Interpreting The Engine Compression Test Results
There's a good chance that your compression test results show that some of the cylinders have a low compression value compared to the rest.
Up to a certain point this could be normal and that these compression values are not causing a problem.
It's when these low compression values vary by more than 15% of the highest compression value (the engine is producing), that you're going to have engine performance problems.
Why? Because if the low compression value varies by more than 15%, then this cylinder is going to misfire and can be considered ‘dead’.
You can do this 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 my 3.3L V6 Dodge Grand Caravan produced 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’ Engine Compression Test.
TEST 2: Wet Compression Test
After you've found the ‘dead’ cylinder (the cylinder with very low or no compression), the next step is to find out if the compression problem is due to bad cylinder head valves or bad piston rings. To find out you need to do a ‘wet’ compression test.
You might be asking yourself, “What's a wet compression test?”, this is a variation of the same compression test you did in the previous page.
The difference being that you're gonna' add about two tablespoons of oil to the engine cylinder that showed a low compression reading in the ‘dry’ compression test.
If the low compression test result you got (in a specific cylinder) is the result of bad piston rings, the oil will help in sealing the rings and the compression value will shoot up/catch up to the rest of the other values.
If the problem is in the cylinder head valves, then no amount of oil will help them seal and the compression value will stay the same.
These are the test steps:
Add a tablespoon (or two) of engine oil in the cylinder you need to retest.
Use a funnel to make sure that the oil reaches the inside of the cylinder.
Install the compression gauge on the cylinder and 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.
If you have another cylinder to test, repeat steps 1 thru' 4 on it now.
Let's take a look at what test results mean:
CASE 1: The compression value increased. This tells you that the low compression problem is due to worn piston compression rings.
CASE 2: The compression value DID NOT increase (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!