Interpreting The Compression Test Results
If in TEST 1 you got one or more cylinders that had a low compression value then the next step is to find out if this is causing an engine performance problem.
Up to a certain point it is normal to have the compression value of each cylinder vary by a small amount since they don't always wear out at the exact same time.
It's when a low compression value varies by more than 15% of the highest value your compression test results gave you that you'll have an engine performance 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
Whenever a compression test result varies by more than 15% of the highest compression value then it's a good idea to retest that specific cylinder after adding a drop or two of engine oil.
This will let us know if the low compression problem is due to worn piston rings or worn/damaged cylinder head valves (of the affected cylinder).
You don't have to add a lot of oil to the low compression cylinder since all you'll need is about 2 tablespoons of engine oil.
If the compression value shoots up, after adding the oil and retesting the cylinder's compression, then you can conclude that the low compression value you got for that specific cylinder during TEST 1 is due to worn piston rings.
If the value does not shoot up and remains the same, then you can conclude that the problem is due to worn/damaged cylinder head valves (of the affected cylinder).
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.
If this info saved the day, buy me a beer!
