Interpreting Your Compression Test Results
If you got a low compression reading (in TEST 1), the next step is to find out if this compression value is causing a problem.
What you have to do is find out if this low compression value varies by more than 15% of the highest compression value you obtained from your tests.
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 3.3L 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
If you got a cylinder with low or no compression (either would be considered ‘dead’), the next step is to do a wet compression test.
This test will help pin-point the problem to either bad cylinder head valves or bad piston compression rings.
Now, to explain what's involved in a wet compression test, you're gonna' add about two tablespoons of oil to the confirmed ‘dead’ engine cylinder or cylinders.
Depending on the compression test result of the cylinder you added oil to, you'll be able to determine if the low compression reading you recorded in the dry compression test is caused by worn piston rings or worn cylinder head valves.
OK, this is what you need to do:
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.
When all is set up, have your helper crank the engine.
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 result means:
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 Chrysler mini-van.
The reason why is that the oil you just added helped the piston rings seal better, thus causing the compression reading to rise.
CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.
The reason for this is that nothing, not even the engine oil, can help the cylinder head valves seal the compression in the cylinder. So, if after adding oil and retesting the compression the compression value doesn't increase, then you now know that the problem is in the cylinder head valves.
More Diagnostic Test Tutorials
You can find all of the 3.3L V-6 Chrysler/Dodge/Plymouth equipped mini-van diagnostic tutorials here:
Here's a sample of the tutorials you'll find in the index:
- How To Test The Alternator (2001-2004 3.3L Chrysler).
- How To Test The MAP Sensor (2001-2004 3.3L V6 Chrysler And Dodge Mini-Van).
- How To Test Engine Compression (2001-2007 3.3L Chrysler).
- How To Test The Coil Pack 3.3L, 3.8L Chrysler, Dodge, Plymouth (1990-1998) (at: easyautodiagnostics.com).
- How To Test The Coil Pack 3.3L Chrysler, Dodge, Plymouth (1999-2001) (at: easyautodiagnostics.com).
- How To Test The Coil Pack 3.3L, 3.8L Chrysler, Dodge, Plymouth (2001-2008) (at: easyautodiagnostics.com).
If this info saved the day, buy me a beer!