If you're trying to nail down a hard to diagnose misfire condition or misfire codes and need to know how to do and interpret an engine compression test... you've come to the right article.
This article will walk you thru' the engine compression tests as it pertains to troubleshooting a misfire condition that is lighting up the check engine light ( and storing any of the misfire codes: P0300, P0301, P0302, P0303, P0304, P0305, P0306).
Here are the contents of this article at a quick glance:
You can find this tutorial in Spanish here: Cómo Probar la Compresión del Motor (GM 3.1L, 3.4L) (at: autotecnico-online.com).
Testing the engine compression of the 3 cylinders that face the firewall of your 3.1L or 3.4L GM car or mini-van (Chevrolet, Buick, Oldsmobile, or Pontiac) can be quite a challenge, since removing those spark plugs is not the easiest thing in the world. In most cases the alternator has to be removed to gain access to them. If you've never removed these spark plugs from your vehicle, I recommend buying a repair manual to brush up on the steps to do it.
This test article assumes that the engine starts and runs and that you're testing a misfire condition. OK, here's the test:
It's important the engine be slightly warmed up for this test, yet it can not be hot (technically: normal operating temperature) So, if the engine is completely cold, start her up and let it idle for no more than 15 minutes. If the engine has been running for a long period of time, let her cool down for about 1 hour.
The Fuel System and the Ignition System must be disabled, and you can easily kill these two birds with one stone by disconnecting all three of the Ignition Control Module electrical connectors. It's important that no fuel is being injected and that no spark is being fired when doing this test.
Remove all of the six spark plugs. Be careful and don't drop any of the spark plugs. Dropping them could cause their Ceramic Insulator to break... and this will cause a misfire. Important: Before disconnecting the spark plug wires, to remove the spark plugs, tag each one to identify where it goes. This will help you to connect the right spark plug wire to the right spark plug once you're done with the test.
Hand-thread the engine compression gauge into the spark plug hole that you've chosen to test first. Do not use any type of tool to get it tight, hand tight is enough!
Once everything is set, have your helper crank the car or mini-van till the needle on the compression tester gauge stops climbing. It normally takes about 10 seconds or engine cranking to get to this point. When the needle stops moving, you have reached the maximum compression pressure of that cylinder.
Record the reading on a piece of paper along with the cylinder the reading belongs to. Now, repeat this exact same test on the remaining engine cylinders. The illustration in the image viewer will help you to identify what cylinders you're testing.
Now, look at all of the four compression readings that you just wrote down. They should be within 15% of each other. Now, if you don't understand this 15% part, don't worry, the next part of this article will show you how to interpret what you have just done.
Now, I'm gonna' show you how to interpret all those compression pressure values you wrote down for each specific engine cylinder. Here's what you need to do with them:
Grab a calculator and multiply the highest compression reading recorded by .15 (.15 is the decimal equivalent of 15%). So let's say, for the sake of this example, that the highest reading was 165 PSI (Pounds per Square Inch). Multiplying 165 by .15 gives us 25 (24.75 rounded off).
Subtract 25 from the highest reading. In my example, the highest compression reading is 165... so subtracting 25 from this reading, I get: 140.
So then, 140 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.
Let me give you another, but more specific, example: Let's say that my 3.4L Chevy Venture (or 3.1L Pontiac Grand Am or my 3.1L Buick Regal, or 3.4L whatever), produced the following compression readings:
The next step would be to apply the formula above and I get 145 PSI as the lowest possible reading (170 x .15= 25, 170-25= 145). So, now I know that cylinder #2 is the one causing the misfire!!
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