How To Test Engine Compression (1987-2000 3.0L V6 Chrysler, Dodge, Plymouth)

Testing the compression of all 6 cylinders on the 3.0L equipped Chrysler, Dodge, or Plymouth cars/mini-vans is becoming a standard test when diagnosing a hard to find misfire (rough idle) or a no-start condition, especially if the vehicle has a lot of miles on it.

In this tutorial, I'll walk you through the entire process in a step-by-step fashion and help you interpret your compression test results to see if one or several cylinders have low (or no) compression.

CYLINDER MISFIRE DIAGNOSTICS:

• How To Troubleshoot A Cylinder Misfire (1990-2000 3.0L V6 Dodge And Plymouth Minivan).

ENGINE NO-START DIAGNOSTICS:

• How To Test An Engine No-Start (1990-2000 3.0L V6 Dodge And Plymouth Minivan).

Symptoms Of Low Or No Engine Compression

Engine compression problems usually fall into two specific categories:

1. One or more (but not all) cylinders have low or no compression.
2. No compression in all 6 cylinders.

Here are some more specifics about the above two conditions.

One or more (but not all) cylinders have low or no compression: This is probably the most common problem affecting vehicles with a lot of miles or that have had a life of severe wear and tear and is usually due to worn cylinder head valves or worn piston rings.

The most obvious symptom of this type of problem is a rough idle that usually stops as you accelerate the engine.

Here are some more specific symptoms you'll see:

1. Misfire codes (if the vehicle is OBD II equipped) lighting up the check engine light:
   • 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.
2. Engine misses at idle.
3. Lack of power.
4. Blue smoke coming out of the tailpipe at idle and/or when accelerating (this is engine oil burning in the cylinders).

No compression in all 6 cylinders and your 3.0L equipped Chrysler car or mini-van won't start. It'll crank but not start. This usually indicates that your vehicle's timing belt is broken, or one or both of the head gaskets have failed, or that the engine is blown.

When you have a situation where you have no compression on ALL 6 cylinders, you'll see:

1. The engine cranks very fast and this fast cranking speed is very noticeable.
2. The ignition system is not creating spark (only occurs if timing belt is broken).
3. Fuel pump is working and providing pressure.

Let's get testing to see what's happening to your 3.0L equipped Chrysler, Dodge, or Plymouth car/mini-van.

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: The 'Dry' Engine Compression Test

To do an engine compression test, you'll need an engine compression tester.

You don't have to buy one since you can borrow one from your local auto parts store (like AutoZone or O'Reilly Auto Parts) for a cash deposit, which you'll get back once you return the tester.

If you need help deciding where to buy one or which one to buy, take a look at my recommendations: 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!

This is what you'll need to do:

1. 1

   Disconnect the ASD relay. This will prevent fuel from being injected into the cylinders as you crank the engine.

2. 2

   Disconnect the ignition coil from its electrical connector. This is just to make sure that the ignition coil does not fire off spark during the compression testing of the cylinders.

3. 3

   Remove all 6 spark plugs. As your 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!
   
   IMPORTANT: You should label each spark plug wire with its location before you remove the spark plugs so that you can connect them (the spark plug wires) back to the correct spark plug once you're done with the compression test.

4. 4

   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.

5. 5

   Have a helper crank the engine, when the test is set up, as you observe the needle on the compression tester's gauge.

6. 6

   Once the needle on the gauge stops climbing, have him or her stop cranking the engine.

7. 7

   Write down the compression test result and the cylinder the test result belongs to.

8. 8

   Repeat the test in the other 5 cylinders.

OK, let's interpret your compression test results.

CASE 1: Low or no compression in 2 side by side cylinders (of the same bank). This indicates a problem that's usually caused by:

• Blown head gasket.

CASE 2: No compression in ALL cylinders. This isn't good and indicates that your 3.0L equipped Chrysler car or mini-van has serious engine mechanical problems.

The most common issues would be:

• Broken timing belt. You can find the timing belt test here: How To Test For A Broken Timing Belt (1998-2000 3.0L SOHC Chrysler).
• Engine threw a rod.

CASE 3: Low or no compression in one or two cylinders (that are NOT side by side on the same bank). Up to a certain point, the low compression values could be normal.

To find out if they are normal or not, we now need to a simple math calculation to find out if this test result is within normal specs or not. Go to: Interpreting Your Compression Test Results.

Interpreting Your Compression Test Results

It's not unusual to obtain compression values, from the compression test, that vary between each other. This could be normal, or it could be causing an engine performance problem.

The rule of thumb is that if the low compression value or values vary by more than 15% of the highest compression value you obtained from your tests, then you're gonna' have an engine performance problem.

Why? Because if the low compression value varies by more than 15%, then this cylinder is going to misfire and can be considered 'dead'.

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 (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 my 3.0L 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

The two most common causes of a low compression test result for one or two cylinders (out of all 6) on your 3.0L equipped Chrysler car or mini-van is either worn cylinder head valves or worn piston rings.

The best way to find out, without tearing down the engine is by doing a wet compression test.

All that the wet compression test involves is adding about two tablespoons of oil to the engine cylinder that showed a low compression reading in the 'dry' compression test and testing its compression once again.

If the piston rings are worn, the engine oil that you just added will cause the compression value to shoot up to whatever the average compression value is for the rest of the cylinders.

If the cylinder head valves are worn (for that particular cylinder), the compression value will stay the same.

Here are the steps:

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

   Crank the engine after you've set up the test.

4. 4

   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.

Let's analyze your 'wet' compression test results:

CASE 1: The compression value shot up. This tells you that the piston compression rings are worn out and thus the problem is in the bottom end (block) of the engine in your 3.0L equipped Chrysler car or mini-van.

CASE 2: The compression value stayed the same. This confirms that the problem is in the cylinder head valves.

More 3.0L V6 Dodge And Plymouth Minivan Tutorials

You can find a complete list of tutorials in this index:

• 3.0L V6 Dodge And Plymouth Minivan Index Of Articles.

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

• How To Replace The Distributor (1988-2000 3.0L Chrysler).
• How To Test The MAP Sensor (1991-1995 3.0L Chrysler).
• How To Test The Throttle Position Sensor (1991-1995 3.0L Chrysler).
• How To Test For A Broken Timing Belt (1998-2000 3.0L SOHC Chrysler).
• Ignition Distributor System Tests 3.0L Chrysler, Dodge, Plymouth (at: easyautodiagnostics.com).