Interpreting The Results Of The Engine Compression Test
It's not unusual to obtain compression readings that vary from each other. But if they vary too much, you'll have a rough idle or a misfire condition on your hands. So the next step is to find out if the low compression value or values are causing a problem.
You can do this by figuring out (mathematically) if the low compression value varies by more than 15% of the highest compression value you obtained from your tests.
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 engine compression test produced the following compression readings:
| Cylinder | Pressure |
|---|---|
| #1 | 165 PSI |
| #2 | 95 PSI |
| #3 | 155 PSI |
| #4 | 175 PSI |
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 Engine Compression Test
The above compression test is known as a ‘dry’ compression test. The next step (after finding cylinders with low compression) is to do a ‘wet’ compression test.
This involves adding a few drops (2 tablespoons) of engine oil to the cylinders with the low engine compression result and repeating the compression test.
The results you obtain from this second ‘wet’ compression test will help you determine if the low compression you recorded in the ‘dry’ compression test are caused by worn piston rings or worn cylinder head valves.
OK, this is what you need to do:
- 1
Add a small amount of engine oil to the cylinder that reported low compression or no compression in the ‘dry’ compression test
The amount should be about 1 to 2 tablespoons of oil. - 2
Install the compression tester onto the cylinder.
Do not use any type of tool to tightened the compression tester. Hand tight is fine. - 3
When all is set up, have your helper crank the engine.
- 4
You'll get one of two results:
1.) The compression value will go up (from the one you recorded before).
2.) The compression value will stay the same.
Let's take a look at what your test results mean:
CASE 1: The compression value shot up. This tells you that the piston compression rings are worn out and thus the problem is in the bottom end (block) of the engine in your 2.0L or 2.4L Chrysler vehicle.
Here's why: The engine oil helped the piston rings seal better, thus bringing up the compression value almost back to normal.
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
Here's why: If the cylinder head valves and their seats are worn out (or maybe even bent from a broken timing belt), no amount of engine oil is gonna help seal the compression in, in the cylinder. So, if the compression value, for the specific cylinder you're testing did not go up (after you added oil to it) then this is a dead giveaway that you've got cylinder head valve damage.
