How To Test The Engine Compression (1989-2003 3.9L V6 Dodge Ram Van)

Interpreting Your Compression Test Results

It's totally normal for compression numbers to be a bit different, especially if the 3.9L V6 engine in your Dodge Ram van has racked up some serious miles.

Most of the time, the differences are small and don't cause any trouble. But if one or more cylinders drop more than 15% below the highest reading, you're gonna be dealing with a misfire or rough idle issue.

Figuring out if a cylinder's "dead" because of low compression is super simple —you just need to do a little math.

You've got two options for this 15% check: grab a pen and paper and do the math yourself, or save time by using my low compression calculator. You can find it here:

• Online Low Engine Compression Calculator (at: easyautodiagnostics.com).

If you want to do it by hand, here's how to break it down:

• STEP 1: Take the highest reading and multiply it by 0.15 (that's 15%).
• STEP 2: Round that number to the nearest whole number. (Like, 25.6 becomes 26.)
• STEP 3: Subtract that rounded number from your highest compression reading.
• ANSWER: That final number is the lowest pressure any cylinder should have. If one falls below it, that cylinder has a compression problem.

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 only showed 95 PSI, that tells me it's "dead" and the reason for the misfire.

To see if your lowest reading's still in the safe zone, you'll need to run that same 15% calculation. Just make sure you're using the highest number from your own test, not the one from this example.

Once you've nailed down which cylinder's got low compression, the next move is figuring out why. To do that, head over to: TEST 2: Wet Engine Compression Test.

TEST 2: Wet Engine Compression Test

If you saw one or more cylinders with lower compression in TEST 1, the next thing to do is run a wet compression test.

This test helps us figure out whether the low reading's coming from worn piston rings or bad cylinder head valves in that cylinder.

All you're gonna do is pour a bit of engine oil into the low-compression (or zero-compression) cylinder and run the test again.

This is what you're looking for:

• The compression increases: The low or zero compression problem is caused by worn piston rings.
• The reading doesn't change: The issue is caused by the cylinder's intake or exhaust valves.

Alright, here's how you do it:

1. 1

   Pour in a tablespoon or two of engine oil into the cylinder you're testing again.
   
   Use a funnel so the oil actually gets into the cylinder and not all over the place.

2. 2

   Screw the compression tester into place and tighten it by hand.

3. 3

   Have someone crank the engine until the gauge needle stops rising.

4. 4

   You're gonna see one of two things:
   
   1.) The needle climbs higher than before — meaning compression improved.
   
   2.) The needle barely moves or stays the same as your first test.
   
   Whatever number you get, write it down again.

5. 5

   If there's another low cylinder, repeat steps 1 through 4 on that one too.

Here's what the results mean:

CASE 1: Compression went up. That means the piston rings are worn, and that's what caused the low number in TEST 1.

CASE 2: Compression didn't change. That points to worn or damaged cylinder head valves being the issue.

More 3.9L V6 Dodge Ram Van Tutorials

You can find a complete list of 'how to' tutorials and wiring diagrams for your 3.9L V6 Dodge Ram van in this index:

• 3.9L V6 Dodge Ram Van Index Of Tutorials.

Here's a sample of the tutorials you'll find in the index:

• How To Test For A Blown Head Gasket (1989-2003 3.9L V6 Dodge Ram Van).
• How To Test The Fuel Pump (1992-2003 3.9L V6 Dodge Ram Van).
• How To Test The TPS (1997-2003 3.9L V6 Dodge Ram Van).
• Fuel Pump Circuit Wiring Diagram (2000-2003 3.9L V6 Dodge Ram Van).