How To Test Engine Compression (3.8L V6 Chrysler, Dodge, Plymouth Min-Van)

Interpreting Your Compression Test Results

It's not unusual to find that the compression of the six cylinders varies a bit between them, especially in engines that have a lot of mileage on them.

In most cases, the variation in the compression values between them does not affect engine performance.

But if a compression value of a cylinder varies by more than 15% of the highest compression value, then that engine cylinder will misfire.

Thankfully, finding out if the lowest compression value you got from TEST 1 is causing a problem is not hard.

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.

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 3.8L V6 Dodge Grand Caravan produced the following compression readings:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 PSI
#5	160 PSI
#6	165 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 a cylinder or cylinders with low engine compression, the next step is to do a wet compression test on them.

A wet compression test simply involves adding about 2 tablespoons of engine oil to the affected cylinder and then retesting its compression.

If the compression value increases, then you can conclude that the low compression value produced by the cylinder in TEST 1 is due to worn piston rings.

If the compression value does not increase, then you can conclude that the low compression value is due to worn or damaged cylinder head valves (specifically the cylinder head valves of the affected cylinder).

NOTE: If you're having to do a wet compression test on cylinder 1, 3, or 5, you'll have to be creative to add the oil. I suggest attaching a vacuum hose to your funnel and then inserting the vacuum hose into the spark plug hole. Then let the oil drip into the spark plug hole for approximately 2 to 3 minutes, to make sure the oil actually enters the spark plug hole.

OK, let's start:

1. 1

   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.

2. 2

   Install the compression tester onto the cylinder.
   
   Do not use any type of tool to tightened the compression tester. Hand tight is fine.

3. 3

   When all is set up, have your helper crank the engine while you observe the gauge.

4. 4

   Once the needle on the gauge stops climbing, have your helper stop cranking the engine.

5. 5

   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.

6. 6

   Repeat steps 1-4 on any other cylinder with a low compression test result.

Let's examine your test results:

CASE 1: The compression value shot up. This tells you that that cylinder's piston compression rings are worn out and are behind the low compression test result you got in TEST 1.

CASE 2: The compression value stayed the same. This confirms that that cylinder's valves are behind the low to 0 PSI compression result you obtained in TEST 1.