Interpreting Your Compression Test Results
Low engine compression in one or more cylinders can have an impact on engine performance and cause a cylinder misfire problem (especially at idle RPMs).
And sometimes, these low compression values don't cause any adverse problem at all. How can you find out? -By calculating the difference between the low and highest compression value and seeing if it varies by more than 15%.
If it varies by more than 15%, then you've got a cylinder that will cause a misfire.
Alright, finding out if the low compression value is causing a problem can be done 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.
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 a 3.8L Buick LeSabre produced the following compression readings:
| Cylinder | Pressure |
|---|---|
| #1 | 165 PSI |
| #2 | 95 PSI |
| #3 | 155 PSI |
| #4 | 175 PSI |
| #5 | 165 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
Once you've found the cylinder or cylinders with low or zero compression, the next step is to find out what is causing the problem.
Either the cylinder's low compression value will be caused by its piston rings or by its cylinder head valves.
Thankfully, we can find out by doing a 'wet' compression test, instead of having to tear the engine apart (at this point).
You may be asking yourself, 'what's a wet compression test?' -Well, it's a compression test but with one minor difference. You're gonna' add a small amount of engine oil to the cylinders with low or zero compression and then test their compression ability again.
Depending on the result of the wet compression test, you can conclude that the compression problem is due to bad piston rings or bad cylinder head valves.
OK, let's get this show on the road:
- 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 while you observe the gauge.
- 4
Once the needle on the gauge stops climbing, have your helper stop cranking the engine.
- 5
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. - 6
Repeat steps 1-4 on any other cylinder with a low compression test result.
Let's examine your test results:
CASE 1: The compression value shot up. This tells you that that cylinder's piston compression rings are worn out and are behind the low compression test result you got in TEST 1.
CASE 2: The compression value stayed the same. This confirms that that cylinder's valves are behind the low to 0 PSI compression result you obtained in TEST 1.
More 3.8L GM V6 Tutorials
You can find a complete list of 3.8L GM V6 tutorials in this index:
Here's a small sample of the tutorials you'll find in the index:
- How To Test The MAF Sensor (GM 3.8L V6 1996-2005).
- How To Test A Does Not Crank Condition -Case Study (GM 3.8L).
- GM 3.8L Ignition Control Module And Crank (3X, 18X) Sensor Test.
- How To Test The Ignition Coil Pack -Misfire Troubleshooting Tests (GM 3.8L).
If this info saved the day, buy me a beer!
