Interpreting Your Compression Test Results
It's not unusual to see cylinder compression values that differ in high-mileage engines.
The difference in the compression values will not cause any engine performance problems within a specific range.
But if the difference exceeds a specific value, you'll definitely see the engine misfiring (running rough).
The cool thing is we can easily find out if the low compression values are causing an engine performance problem or not.
To find out, we need to figure out if the low compression values are lower than 15% of your highest compression value.
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 (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 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
One of two things usually cause a low or 0 PSI compression value:
- The affected cylinder's piston rings are worn out or damaged.
- The affected cylinder's intake/exhaust valves are worn out or damaged.
Thankfully, you don't need to tear the engine apart to find out where the cause of the low compression value lies.
We can find out by doing a simple 'wet' compression test which involves adding one to two tablespoons of engine oil into the affected cylinder.
Once the oil is added to the cylinder, you'll test its compression again.
Depending on the compression test result, you'll know that the problem is due to an issue with the piston rings or the cylinder head valves.
Specifically, if the compression value increases, you now know that the affected cylinder's piston rings are severely worn or damaged.
If the compression value does not increase, you now know that the problem is due to worn or damaged intake or exhaust valves (of the affected cylinder).
These are the test steps:
- 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 Buick Century And Oldsmobile Cutlass Ciera Test Tutorials
If this tutorial was helpful, be sure and take a look at all of the 2.2L Buick Century and Oldsmobile Cutlass Ciera diagnostic tutorials in this index:
Here's a sample of the tutorials you'll find there:
- How To Test The Fuel Pump (1993-1996 2.2L Buick Century, Oldsmobile Cutlass Ciera).
- How To Test The TPS (1993-1996 2.2L Buick Century, Oldsmobile Cutlass Ciera).
- How To Test The EGR Valve (1993-1995 2.2L Buick Century, Oldsmobile Cutlass Ciera).
- How To Test The MAP Sensor (1993-1996 2.2L Buick Century, Oldsmobile Cutlass Ciera).
If this info saved the day, buy me a beer!
