Interpreting The Compression Test Results
If you're reading this section then your compression test results show that one or more cylinders have a lower compression value than the others.
Up to a certain point this might be normal. But if the value varies too much (more than 15%), it'll cause a misfire or rough idle problem.
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.
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 low compression value is usually caused by one of two things: either the cylinder's piston rings are worn out or its cylinder head valves are worn or damaged.
The cool thing is that we can find out without having to tear the engine apart by simply doing a wet compression test on the cylinder with the low compression value.
Now, if the compression value shoots up, after adding oil to the low compression cylinder, then you can conclude that the compression problem is due to worn out piston compression rings.
If the compression value does not increase, after adding oil to the low compression cylinder, then you can conclude that the problem is due to worn or damaged cylinder head valves.
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 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 2.4L Chevy And Pontiac Tutorials
You can find a complete list of 2.4L Chevy and Pontiac tutorials here: GM 2.4L Index Of Articles.
Here's a list of articles, you'll find there:
- How To Test The MAP Sensor (2008-2012 2.4L Chevrolet Malibu).
- How To Test For A Blown Head Gasket (2.4L Cobalt, HHR, Malibu, G5, G6, Solstice).
- How To Do A Cylinder Balance Test (2.4L Cobalt, HHR, Malibu, G5, G6, Solstice).
- When Should I Replace The Spark Plugs (2.4L Cobalt, HHR, Malibu, G5, G6, Solstice).
If this info saved the day, buy me a beer!
