Interpreting The Compression Test Results
Up to a certain point, it's normal for the compression of each of the six cylinders of your vehicle's 3.5L V6 engine to vary a bit.
But if the variation is too big, then you're going to have a bonafide misfire or rough idle condition on your hands.
The cool thing is that 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 3.5L 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 Compression Test.
TEST 2: Wet Engine Compression Test
If you're reading this, then the test results you got from TEST 1 indicate that you have one or more engine cylinders with low or 0 PSI compression.
Having a low or 0 PSI engine compression test result usually means that the affected cylinder has worn-out or damaged valves or piston rings.
We can easily find out without having to tear the engine apart by doing a wet compression test on the affected cylinder or cylinders.
To do a wet compression test all we have to do is add about 1 or 2 tablespoons of engine oil to the affected cylinder and then retest its compression.
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, or...
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.
More 3.5L V6 Test Tutorials
You can find a complete list of 3.5L V6 test tutorials here: GM 3.5L V6 Index Of Articles.
Here's a list of articles, you'll find there:
- How To Do A Cylinder Balance Test (2004-2007 3.5L Malibu).
- How To Test A Blown Head Gasket (2004-2010 3.5L V6 GM Engine).
- How To Test The MAF Sensor (2004-2008 3.5L Malibu).
- Fuel Pump Wiring Diagram (2004-2005 3.5L Malibu).
If this info saved the day, buy me a beer!
