Interpreting Your Compression Test Results
If you're reading this section, more than likely your engine compression test results indicate you've got one or more cylinders with a low compression value.
Up to a certain range, a low compression value will not cause a problem. It's when the value is too low that you'll have a rough idle or a misfire condition on your hands.
To find out if the low compression value is causing a problem, you need to find out if it's lower than 15% of the highest compression value you got.
You can do this (figuring out the 15%) in one of two ways: You can calculate this 15% difference with pen and paper or you can use my low compression calculator. You can find the low compression calculator here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).
If you want to manually calculate the 15% difference, here's what you'll need to do:
- STEP 1: Multiply the highest compression value by 0.15 (this is the decimal value of 15%).
- STEP 2: Round the result to the nearest one (for example: 25.6 would become 26).
- STEP 3: Subtract the result (the number that was rounded) from the highest compression value.
- ANSWER: The result of this subtraction is the lowest possible compression value any cylinder can have.
Now, let me give you a more specific example: Let's say that I got the following compression readings:
My next step is to do the following calculation:
- STEP 1: 175 x 0.15 = 26.25.
- STEP 2: 26.25 = 26 (rounded to nearest one).
- STEP 3: 175 - 26 = 149.
- ANSWER: 149 PSI. Any cylinder with this compression (or lower) value will misfire.
Since cylinder #2 is only producing 95 PSI, I can now conclude that it's 'dead' and causing a misfire.
To find out if the lowest compression value you got from your engine compression test is within a good range, you'll need to do the same calculation. Of course, you'll need to use the highest compression value you got and not the one in the example.
Once you'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 Compression Test
If your ‘Dry’ compression test results indicate one or more cylinders with low or 0 compression, then the next step is to find out if that low compression value is due to worn/damaged cylinder head valves or worn/damaged piston rings.
To find out all we need to do is add a small amount of motor oil to the low compression cylinder and checking its compression once again. This is what is known as a ‘Wet’ compression test.
If the problem is bad piston rings, the motor oil that we're gonna' add will help bring the compression value up when we check that cylinder's compression again.
If the problem is bad cylinder head valves, the motor oil WILL NOT help raise the compression value of the cylinder. It's as simple as that!
OK, this is what you'll need to do:
Add a tablespoon (or two) 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.
Once you've added the oil, install the compression gauge, and as before just hand tighten it.
Have your helper crank the engine till the needle stops climbing on the compression gauge.
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.
Repeat steps 1 thru' 4 on any other cylinder that needs to be tested.
Let's take a look at what test results mean:
CASE 1: The compression value went up after adding motor oil and retesting. This tells you that the low compression problem is due to worn piston compression rings.
Here's why: The motor oil you just added helped the piston rings to create a tighter seal. With the piston rings now sealing the compression inside the cylinder, the compression value on your compression tester went up. This type of test result only happens when the problem is due to worn piston rings.
CASE 2: The compression value DID NOT go up after adding oil and retesting (in other words, it stayed the same). This result tells you that the low compression value registered in this cylinder (in the dry test) is due to worn/damaged cylinder head valves.
If this info saved the day, buy me a beer!