How To Test Engine Compression (2.5L V6 Chrysler)

One of the most overlooked tests, when diagnosing a rough idle or no-start condition, is the engine's compression.

In this tutorial, I'll show you how to do and interpret the results of your 'dry' and 'wet' engine compression test results.

NOTE: This tutorial applies only to the indicated 2.5L V6 equipped vehicles listed in the Applies To: box on the right column.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (2.5L V6 Chrysler-Dodge) (at: autotecnico-online.com).

Important Tips And Suggestions

TIP 1: You should warm up the engine for about 10 minutes (from a completely cold condition) if your 2.5L V6 Stratus (Cirrus, Sebring, or Avenger) starts and runs. The key words here are 'slightly warmed up engine' since the engine should not be hot.

A slightly warmed up engine will improve the accuracy of your compression test results.

TIP 2: Take all necessary safety precautions as you work around a cranking engine. Your safety is your responsibility, so use common sense and think safety all of the time.

TIP 3: Your 2.5L V6 Stratus (Cirrus, Sebring, or Avenger) engine's cylinder head is made of aluminum, so do not remove the spark plugs with a hot engine. Removing the spark plugs from a hot engine can/will damage the spark plug hole threads in the aluminum cylinder head.

Symptoms Of Low Or No Cylinder Compression

Engine compression problems fall into 2 basic categories: Low engine compression in one or two cylinders and no compression in all cylinders.

Low engine compression in one or more cylinders will cause a rough idle issue that can seem very hard to diagnose.

No compression in all cylinders results in a 'cranks but does not start' condition.

Here are some other specific symptoms you may see with low cylinder compression:

• 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

You'll need to remove the spark plugs for the engine compression test and this will afford you the chance to see if the spark plug wire boots and spark plugs are swimming in motor oil (from leaking valve cover gaskets).

Leaking valve cover gaskets that are letting the spark plug tubes fill up with motor oil are a very common problem on the 2.5L V6 engines. This problem will eventually result in a misfire condition as the oil cooks and creates carbon tracks on the spark plug or spark plug boot.

So, if you do spot a spark plug wire boot covered/dripping in oil, you've found a the potential problem behind the misfire you're trying to diagnose with the compression test (you should still proceed with the compression test though).

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

1. 1

   Disable the ignition system by disconnecting the ignition distributor from its electrical connector. This will prevent the ignition coil from sparking during the test.

2. 2

   Remove the spark plugs from a slightly warmed up engine (if it starts and runs). Remember, the engine can not be hot!
   
   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 number 1 engine cylinder.
   
   NOTE: 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.

5. 5

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

6. 6

   Release the pressure on the gauge and check this cylinder's compression one more time.

7. 7

   Remove the compression tester and check the compression on the remaining 5 cylinders.

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

CASE 1: You got 0 PSI on all 6 cylinders. This is not good since it confirms that the engine has suffered one of the following conditions:

• Timing belt problem.
• Blown head gasket.
• Blown engine.

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

CASE 2: The compression values are different from one another. Within a certain range, this could be normal.

What is NOT normal is if the values vary too much. The cool thing is that we can find out if the variations in the values, you wrote down, indicate a problem (with that cylinder) or not. Go to: Interpreting The Results Of The Engine Compression Test.

Interpreting The Results Of The Engine Compression Test

It's completely normal for cylinder compression numbers to vary a little from one to another, and small differences usually won't cause any performance problems.

The trouble starts when a cylinder (or more than one) drops by more than 15% compared to the strongest reading. That's when you'll notice issues like misfires, rough idle, or poor engine performance.

So how do you know if your results fall into that problem zone? The quickest way is to use my free online calculator here: Online Low Engine Compression Calculator. If you'd rather crunch the numbers yourself, here's how to do it manually:

• STEP 1: Multiply your highest compression value by 0.15 (this represents 15%).
• STEP 2: Round the result to the nearest whole number (for example, 25.6 becomes 26).
• STEP 3: Subtract that rounded number from the highest compression reading.
• ANSWER: Whatever number you end up with is the lowest compression a cylinder can have before it's considered too low.

To make it easier, let me walk you through an example with real compression test numbers:

• Cylinder #1 170 PSI.
• Cylinder #2 165 PSI.
• Cylinder #3 160 PSI.
• Cylinder #4 110 PSI.
• Cylinder #5 165 PSI.
• Cylinder #6 175 PSI.

The next step is to do the math:

• STEP 1: 175 x 0.15 = 26.25.
• STEP 2: 26.25 = 26.
• STEP 2: 175 - 26= 149.
• ANSWER: 149 PSI is the lowest possible compression value.

Cylinder #4 produces a compression value of just 110 PSI, which tells us it's the one dragging things down and causing the misfire. The next step is to troubleshoot the reason for that low reading. For that, go to: TEST 2: Wet Compression Test.

TEST 2: Wet Compression Test

If one or several of the cylinders on your 2.5L V6 have low or no compression, then the next step is to find out if the problem is due to worn/damaged cylinder head valves or worn piston rings.

To find out, we'll do a 'Wet' engine compression test on the affected cylinders.

This test simply involves adding a small amount of motor oil to the low compression cylinder and repeating the compression test.

At the bottom, I'll show you how to interpret your test results.

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. This confirms that the low compression value registered in this cylinder in the dry test is due to worn 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). 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 2.5L V6 Chrysler Diagnostic Tutorials

You can find a complete list of Chrysler 2.5L V6 tutorials in these two indexes:

• Chrysler 2.5L V6 Index Of Articles.

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

• How To Test The Throttle Position Sensor (2.5L Chrysler).
• How To Test The Fuel Injectors (2.5L Chrysler).
• 3-Wire MAP Sensor Diagnostic Test Chrysler, Dodge, Plymouth.