Troubleshooting a misfire (P0300, P0301, P0302, P0303, P0304) or rough idle condition on your Quad 4, whether it's a Chevy, or a Buick, or a Pontiac, or an Olds, can be a challenge that can have you pulling out your hair (I know from first hand experience). There's a good chance you have already replace all of the common things that can go BAD (like: spark plugs, plug boots, plastic coil cover, ignition coil pack, etc.) and the car is still running rough or with a misfire.
Well, an engine compression test will help you to either pin the blame on engine mechanical wear and tear or completely eliminate the engine's mechanical condition as the source of the issue. So whether you car Does Not Start or misfires, this article will take you step-by-step thru' the whole test.
Here are the contents of this article at a quick glance:
- The Engine Compression Test.
- Interpreting the Results of the Engine Compression Test.
- ‘Wet’ Compression Test.
- Why an Engine Compression Test?
- Which Compression Tester Should I Buy?
- Related Test Articles.
Tools You'll Need:
- Compression Gauge Tester.
- A helper
- Pen and Paper
The Engine Compression Test
Just a friendly reminder: You're gonna' be working around a cranking engine, so you have to be alert and think safety all of the time.
The compression test on your GM 2.4L Quad 4 engine can be done in under 20 minutes and at the end of the step-by-step testing instructions, I've included test interpretations so that you can interpret your test results.
OK, to get this party started... this is what you need to do:
If your car starts and runs, it's important that the engine not be hot (technically: normal operating temperature). So, if you've had the car running for an extended period of time, let the engine cool down for about an hour. If the engine is completely cold, start and run the engine for about 20 minutes.
If your car doesn't start, then don't worry about the engine being warm or hot or cold.
Disable the fuel system. You can do this by either disconnecting all four fuel injectors or by removing the fuel pump relay. This will prevent fuel from being injected into the engine while you do the compression test.
OK, remove the aluminum cover that holds the plastic ignition coil cover (this is the Cover that has the two ignition coils attached to it).
Now, remove all four spark plugs. As you're 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 (this is the spark plug hole closest to the drive belt). Hand tighten the compression gauge only! Do not use any type of tool to get it tight.
When all is ready, have an assistant crank up the car. Here's what should happen: The compression tester's needle will climb to a certain point and then stop, while the engine cranks. Once the needle stops climbing on the gauge, have your helper stop cranking the engine.
At whatever value the compression tester's needle stopped at (even if it did not move), write it down on a piece of paper. Include the number of the cylinder this reading belongs to. Now repeat steps 1 thru' 6 on the other 4 cylinders.
When you're done testing all four cylinders, now you need to interpret the results, for this, turn to the next page...
Interpreting the Results
of the Engine Compression Test.
The idea behind the engine compression test (in the case of a misfire condition or rough idle condition) is to find out if any one engine cylinder is not contributing 100% of its power to the overall engine output. If only one cylinder has lower than normal compression, your 2.4L Quad 4 will run rough or cause a misfire code to set and turn on the Check engine Light.
So then, to wrap things up, the individual cylinder compression readings of each engine cylinder can not vary more than 15%... and 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 2.4L Quad 4 Pontiac Grand Am (or Sunfire, or 2.4L Chevy Cavalier, etc.) gave me the following compression readings:
- Cylinder #1 175 PSI.
- Cylinder #2 160 PSI.
- Cylinder #3 165 PSI.
- Cylinder #4 30 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!!