Calculating The Compression Values To See If They Are Within Range
It's not unusual to see compression values that are not similar when doing an engine compression test (particularly in high-mileage engines).
This is usually not a cause for concern since engine cylinders do not wear out at the exact same rate as time and miles accumulate on your Chevrolet TrailBlazer or GMC Envoy's engine.
When a low engine compression value does cause a problem, it's when its compression value varies more than 15% from the highest engine compression value.
You can do this (figuring out the 15%) in one of two ways: You can calculate this 15% difference with pen and paper or use my low compression calculator.
If you want to use the calculator, you can find it 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 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 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 Compression Test.
TEST 2: Wet Compression Test
A cylinder will produce a low engine compression value if it has a problem with its piston rings or its cylinder head valves.
The cool thing is we don't have to remove and disassemble the engine to find out where the problem lies.
How? You might be wondering. Well, we can do this by adding a small amount of engine oil into the cylinder with low compression and retesting its compression.
If the low compression problem is due to worn-out piston rings, the oil we add to the cylinder will cause the compression reading to shoot up.
If the compression value does not increase, after adding oil to the low compression cylinder, then we can conclude the problem is due to worn-out or damaged cylinder head valves.
These are the test steps:
- 1
Add a tablespoon (or two) of engine oil in the cylinder you need to retest.
Use a funnel to make sure the oil reaches the inside of the cylinder. - 2
Install the compression gauge on the cylinder and hand tighten it.
- 3
Have your helper crank the engine till the needle stops climbing on the compression gauge.
- 4
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. - 5
Write the compression value down.
- 6
If you have another cylinder to test, repeat steps 1 thru' 4 on it now.
Let's take a look at what test results mean:
CASE 1: The compression value increased. This tells you the low compression problem is due to worn piston compression rings.
CASE 2: The compression value DID NOT increase (in other words, it stayed the same). This result tells you the low compression value registered in this cylinder (in the dry test) is due to worn/damaged cylinder head valves.
More GM 4.2L Test Tutorials
If this tutorial was helpful/informative, you can find a complete list of tutorials here:
Here's a sample of the tutorials you'll find there:
- How To Test For A Blown Head Gasket (2002-2009 4.2L Chevrolet TrailBlazer).
- How Often Should I Replace The Spark Plugs? (2002-2009 4.2L Chevrolet TrailBlazer).
- How To Test APP Sensor 1 And 2 (2002-2003 4.2L Chevrolet TrailBlazer).
- Electronic Throttle Body Tests (2002-2007 4.2L Chevrolet TrailBlazer, GMC Envoy).
If this info saved the day, buy me a beer!
