Interpreting Your Compression Test Results
Within a certain range, a difference in compression between cylinders does not affect engine performance.
Your 2.8L V6 Chevy S10 pickup or GMC S15 pickup or GMC Sonoma's engine performance will only be affected if the compression values vary too much.
The cool thing is that we can find out whether or not the differences in the values you noted in TEST 1 indicate a problem (with that cylinder).
The rule of thumb is that the compression values can not vary more than 15% from each other and if they do, you're gonna' have a genuine misfire condition on your hands or possibly a no-start condition (if more than one cylinder is affected).
The rule of thumb is that the compression readings can't differ by more than 15%, and if they do, you'll have a cylinder misfire problem or possibly an engine no-start issue (if more than one cylinder is affected).
You can calculate the 15% difference in one of two ways:
- Do the math with pen and paper.
- Use my low compression calculator.
You can find my 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 |
| #3 | 165 PSI |
| #4 | 160 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 reading you got from your engine compression test is in a good range, you need to do the same calculation. Of course, you must use the highest compression value you got, not the one in the example.
Once you've found the 'dead' cylinder, the next step is to figure out what's causing the low compression reading. For this step, go to: TEST 2: Wet Engine Compression Test.
TEST 2: Wet Engine Compression Test
If in TEST 1 you found that you do have one or more cylinders with low (or no) compression, then the next step is to do a 'wet' compression test.
The 'wet' compression test will let us pinpoint the problem to worn cylinder head valves or worn piston rings (as the cause of the low or no compression test result).
At the bottom, I'll show you how to interpret your test results.
OK, let's get testing:
- 1
Add a tablespoon (or two) of engine oil in the cylinder you need to retest.
I suggest using a small and long funnel so that the oil will reach the inside of the cylinder. - 2
Once you've added the oil, install the compression gauge and as before, just hand tighten it.
- 3
Have your helper crank the engine.
- 4
Stop cranking the engine once the needle stops climbing on the compression gauge.
- 5
You'll see one of two results:
1.) The needle will climb higher than the previous compression number you recorded for this specific cylinder.
2.) The needle will not move at all or stay at the same number you recorded earlier. - 6
Write the compression value down on a piece of paper.
- 7
Repeat steps 1 through 6 on any other cylinder you need to test.
Let's take a look at what your compression test results mean:
CASE 1: The compression value shot up. This compression test result confirms that the low compression reading recorded on this cylinder in the dry test is due to worn piston rings.
The reason the compression reading went up is because the engine oil you just added helped the piston rings create a tighter seal. This type of test result only occurs if the problem is due to worn piston rings.
CASE 2: The compression value DID NOT shoot up (stayed the same). This result tells you that the low compression value registered in this cylinder (in the dry test) is due to worn/damaged cylinder head valves.
More 2.8L Chevy S10 Pickup, GMC S15 Pickup, And GMC Sonoma Tutorials
You can find a complete list of tutorials for the 2.8L V6 Chevrolet S10 pickup, GMC S15 pickup, and GMC Sonoma in this index:
Here's a sample of the articles, you'll find in the Index of Articles:
- How To Test The Fuel Pump (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
- How To Diagnose An Engine No-Start Problem (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
- How To Test The EGR Valve (1988-1993 2.8L Chevrolet S10 Pickup, GMC S15 Pickup, GMC Sonoma).
- How To Test The Starter Motor (1988-1993 2.8L V6 Chevrolet S10 Pickup, GMC S15 Pickup).
If this info saved the day, buy me a beer!
