How To Test Engine Compression (1993-2002 2.5L Dodge Dakota)

Testing the engine compression is probably one of the most overlooked diagnostic tests when trying to find the source of a hard to find rough idle condition or a misfire trouble code.

In this tutorial, I'm going to explain how to do the compression test and more importantly, how to interpret its results and find out if you have a dead cylinder on your hands.

This tutorial applies to both the 1993-1995 SOHC and 1996-2002 OHV 4 cylinder 2.5L Dodge Dakota engines.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (1993-2002 2.5L Dodge Dakota) (at: autotecnico-online.com).

Symptoms Of Low Or No Cylinder Compression

It's been my experience, over the years that I have been working on cars, that the majority of engine compression problems can be divided into two distinct categories.

In the first category, you have the type of compression problems that provoke a rough idle condition or a misfire condition.

In the other category, compression problems will make the engine crank but not start.

Here are some of the symptoms you may see when your 2.5L Dodge Dakota has engine compression problems:

• Engine cranks but does not start (0 compression in all cylinders).
• Blue smoke coming out of the tailpipe.
• Rough idle (engine misfires).
• Bad gas mileage.
• Engine 'misses' at idle but 'miss' disappears as you accelerate.
• Check engine light is illuminated with a MAP sensor trouble code (even tho' the MAP sensor is good).

With this info under our belts, let's head down to the next subheading and get testing.

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

The engine compression test is not that hard to do, but there are some precautions you need to take before you start.

The most important precaution being that you need to remove the spark plugs with a cold engine. So if the engine in your 2.5L Dodge Dakota has been running for any amount of time, you need to let it cool down completely.

Removing the spark plugs from a hot engine can damage to the spark plug threads in the spark plug holes.

The other precaution I recommend you take, is to label all of the spark plug wires so that you won't lose their firing order.

OK, these are the test steps:

1. 1

   Disable the ignition system by disconnecting the distributor and ignition coil from their electrical connectors. This will prevent the ignition coil from sparking during the test.

2. 2

   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.
   
   If the engine does not start, don't worry about it being warmed up.

3. 3

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

4. 4

   Have your helper crank the engine till the needle on the compression gauge stops climbing.
   
   Now, record on paper the value at which the needle stopped and the number of the engine cylinder on a piece of paper. Release the pressure on the gauge and repeat this step one more time.
   
   Repeat this test step on the remaining 3 cylinders.

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

CASE 1: You got 0 compression on all 4 cylinders. This test result is usually due to one of the following conditions:

• Timing chain (or timing belt on SOHC engines) problem.
• Blown head gasket.
• Blown engine.

Any compression value below 100 PSI (even if it doesn't go as low as 0 PSI) means internal mechanical engine trouble.

CASE 2: You got uneven compression values between engine cylinders. To a certain extent, having an uneven compression values might be normal.

The next step now is to find out if your compression test results are within normal operating parameters. For this info go to: Interpreting The Compression Test Results.

Interpreting The Compression Test Results

Now that you've done the compression test, the next important thing is to interpret the test results. If you got 4 different compression values, let me tell you that this could be normal.

The engine cylinders, particularly in high-mileage engines, don't wear out exactly the same. But if the compression values vary too much, then you've got a problem that'll cause a misfire condition.

The rule of thumb is that the lowest compression value can not vary more than 15% of the highest value (that you got in TEST 1). If any value is lower by more than 15%, then that engine cylinder can be considered dead and is going to misfire.

How do you figure this out? You can find out by using my online low compression calculator here: Online Low Engine Compression Calculator or by calculating this 15% difference manually.

To understand how to figure out this 15% thing manually, on your 2.5L Dodge Dakota, let's say that I got the following compression values:

• Cylinder #1 175 PSI.
• Cylinder #2 165 PSI.
• Cylinder #3 160 PSI.
• Cylinder #4 140 PSI.

The next step is to do the following math:

• Multiply .15 (15%) by the highest value: 175 x 0.15. This gives us 26.25, but we'll round it out to 26.
• Next, we subtract 26 from 175: 175 - 26 = 144.
• So now we know that the lowest possible compression value is: 144 PSI.

This means that cylinder #4 (at 140 PSI) is the one causing the misfire because it's below the 144 PSI minimum.

Once we'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 Compression Test.

TEST 2: Wet Compression Test

There are usually two main causes of low compression (or zero) compression in a cylinder.

The first one is that the dead cylinder's piston rings are severely worn out. The other one is that the dead cylinder's head valves are severely worn-out or damaged.

In either case, we don't have to tear the engine apart to find out. We can find out by doing a wet compression test. This test simply involves adding about two spoonfuls of oil into the cylinder (with bad compression) and retesting it again.

OK, this is what you'll need to do:

1. 1

   Add a tablespoon (or two) of engine oil in the cylinder you need to retest. I suggest using a small and long funnel so that the oil will reach the inside of the cylinder.

2. 2

   Once you've added the oil, install the compression gauge, and as before just hand tighten it.

3. 3

   Now, 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 that needs to be tested, repeat steps 1 thru' 4 on it now.

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

CASE 1: The compression value shot up for the low compression cylinder. Since the compression value shot up, you can conclude that low compression value you got in TEST 1 is due to extremely worn out piston rings.

The reason the compression value shot up is due to the fact that the motor oil you just added helped the piston rings to create a tighter seal. This type of test result only happens when the problem is due to worn piston rings.

CASE 2: Your compression value DID NOT shoot up (stayed the same). Since the compression value did not shoot up, you can conclude that the bad compression value you got in TEST 1 is due to extremely worn-out or damaged cylinder head valves.