How To Test The Engine Compression (1989-2001 3.9L V6 Dodge Ram Pickup)

Engine compression problems can cause a no-start problem or a rough idle/misfire problem.

The cool thing is that testing and interpreting an engine compression test is not that hard.

In this tutorial, I'll explain the test and how to interpret your specific test results so that you can find out if an engine compression problem is behind the engine performance problem you're trying to diagnose.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (1991-2003 3.9L Dodge Ram Pickup y Van) (at: autotecnico-online.com).

APPLIES TO: This tutorial applies to the following Dodge Ram pickups:

• 3.9L V6 Dodge Ram 150 Pickup: 1989, 1990, 1991, 1992, 1993.
• 3.9L V6 Dodge Ram 250 Pickup: 1989, 1990, 1991, 1992, 1993.
• 3.9L V6 Dodge Ram 1500 Pickup: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001.
• 3.9L V6 Dodge Ram 2500 Pickup: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001.

NOTE: You can find all of the B-Series Ram van tutorials here:

• 3.9L V6 Dodge Ram Van Index Of Articles.

Symptoms Of Engine Compression Problems

It's been my experience that engine compression problems usually cause one of two things.

Either the engine is not going to start or the engine will start and run but run with a misfire problem.

When an engine compression problem causes a misfire condition, you're going to see one or more of the following trouble codes (if your 3.9L V6 Dodge Ram pickup is OBD II equipped):

• P0300: Random Cylinder Misfire.
• P0301: Cylinder #1 Misfire.
• P0302: Cylinder #2 Misfire.
• P0303: Cylinder #3 Misfire.
• P0304: Cylinder #4 Misfire.
• P0305: Cylinder #5 Misfire.
• P0306: Cylinder #6 Misfire.

You're also going to see one or more of the following symptoms:

• Rough idle condition that does not set a misfire trouble code.
• Misfire felt only when accelerating the vehicle.
• Misfire felt when the engine is idling but goes away when engine is accelerated.
• Bad gas mileage.
• Heavy exhaust smell coming out of the tailpipe when the engine is running.

Important Tips And Suggestions

TIP 1: If the engine has been running for any length of time, let it cool down completely before you remove the spark plugs.

Attempting to remove the spark plugs from a hot engine can damage the threads in the spark plug holes. This is a nightmare you want to avoid.

TIP 2: To disconnect the spark plug wires from the spark plugs, use a spark plug wire puller.

Using a spark plug wire puller will help you avoid damaging the spark plug wires when disconnecting them from the spark plugs.

To see an example of this tool, check out this article: How To Use A Spark Plug Wire Puller And Where To Buy One (at: easyautodiagnostics.com).

Which Compression Tester Should I Buy?

There are lot of engine compression testers to choose from and many places to buy them. I'm gonna' make some recommendations to you:

TEST 1: Dry Compression Test

To find out if an engine compression problem is behind a no-start or misfire problem, it's important to test all 6 cylinders.

If you don't have an engine compression tester, you can run down to your local auto parts store and buy or rent it there. If you'd like to save a few bucks on its purchase, check out my recommendations here: Which Compression Tester Should I Buy?

Let's get testing:

1. 1

   Remove the spark plugs.
   
   When removing the spark plugs, be careful not to drop any of them on the floor, or you run the risk of having the spark plugs porcelain insulator crack and then you'll have a misfire on your hands.

2. 2

   Thread the engine compression gauge into the spark plug hole for the number 1 engine cylinder.
   
   Hand tighten the compression gauge only! Do not use any type of tool to get it tight.

3. 3

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

4. 4

   Record the value at which the needle stopped and the number of the engine cylinder on a piece of paper.

5. 5

   Repeat steps 2-4 on the remaining cylinders.

Let's take a look at what your compression test results mean:

CASE 1: You got 0 PSI in the majority of the cylinders. This test result confirms that your 3.9L V6 engine has serious internal problems. This is usually due to:

• Busted timing chain.
• Engine threw a rod.

Any compression value below 100 PSI (even if it's not 0 PSI) means internal mechanical engine trouble.

CASE 2: All cylinders have compression but their values are not the same. It's normal for each cylinder's compression value to vary slightly from one another. But if the compression values vary too much, they'll cause a rough idle or misfire problem.

To find out, the next step is to go to: Interpreting Your Compression Test Results.

Interpreting Your Compression Test Results

It's not unusual for the compression values to vary, especially if your Dodge pickup'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 its 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.

More 3.9L V6 Dodge Ram Pickup Tutorials

I've written quite a few 3.9L V6 Dodge Ram pickup 'how to test' tutorials. You can find them in this index:

• 3.9L V6 Dodge Ram Pickup Index Of Articles.

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

• How To Test For A Blown Head Gasket (1989-2001 3.9L V6 Dodge Ram Pickup).
• How To Test The Fuel Pump (1992-2001 3.9L V6 Dodge Ram Pickup).
• How To Test The TPS (1992-1996 3.9L V6 Dodge Ram Pickup).
• How To Test The MAP Sensor (1992-1996 3.9L V6 Dodge Ram Pickup).