How To Test Engine Compression (1992-1993 3.3L V6 Pontiac Grand Am)

Interpreting The Compression Test Results

It isn't unusual to see a variation in the compression values between cylinders after a compression test. Especially if the engine has quite a bit of wear and tear.

If the difference is small enough, this variation in the compression values will not cause any engine performance or driveability problems.

But if the difference is too great, you're definitely going to have engine performance problems.

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). If any value is lower by more than 15%, then that engine cylinder is going to misfire. This misfire will cause your Pontiac Grand Am's 3.3L 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

A low or 0 PSI compression value is usually caused by one of two things:

1. Either the affected cylinder's piston rings are severely worn or damaged.
2. Or the affected cylinder's intake/exhaust valves are severely worn or damaged.

Thankfully we can find out without having to disassemble the engine. How? By simply by doing a wet compression test.

To perform a wet compression test, we're going to add 1 to 2 tablespoons of engine oil to the cylinder that produced the low or 0 PSI compression value.

We'll then test that cylinder's compression. If the compression value shoots up, then you can conclude that the low or 0 PSI compression value is due to worn-out piston rings.

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

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 3.3L Pontiac Grand Am Tutorials

You can find a complete list of 3.3L Pontiac Grand Am tutorials in this index:

• GM 3.3L Index Of Articles.

Here's a small sample of the tutorials you'll find in the index:

• What Does The MAF Sensor Do? (3.3L V6 Pontiac Grand Am).
• How To Test A Blown Head Gasket (3.3L V6 Pontiac Grand Am).
• How To Test The Throttle Position Sensor (3.3L V6 Pontiac Grand Am).
• How To Test The MAF Sensor (3.3L V6 Pontiac Grand Am).