How To Test Engine Compression (2.7L V6 Chrysler)

Testing engine compression can reveal quite a lot about the state of health of your 2.7L engine. Doing an engine compression test will:

1. Help diagnose a hard to troubleshoot misfire.
2. Help to find out if no compression is behind a 'cranks but does not start' condition.
3. Tell you the overall health of your engine.

In this tutorial, I'll explain how to do and interpret an engine compression test.

You can find this tutorial in Spanish here: Cómo Probar La Compresión Del Motor (2.7L V6 Chrysler, Dodge) (at: autotecnico-online.com).

NOTE: This tutorial applies only to the indicated 2.7L V6 equipped vehicles listed in the Applies To: box on the right column.

Important Tips And Suggestions

TIP 1: To get a more accurate test result, from your compression test, you should perform it with a slightly warmed up engine. I want to emphasize slightly warmed up engine because the engine should not be hot.

TIP 2: Be careful and 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: Both cylinder heads are made of aluminum metal, 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.

TIP 4: On the rear wheel drive (RWD) 2.7L equipped vehicles you'll need to remove the plastic manifold plenum (also known as the upper intake manifold). Before you start, take a look at the section Precautions To Take When Removing The Intake Manifold Plenum.

Symptoms Of Low Or No Cylinder Compression

Engine compression problems usually fall into one of two categories: Good compression in most but not all 6 cylinders or no compression in all 6 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:

• Misfire codes: P0300, P0301, P0302, P0303, P0304, P0305, P0306.
• Engine won't start (0 compression in all cylinders).
• Engine smokes when running (blue smoke).
• 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 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

Before you start, let me tell you that the 2.7L V6 engine in your Chrysler/Dodge vehicle suffers from one very common problem. This is valve valve covers leaking and letting oil into the spark plug tubes.

The end result is spark plug wire boots and spark plug drowning in motor oil. This eventually leads to carbon tracks on the side of the spark plugs and on the inside of the plug wire boots and eventually a misfire condition.

So, if you remove a spark plug wire boot or spark plug 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

   Pull out the ignition coils. As you remove each one, check the coil boot (spark plug boot) for signs of oil contamination —if it's soaked or dripping with engine oil, that's a clue you'll want to note.

2. 2

   Remove all the spark plugs. Do this with the engine slightly warm (if it starts and runs), but not hot.
   
   Be extra careful not to drop any plugs on the floor, since the porcelain insulator can crack and cause a brand-new misfire.
   
   If the engine won't start, don't worry about warming it up first —just go ahead and remove the plugs cold.

3. 3

   Thread your compression tester into cylinder #1's spark plug hole. Only hand-tighten the gauge —do not use tools to force it tight.

4. 4

   Have your helper crank the engine until the gauge needle stops moving upward.

5. 5

   Write down the reading. Record the PSI value and the cylinder number on paper so you can compare results later.

6. 6

   Bleed off the pressure from the gauge and retest the same cylinder one more time.

7. 7

   Move on to the next cylinder. Remove the tester and repeat the same steps for the remaining cylinders until all have been tested.

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

CASE 1: All cylinders have 0 PSI. This is bad news since this test result tells you the engine has suffered one of the following conditions:

• Timing chain 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 you wrote down for each cylinder are different from one another. Within a certain range, a variation in the compression values is 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. Head over to: Interpreting Your Compression Test Results.

Interpreting Your Compression Test Results

If your compression test shows that one or more cylinders came back with low numbers, the first thing to keep in mind is that not every low reading is a problem.

As long as the difference is small, the engine will usually run fine and you won't notice anything unusual while driving.

But once a cylinder drops past the acceptable limit, that's when drivability issues show up —rough idle, loss of power, or even a misfire.

So how do you know if the readings you got are still in the safe zone? The rule of thumb is that no cylinder should be more than 15% lower than the highest reading you recorded.

You can figure this out two ways: either by doing the math on paper, or by using my free calculator here: Online Low Engine Compression Calculator (at: easyautodiagnostics.com).

If you'd rather calculate it yourself, here's how:

• STEP 1: Take the highest compression value and multiply it by 0.15 (which equals 15%).
• STEP 2: Round the answer to the nearest whole number (for example, 25.6 becomes 26).
• STEP 3: Subtract that number from your highest compression value.
• ANSWER: The result is the lowest compression value any cylinder can have before it's considered too low.

To make this clearer, here's a real-world example using actual compression test results:

Cylinder	Pressure
#1	165 PSI
#2	95 PSI
#3	155 PSI
#4	175 PSI
#5	175 PSI
#6	170 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.

With cylinder #2 showing only 95 PSI, it's safe to say that this cylinder is effectively 'dead' and the reason for the misfire.

To check if your own lowest reading is still within an acceptable range, you'll need to run the same 15% calculation —but make sure to use the highest number from your test, not the values from my example.

Once you've identified the weak cylinder, the next step is to figure out why the compression is low. For that, continue on to: TEST 2: Wet Engine Compression Test.

TEST 2: Wet Compression Test

If your compression test results confirm that one or more cylinders has low or no compression, then the next step is to find out if the problem is due to bad cylinder head valves or bad piston rings (of the affected cylinders).

To find out, we need to do a 'Wet' compression test on the cylinders that tested with low or no compression in TEST 1.

A 'Wet' compression 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: Your compression test gauge registered a higher compression value on the affected 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 test gauge DID NOT register a higher compression value on the affected cylinder (compression 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.

Precautions To Take When Removing The Intake Manifold Plenum

Before you remove the intake manifold's plenum (on the 2001 and newer 2.7L engines only), keep in mind several important things:

1. Buy or borrow a repair manual (preferably a Haynes Repair Manual -Chilton manuals suck) so that you can have the intake manifold removal process explained in detail.
   • The manual will also give you the torque specifications for the bolts.
2. Be careful that no foreign object, like a bolt, a nut, or any metal piece/part, falls into the open manifold port runners.
3. Once the plenum has been removed, place a clean rag or rags on the open intake runners. This will keep things from falling into them.
4. As you're removing bolts, nuts and stuff from the intake plenum to remove it, place them in a container and away from the engine compartment.

Anything falls into the open intake runners, without you knowing, and the engine is started, you're going to be in a big world of hurt! Since it may require the removal of the cylinder head to retrieve the part that fell in. So be alert and be careful.

Removing the plenum is not an out of this world thing that no one can do, it's done around the country in many shops without complications or unhappy endings by simply following some precautions.