Testing the Engine Compression is not as hard as people think and in this article, I’ll show you how to do it and more importantly I’ll also show you how to interpret the results of the Compression Test.
Why do an Engine Compression Test? Well, it may help you to find out if the engine is shot and if this is the reason why it won't start or the reason you have a hard to diagnose Misfire Condition.
OK, to make it as easy as possible to navigate this article, here are its contents at a quick glance:
The image viewer on the left, has two images that will help you to identify the individual Engine Cylinders. One tells you the id number of each cylinder for the 3.9L V6 and the other for the 5.2L and 5.9L V8s.
OK, to get this show on the road, I’ll first explain the test steps. At the end of the test steps, you’ll find two possible test results that will help you to interpret your specific test results. Let’s get started:
The best way to do the Compression Test is with the Engine slightly warmed up... if it starts and runs. If it doesn't , well it's no big deal... you'll still be able to accomplish the test.
So, if it does start... crank her up and let it run for a few minutes to warm it up. If it has been running for an extended amount of time, let her cool down.
The Fuel System has to be disabled and you can easily do this by simply disconnecting all of the Fuel Injectors. This step is important, so don’t skip it.
The next step is to disconnect the Ignition Coil's electrical connector. This step is also very important, since you can not have the Distributor delivering Spark to each Spark Plug Wire during the test.
Now, remove all six (or eight spark plugs) 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!
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.
OK, when you’re ready, have a helper crank up the engine while you keep your eyeballs on the Compression Tester’s gauge. Once the needle on the gauge stops climbing, stop cranking the Engine.
Before moving on to the next Cylinder, write down this Compression reading on a piece of paper. Include the number of the Cylinder this reading belongs to. Now repeat steps 1 thru’ 6 on the other cylinders.
After testing all cylinders and having written down all of your Compression Test readings, now you need to interpret the results.
You're not gonna' see the exact same compression value on each of the 6 or 8 Cylinders. What you're looking for is a compression value or compression values that are radically different than the average compression of the good Cylinders.
This means that Compression between cylinders should not vary more than 15%... If you're scratching your head and wondering how to do this, don't worry... this is how you can find out:
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.
Now, so that this calculation can make more sense to you... let’s say that my Dodge Ram pick up (with a 3.9L) gave me the following compression readings:
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!!