How To Test The Engine Compression (3.9L Dodge Ram Pickup And Van)

Interpreting Your Compression Test Results

It's not unusual for the compression values to vary, especially if your Dodge pickup or van's 3.9L V6 engine has a lot of miles.

Generally the variation in the compression values is small and they don't cause any problems. But when these values varies by more than 15% of the highest cylinder compression value, then you'll have a misfire or a rough idle problem on your hands.

Figuring out if the cylinder is 'dead' due to low compression is not difficult at all and it simply involves doing a little math.

You can do this (figuring out the 15%) 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 (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 I got the following compression readings:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 PSI
#5	160 PSI
#6	175 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

If in TEST 1, you got one or more cylinders with a lower compression value than the rest of them, then the next step is to do a wet compression test.

A wet compression test will let us know if the low compression value is due to worn cylinder head valves or worn piston rings (in the affected cylinder).

This test simply involves adding a small amount of engine oil to the cylinder with low compression and then retesting it's compression.

If the compression value shoots up, then you can conclude that the problem is due to worn piston rings.

If the compression value does not increase, then you can conclude that the problem is due to worn or damaged cylinder head valves.

Let's get started:

1. 1

   Add a tablespoon (or two) of engine oil in the cylinder you need to retest.
   
   Use a funnel to make sure that the oil reaches the inside of the cylinder.

2. 2

   Install the compression gauge on the cylinder and hand tighten it.

3. 3

   Have your helper crank the engine till the needle stops climbing on the compression gauge.

4. 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.
   
   What ever value your compression tester reads, write it down again.

5. 5

   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 test result confirms that the piston compression rings are worn and behind the low compression test result you got in TEST 1.

CASE 2: The compression value DID NOT increase (in other words, it 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.