How To Test Engine Compression (2004-2006 2.8L Chevrolet Colorado, GMC Canyon)

Interpreting The Compression Test Results

Depending on the engine's mileage, the cylinder compression values may differ from one another.

To a certain point, a low compression value will not cause engine drivability or an engine performance problem.

But if the variation between the compression values is beyond a specific range, you'll have a cylinder misfire problem.

The rule of thumb is that if a low compression value that's lower by 15% or more than the highest one, then that low compression cylinder will experience a misfire.

How do you figure this out? You can find out by using my online low compression calculator here:

• Online Low Engine Compression Calculator.

You can also calculate 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.

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 Compression Test.

TEST 2: Wet Engine Compression Test

A wet compression test will help determine if the low or 0 PSI compression value you got in TEST 1 is caused by the affected cylinder's intake/exhaust valves or worn piston rings.

The cool thing is that you don't need to tear the engine apart to find out what's causing the problem.

You can easily determine the cause of the issue 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.

5. 5

   Write the compression value down.

6. 6

   Repeat steps 1 thru' 5 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 confirms that the cylinder's low compression value is due to worn compression rings on its piston.

CASE 2: The compression value of the cylinder you added oil to DID NOT increase. This tells you that the low compression value you registered in TEST 1 is due to the cylinder having worn or damaged cylinder head valves.

More 2.8L Chevrolet Colorado (GMC Canyon) Tutorials

You can find a complete list of 2.8L Chevrolet Colorado (GMC Canyon) tutorials and wiring diagrams here:

• GM 2.8L Vortec 4-Cylinder Index of Articles.

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

• How To Test The Electronic Throttle Body (2004-2006 2.8L Chevrolet Colorado, GMC Canyon).
• APP Sensor Tests (2004-2006 2.8L Chevrolet Colorado, GMC Canyon).
• Blown Head Gasket Tests (2004-2006 2.8L Chevrolet Colorado, GMC Canyon).
• TAC Throttle Body Wiring Diagram (2004-2006 2.8L Chevrolet Colorado, GMC Canyon).