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
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
Install the compression gauge on the cylinder you just added oil to.
- 3
Have your helper crank the engine till the needle stops climbing on the compression gauge.
- 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
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.
If this info saved the day, buy me a beer!
