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

Engine compression problems can cause a cylinder misfire problem or in a worst case scenario, an engine no-start problem.

With the help of this tutorial, you'll be able to find out if a compression problem is causing a misfire problem or a no-start problem on the 1995-1996 3.1L V6 Chevrolet Lumina (Monte Carlo).

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (Vehículos Chevrolet y Pontiac Con Motor 3.1L V6) (at: autotecnico-online.com).

This tutorial applies to the following vehicles:

• 3.1L V6 Chevrolet Beretta: 1994, 1995, 1996.
• 3.1L V6 Chevrolet Corsica: 1994, 1995, 1996.
• 3.1L V6 Chevrolet Lumina: 1995, 1996, 1997, 1998, 1999, 2000, 2001.
• 3.1L V6 Chevrolet Malibu: 1997, 1998, 1999, 2000, 2001, 2002, 2003.
• 3.1L V6 Chevrolet Monte Carlo: 1995, 1996, 1997, 1998, 1999.
• 3.1L V6 Pontiac Grand Prix: 1994, 1995, 1996, 1997, 1998.
• 3.1L V6 Pontiac Grand Prix: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003.

ENGINE NO-START DIAGNOSTICS:

• How To Diagnose A No-Start Problem (1995-1999 3.1L V6 Chevrolet Lumina, Monte Carlo).

Symptoms Of Low Or No Engine Cylinder Compression

It's been my experience, over the years working as an automotive technician, that engine compression problems usually fall into one of two categories:

• The engine compression problem causes the engine to not start.
• The engine compression problem causes the engine to suffer a misfire problem.

When an engine compression problem is causing a misfire condition (and your vehicle is OBD2 equipped), you're going to see the check engine light illuminated by one of the following 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 3.1L V6 engine is going to suffer from:

• Bad gas mileage.
• A heavier exhaust smell coming out of its tailpipe.
• Engine is not 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.

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

Testing the engine compression on your 3.1L Chevrolet Lumina (Monte Carlo) requires removing all of a spark plugs.

Before removing them, it's critical that you label each spark plug wire before unplugging them from their spark plug.

Labeling them will help you to connect them to the correct spark plug/cylinder when you have completed the compression test.

Also, it goes without saying that you're gonna' need a compression tester. If you don't have one, check out my recommendations and where to buy them here: Which Compression Tester Should I Buy?

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

   Disable the ignition system by disconnecting the ignition coil pack from its electrical connectors. This will prevent the ignition coil from sparking 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.

5. 5

   Record on paper the value at which the needle stopped and the number of the engine cylinder.
   
   Release the pressure on the gauge and repeat this step one more time.

6. 6

   Repeat test steps 3 - 5 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. Up to a certain range, this could be normal.

To further interpret these test results go to: Interpreting The Compression Test Results.

CASE 2: Three or more cylinders had almost no compression or 0 PSI compression. Having three or more cylinders with almost no compression or 0 PSI compression will cause your GM vehicle's 3.1L engine to not start.

Having a compression value close to 0 PSI or 0 PSI is usually due to a problem in the cylinder head valves or piston rings of the affected cylinders.

We can find out which of the two it is 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 test result lets you know that an engine compression problem is not behind the misfire condition and/or engine no-start problem you're troubleshooting.

I'll explain 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

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.
   
   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).