet Lumina (Monte Carlo), the engine compression values of each cylinder may vary a bit.

Up to a certain point, this variation in the compression that each cylinder produces does not cause a problem.

Now, if the variations in the compression values are too great, then the engine will definitely misfire or suffer a rough idle condition.

Thankfully, we can easily figure this out by doing a simple math calculation and in this section I'll explain how to do just that.

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 vehicle's 3.1L 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, 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 .15 (15%) by the highest value: 175 x 0.15. This gives us 26.25, but we'll round it out to 26.
• Next, we subtract 26 from 175: 175 - 26 = 144.
• So now we 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

Finding out what's causing a cylinder to produce a low compression value is as simple as adding engine oil to it and retesting it compression. This test procedure is known as a 'wet' compression test.

To further explain this, it's important to know that two different kinds of problems will cause a cylinder to produce 0 psi or a low compression value. These problems are:

• The cylinder's piston has worn out compression rings.
• The cylinder's intake or exhaust valves are worn or damaged.

We can easily find out without having to tear the engine apart by doing a wet compression test on the affected cylinder or cylinders.

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 increased. This test result tells you that the low or near 0 PSI compression value is due to worn out piston rings of that specific cylinder.

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 that cylinder are worn-out or damaged.

More 3.1L V6 Chevrolet, Pontiac Diagnostic Tutorials

You can find a complete list of 3.1L V6 test tutorials here: GM 3.1L V6 Index Of Articles.

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

• How To Test The Fuel Pump (1995-1999 3.1L V6 Chevrolet Lumina, Monte Carlo).
• How To Test The Fuel Injectors (1995-1999 3.1L Chevrolet Lumina, Monte Carlo).
• How To Test The 24X Crankshaft Position Sensor (1995-1999 3.1L V6 Chevrolet Lumina, Monte Carlo).
• Fuel Pump Wiring Diagram (1998-1999 3.1L V6 Chevrolet Lumina, Monte Carlo).