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.
Contents of this tutorial:
NOTE: This tutorial applies only to the indicated 2.5L V6 equipped vehicles listed in the Applies To: box on the right column.
Puedes encontrar este tutorial en Español aquí: Cómo Probar La Compresión Del Motor (2.5L V6 Chrysler-Dodge) (en: 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.
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:
Disable the ignition system by disconnecting the ignition distributor from its electrical connector. This will prevent the ignition coil from sparking during the test.
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.
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.
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 5 cylinders.
Let's take a look at what your test results mean:
CASE 1: If the engine does not start and the results of the engine compression test are 0 PSI on all 6 cylinders then you have 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: If the engine does start, the very first thing you'll notice is that the compression values you wrote down for each cylinder are slightly different from one another. This 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.
The rule of thumb is that they can not vary more than 15% from each other and if they do, you're gonna' have a genuine misfire condition on your hands or possibly a no start condition (if more than one cylinder is affected).
How do you figure this out? This is how you do it:
- Grab a calculator and multiply the highest compression reading that you recorded by .15. So, let's say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .15 gives you 26 (25.5 rounded off).
- Now, the next step is to subtract 26 from 170... which gives us 144 PSI.
- So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this and that engine cylinder will misfire.
To make better sense of the above calculation, let's say that my 2.5L V6 equipped Dodge Stratus produced the following compression test results:
- 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: 175 x .15= 26, 175-26= 149. So, now I know that cylinder #4 is the one causing the misfire!!