Interpreting Your Compression Test Results
If you're reading this section, your compression test results indicate that the compression values vary between each other.
A variation in the compression values does not affect engine performance up to a certain point.
But if the difference exceeds a specific range, you'll have engine performance problems on your hands.
The rule of thumb is that the compression values cannot vary more than 15%.
You can do this (figuring out the 15%) in one of two ways: Calculate this 15% difference with pen and paper or use my low compression calculator. You can find the low compression calculator here: Online Low Engine Compression Calculator.
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:
| 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
A low or 0 PSI compression value is due to one of two issues:
- The piston rings of the affected cylinder are worn out or damaged.
- The cylinder head valves of the affected cylinder are worn or damaged.
Thankfully, you don't need to tear the engine apart to inspect any of these components since a wet engine compression test will help you determine which components are bad.
A wet compression test involves adding about 1 to 2 tablespoons of engine oil to the affected cylinder.
Then the engine compression of that specific cylinder is retested. If the problem lies with the piston rings, the compression value increase.
If the issue lies with the cylinder head valves, the compression value will not increase.
Let's get started:
- 1
Add a small amount of engine oil to the cylinder that reported low compression or no compression in the ‘dry’ compression test.
You don't have to add a lot of oil. 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 tighten 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 increase in the compression value confirms that the piston compression rings are worn out and causing the low compression value you got for this cylinder in TEST 1.
CASE 2: The compression value stayed the same. This confirms that the low compression problem of the affected cylinder is due to worn or damaged cylinder head valves.
More 2.2L Beretta, Cavalier, Corsica, And Sunfire Diagnostic Tutorials
If this tutorial was helpful, be sure and take a look at all of the 2.2L Beretta, Cavalier, Corsica, And Sunfire diagnostic tutorials in this index:
Here's a sample of the tutorials you'll find there:
- How To Test The Thermostat (1992-1997 2.2L Beretta, Cavalier, Corsica, Sunfire).
- How To Test The Fuel Pump (1992-1997 2.2L Beretta, Cavalier, Corsica, Sunfire).
- How To Test The TPS (1992-1997 2.2L Beretta, Cavalier, Corsica, Sunfire).
- How To Test The GM 2.2L Ignition Coil Pack.
If this info saved the day, buy me a beer!
