Interpreting Your Compression Test Results
It's not unusual to obtain compression values, from the compression test, that vary between each other. This could be normal, or it could be causing an engine performance problem.
The rule of thumb is that if the low compression value or values vary by more than 15% of the highest compression value you obtained from your tests, then you're gonna' have an engine performance problem.
Why? Because if the low compression value varies by more than 15%, then this cylinder is going to misfire and can be considered ‘dead’.
You can do this 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 my 3.0L V6 Dodge Grand Caravan produced the following compression readings:
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
The two most common causes of a low compression test result for one or two cylinders (out of all 6) on your 3.0L equipped Chrysler car or mini-van is either worn cylinder head valves or worn piston rings.
The best way to find out, without tearing down the engine is by doing a wet compression test.
All that the wet compression test involves is adding about two tablespoons of oil to the engine cylinder that showed a low compression reading in the ‘dry’ compression test and testing its compression once again.
If the piston rings are worn, the engine oil that you just added will cause the compression value to shoot up to whatever the average compression value is for the rest of the cylinders.
If the cylinder head valves are worn (for that particular cylinder), the compression value will stay the same.
Here are the steps:
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.
Install the compression tester onto the cylinder. Do not use any type of tool to tightened the compression tester. Hand tight is fine.
Crank the engine after you've set up the test.
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 analyze your 'wet' compression test results:
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 3.0L equipped Chrysler car or mini-van.
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
More 3.0L Chrysler Tutorials
You can find a complete list of tutorials in this index: Chrysler 3.0L Index Of Articles.
- How To Replace The Distributor (1988-2000 3.0L Chrysler).
- How To Test The MAP Sensor (1991-1995 3.0L Chrysler).
- How To Test The Throttle Position Sensor (1991-1995 3.0L Chrysler).
- How To Test For A Broken Timing Belt (1998-2000 3.0L SOHC Chrysler).
- Ignition Distributor System Tests 3.0L Chrysler, Dodge, Plymouth (at: easyautodiagnostics.com).
If this info saved the day, buy me a beer!