How To Interpret The Engine Compression Test Results
On high mileage engines, you're not gonna' see the exact same compression value on each of the cylinders. Up to a certain point this is normal and you're not gonna' see any engine performance problems as a result of this variation in the compression values.
What is not normal is to have one or several a compression values that are radically different than the average compression of the good cylinders.
To be a bit more specific, the compression values between the cylinders should not vary more than 15%. If you're scratching your head and wondering how to do this, don't worry, this is how you can find out:
- STEP 1: Multiply the highest compression value by 0.15 (this is the decimal value of 15%).
- STEP 2: Round off the result (for example: 25.6 would become 26).
- STEP 3: Subtract the result (the number that was rounded off) from the highest compression value.
- ANSWER: The result of this subtraction is the lowest possible compression value any cylinder can have.
NOTE: You can also use my online low compression calculator if you want to avoid doing the math. You can find it here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).
To make better sense of this calculation, I'll give you a specific example on how to do this 15% calculation. Let's say that my 2.2L Chevy Cavalier gave me following compression values when I did a compression test on all 8 cylinders:
| Cylinder | Pressure |
|---|---|
| #1 | 175 |
| #2 | 170 |
| #3 | 170 |
| #4 | 140 |
The next step is to do the math:
- 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 #4 gave me a compression value of 140 PSI, I now know that this is the one causing the misfire I'm trying to diagnose.
The next step, after finding out that a cylinder's low engine compression value is not within range, is to do a ‘wet’ compression test on it.
TEST 2: ‘Wet’ Engine Compression Test
If in the previous test (the ‘Dry’ compression test), you found a cylinder with low or no compression, then you should do a ‘Wet’ compression test.
All that's involved is the addition of about two tablespoons of oil to the engine cylinder that showed a low or no compression.
The engine oil that you're gonna' add to the cylinder will help determine if the low cylinder pressure or pressures you recorded in the ‘Dry’ compression test are caused by worn piston rings or worn cylinder head valves.
Depending on whether the compression pressure rises (on your compression tester) or not, you'll be able to say that the problem lies in the piston's rings or in the 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 GM 2.2L.
In case you're wondering why, it's because the oil you added has helped the piston rings seal better and this results in the compression value/pressure going up.
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
In case you're wondering why the compression pressure/value didn't shoot up, this is due to the fact that oil can not help the valve seal better. So, when the ‘Wet’ compression test shows the same test result (no compression), this is a dead give away that the cylinder valves are bent or severely damaged.
Why An Engine Compression Test?
Now, usually all four engine cylinders wear out evenly. But every now and then, either thru' lack of maintenance or other repair issues that happen along the way of several thousands of miles and/or years, one or several engine cylinders wear out at an accelerated pace. When this happens, no matter what gets replaced, like: spark plugs, ignition coils, fuel injectors, spark plug cables, etc, nothing solves the misfire codes (P0300, P0301, P0302, P303, P0304) or the misfire condition.
As mentioned earlier, testing the compression of all four cylinders is one of the most overlooked troubleshooting tests. Now, testing the compression of each cylinder is usually done after verifying that each cylinder is getting spark. At the end of this article you'll find some more test articles (GM 2.2L specific test articles) that'll help you to further test a misfire condition.
Related Test Articles
Now, if the compression readings on your Chevy (Cavalier, S10 Pick Up, or GMC Sonoma, or Pontiac Sunfire) indicated that everything is OK, and yet the car or pick up is still misfiring, you can find more test articles here: 2.2L GM Index Of Articles.
- How To Test The Manifold Absolute Pressure (MAP) Sensor (GM 2.2L)
- GM 2.2L Fuel Injector Noid Light Test.
- How To Diagnose Misfire Codes (GM 2.2L)
- How To Test The GM 2.2L Ignition Coil Pack (this info is found at easyautodiagnostics.com).
If this info saved the day, buy me a beer!
