How To Test Engine Compression (1994-2010 3.8L V6 Chrysler, Dodge, And Plymouth Minivan)

Engine compression problems can cause the engine to not start or cause the engine to run with a misfire (rough idle).

In this tutorial, I'll explain the engine compression test and more importantly how to interpret your test results.

So if engine compression problems are causing a no-start problem or a misfire problem, this tutorial will help you find out.

APPLIES TO: This tutorial applies to the following vehicles:

• 3.8L V6 Chrysler Town & Country: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
• 3.8L V6 Dodge Grand Caravan: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010.
• 3.8L V6 Plymouth Voyager: 1994, 1995, 1996.
• 3.8L V6 Plymouth Grand Voyager: 1994, 1995, 1996, 1997, 1999.

ENGINE NO-START DIAGNOSTICS:

• How To Diagnose A No-Start Problem (1994-2010 3.8L V6 Chrysler, Dodge, And Plymouth Minivan).

Symptoms Of Low Or No Engine Compression

Over the years that I've been working I'm cars, I've noticed that engine compression problems usually cause one of two issues:

• The engine is not going to start.
• The engine will start and run but it'll run with a misfire (or a rough idle).

The most common issue is engine compression problems that cause the engine to run with a rough idle or a misfire problem.

On the 1996 and newer 3.8L V6 equipped minivans, the misfire problem will cause the check engine light to illuminate. And a misfire trouble code will be stored in the fuel injection computer's memory:

• 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.

Generally, when an engine compression problem causes a rough idle, you're only going to feel the problem when you come to a stop light. But as soon as you accelerate the engine, the engine feels fine.

You're also gonna' see:

• Bad gas mileage. This is caused by the simple fact that the engine is now running on less than all 6 cylinders.
• Engine pollutes more. This is also caused by the simple fact that the engine is now running on less than its full complement of cylinders. The live cylinders have to compensate for the 'dead' ones.

If your 3.8L V-6 Chrysler equipped mini-van is NOT OBD II equipped, you won't have any codes lighting up the check engine light, but you'll definitely feel a rough idle condition.

OK, having covered the most common scenarios of low compression and no compression, let's get testing to see if this is the case on your 3.8L V-6 Chrysler equipped mini-van.

Spark Plug Access For Cylinders 1, 3, And 5

Testing the engine compression on the 1994-2000 3.8L V6 engine can be a challenge, this is due to the fact that the spark plugs for the 3 cylinders that face the firewall are hard to access (these are cylinders 1, 3, and 5).

Yup, the spark plugs for cylinders 1, 3, and 5 (which face the firewall) are not easy to remove due to the lack of space behind the engine.

I've done this job several times and the one thing that has helped me to remove them (the 3 spark plugs), is to remove the alternator and the alternator bracket first.

Once the alternator and the alternator bracket are removed, you'll able to access them and remove them.

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 Engine Compression Test

If you don't have an engine compression tester, you can run down to your local auto parts store and you can rent one or buy one there.

If you'd like to save a few bucks on its purchase, check out my recommendations here: Which Compression Tester Should I Buy?

IMPORTANT: You'll be working around a cranking engine, so you have to be careful and stay alert at all times. Think safety all of the time!

These are the test steps:

1. 1

   Disconnect the coil pack from its harness connector. This will prevent the ignition coil pack from firing off spark to the spark plug wires.
   
   NOTE: Disconnecting the ignition coil pack's electrical harness connector is important, since it'll prevent damage to the coil pack!

2. 2

   Remove the alternator and the alternator bracket to gain access to the cylinders facing the firewall. This applies to the 1994-2000 3.8L mini-vans.

3. 3

   Remove all 6 spark plugs.
   
   As you're taking them out, be careful and don't drop any of them on the floor, or you could cause the spark plug's ceramic insulator to break, and this will cause a misfire!

4. 4

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

5. 5

   When everything is set up, have your helper to crank the engine. Your job is to keep your eye on the compression tester's gauge.

6. 6

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

7. 7

   Write down the compression value on a piece of paper. Include the number of the cylinder this reading belongs to.

8. 8

   Repeat steps 4-7 on the other 5 cylinders.

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

CASE 1: No compression in 2 or more cylinders. This test result tells you that the engine has serious internal problems.

The most common issues would be: Broken timing chain. Or a blown head gasket. Or the engine threw a rod. To test for a blown head gasket, see this tutorial: How To Test A Blown Head Gasket (3.8L V6 Chrysler, Dodge, Plymouth Min-Van).

CASE 2: Low compression in one or more cylinders. To a certain point, it's normal for the compression to vary a little between cylinders (as the engine accumulates thousands of miles).

But if these values vary too much, then you're gonna' have a bonafide misfire on your hands.

The next step is to do some math to find out if this low compression value is within a normal parameter or not. Go to: Interpreting Your Compression Test Results.

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.