Interpreting Your Compression Test Results
If you're reading this section, then your engine compression test results indicate one or more cylinders with a low compression value (compared to the others).
Up to a certain point, a low compression value will not cause an engine performance problem nor a drivability problem.
But if the compression value is too low, then you'll definitely notice a rough idle condition or a misfire problem.
The cool thing is that 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 the highest compression value you got.
You can do this (figuring out the 15%) in 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 I got the following compression readings:
| Cylinder | Pressure |
|---|---|
| #1 | 165 PSI |
| #2 | 95 PSI |
| #3 | 155 PSI |
| #4 | 175 PSI |
| #5 | 175 PSI |
| #6 | 170 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 cylinder compression test result is usually caused by one of two things.
Either that affected cylinder has severely worn or damaged cylinder head valves.
Or the affected cylinder's piston rings are severely worn or damaged.
Thankfully we can find out by doing a wet compression test.
This test simply involves adding about one or two tablespoons of engine oil to the cylinder with low or zero PSI compression and then retesting it's compression.
If the compression value increases, then you now know for certain that the affected cylinder's piston rings are severely worn or damaged.
If the compression value does not increase, then you now know for certain 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 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 3.3L V6 Buick (Oldsmobile).
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 3.3L V6 Buick, Oldsmobile Tutorials
You can find a complete list of 3.3L V6 Buick and Oldsmobile tutorials in this index:
Here's a small sample of the tutorials you'll find in the index:
- What Does The MAF Sensor Do? (3.3L V6 Buick, Oldsmobile) (3.3L V6 Buick, Oldsmobile).
- How To Test A Blown Head Gasket (3.3L V6 Buick, Oldsmobile).
- How To Test The Throttle Position Sensor (3.3L V6 Buick, Oldsmobile).
- How To Test The MAF Sensor (3.3L V6 Buick, Oldsmobile).
If this info saved the day, buy me a beer!
