Interpreting Your Compression Test Results
Low compression readings, in one or more engine cylinders, are to be expected (especially in high mileage vehicles).
Under a certain threshold, low compression values do not cause any adverse effects on engine performance.
It's when the compression value or values differ more than 15 percent of the highest compression value that they'll cause a problem.
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 vehicle produced the following compression readings:
| Cylinder | Pressure |
|---|---|
| #1 | 165 PSI |
| #2 | 95 PSI |
| #3 | 155 PSI |
| #4 | 175 PSI |
| #5 | 160 PSI |
| #6 | 165 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’ Compression Test.
TEST 2: ‘Wet’ Compression Test
If your compression test confirmed that you've got one or several cylinders with low compression, the next step is to do a ‘wet’ compression test.
The ‘wet’ compression test will help you find out if the low cylinder compression is due to worn out cylinder head valves or worn out piston rings.
This is a very simple test that involves adding about 2 tablespoons of oil to the cylinder (that recorded no or low compression in the previous test) and then checking its compression once again.
What will happen is that if the low compression values is due to worn piston rings, the compression value will go up from the previous one you got doing the Dry compression value.
If the compression value does not go up (from the previous one), then you'll know that the problem lies in the cylinder head valves.
OK, to get this pot of water boiling, 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 analyze your test results:
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.
The reason why the compression value shot up, with the engine oil added, is because the oil is aiding the piston rings to create a near perfect seal.
Since the oil is not letting the compression escape by the rings, this results in your compression gauge reading a higher value than with the ‘dry’ compression test.
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
The reason why the compression reading did not shoot up, is because the engine oil can not help the intake or exhaust valve seal the compressed air. Therefore, the compression value will stay the same.
Related Test Articles
If the engine compression test, you just got done doing, indicates that all engine cylinders have healthy compression, and your 3.1L or 3.4L equipped GM (Chevrolet, Buick, Olds, Pontiac) car or mini-van is still misfiring, the following articles might help:
- GM 3.1L & 3.4L Ignition System Tests This article will help you to troubleshoot the coil packs, spark plug wires, ICM, etc. (at: easyautodiagnostics.com).
- How To Do A Cylinder Balance Test (GM 3.1L, 3.4L).
- How To Troubleshoot Misfire Codes (GM 3.1L, 3.4L)
- How To Do A Fuel Injector Resistance Test (GM 3.1L, 3.4L)
If this info saved the day, buy me a beer!
