How To Test Engine Compression (3.4L V6 Chevrolet, Pontiac)

This tutorial will help you test the engine compression on the 3.4L V6 Chevrolet and Pontiac vehicles.

You'll also be able to interpret your compression test results and find out if you've got a compression problem causing a misfire problem or an engine no-start problem.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (1996-2005 3.4L V6 Chevrolet, Pontiac) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following vehicles:

• 3.4L V6 Chevrolet Impala: 2000, 2001, 2002, 2003, 2004, 2005.
• 3.4L V6 Chevrolet Monte Carlo: 2000, 2001, 2002, 2003, 2004, 2005.
• 3.4L V6 Chevrolet Lumina Minivan: 1996.
• 3.4L V6 Chevrolet Venture: 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005.

• 3.4L V6 Pontiac Aztek: 2001, 2002, 2003, 2004, 2005.
• 3.4L V6 Pontiac Grand Am: 1999, 2000, 2001, 2002, 2003, 2004, 2005.
• 3.4L V6 Pontiac Montana: 1999, 2000, 2001, 2002, 2003, 2004, 2005.
• 3.4L V6 Pontiac Trans Sport: 1996, 1997, 1998.

Symptoms Of Low Or No Engine Cylinder Compression

An engine compression problem will usually manifest itself in one of two ways:

• The engine compression problem will cause the engine not to start.
• -or-
• The engine will start and run, but it will run with a misfire.

If the engine in your vehicle starts and runs, but it's suffering an engine compression problem, you're gonna see one or more of the following symptoms:

• Bad gas mileage.
• A heavier exhaust smell coming out of its tailpipe.
• The engine is not as peppy as it was once.
• Rough idle that goes away as soon as you accelerate the engine.

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

As you're probably already aware, you'll need to remove the spark plugs to test the engine's compression.

Before you remove them, be sure to label the spark plug wires with the cylinder they belong to. This way, you'll avoid losing their firing order (when it comes time to put everything back together).

If you don't have a compression tester, you can run down to your local auto parts store (AutoZone, O'Reilly Auto Parts) and borrow one from them (for a small deposit which they'll return once you return the tool).

If you'd like to buy your own and save a few bucks in the process, check out my recommendations here: Which Compression Tester Should I Buy?

IMPORTANT: If the engine has been running for any length of time, let it cool down completely before removing the spark plugs. Removing the spark plugs from a hot engine can damage the spark plug hole threads!

OK, let's get testing:

1. 1

   Disable the ignition system by disconnecting the ignition coil pack/ignition control module assembly from its electrical connector.
   
   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 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 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.

6. 6

   Repeat test steps 3 - 5 on the remaining cylinders.

Let's examine your test results:

CASE 1: Several 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 Chevrolet (Pontiac) vehicle's 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

If you've arrived at this section, then the results of TEST 1 indicate that some of the cylinders have a low compression value.

If the low value is within a specific range, you won't notice a difference in how the engine runs.

But if the value is more than it should be, the engine will suffer an engine performance problem.

This engine performance problem usually manifests itself as a misfire problem.

Finding out if the low compression values you got from your test are causing a problem is not difficult. 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). Therefore, if any value is lower by more than 15%, that engine cylinder will misfire. This misfire will cause your Chevrolet (Pontiac) vehicle's 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 manually calculating this 15% difference.

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

What usually causes a cylinder to have a low compression value is one of two things:

• The piston rings of the affected cylinder are severely worn or damaged.
• The intake/exhaust valves of the affected cylinder are severely worn or damaged.

The way to find out, without disassembling the engine, is to do a wet compression test.

This test involves adding a small amount of engine oil to the cylinder with the low compression value and then retesting its compression.

If the compression value DOES NOT increase, you can conclude that the affected cylinder's problem is due to worn or damaged intake or exhaust valves.

If the compression value increases, you can conclude that the problem lies in the cylinder's piston rings.

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.