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.
Contents of this tutorial:
Tools You'll Need:
- Compression Gauge Tester.
- A helper
- Pen and Paper
Symptoms Of Low Or No Engine Compression
Engine compression problems will generally cause one of two types of problems.
1) Either the engine will not start or 2) the engine will start but it will run with a misfire.
by far the most common issue that you'll encounter, when the engine has a compression problem, is a cylinder misfire.
If your vehicle is OBD II equipped and the engine compression problem is causing a misfire, then you're going to see the check engine light lit up by one or more of the following trouble codes:
- P0300 Random Cylinder Misfire.
- P0301 Cylinder #1 Misfire.
- P0302 Cylinder #2 Misfire.
- P0303 Cylinder #3 Misfire.
- P0304 Cylinder #4 Misfire.
Generally, when an engine compression problem causes a rough idle, you're only going to feel the problem when you come to a stop light. But as soon as you accelerate the engine, the engine feels fine.
You're also gonna' see:
- Bad gas mileage.
- Engine pollutes more and it won't pass an emissions test.
If your 2.4L Quad 4 GM vehicle is NOT OBD II equipped, you won't have any codes lighting up the check engine light, but you'll definitely feel a rough idle condition.
OK, having covered the most common scenarios of low compression and no compression, let's get testing to see if this is the case on your vehicle.
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 two recommendations to you:
1) Which one to buy: The engine compression tester that I have always used is the Actron CP7827 Compression Tester Kit. My only complaint about this engine compression tester is that it does not come with a case to store it in.
2) Where to buy: You can buy an engine compression tester just about anywhere, but you'll end up paying more for it (especially at your local auto parts store). The above links will help you comparison shop. I think you'll agree it's the better way to save money on the compression tester!
TEST 1: ‘Dry’ 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.
NOTE: If your car starts and runs, it's important that the engine not be hot (technically: it should not be at 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 5 minutes. If your car doesn't start, then don't worry about the engine being warm or hot or cold.
OK, to get this party started, this is what you need to do:
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.
Remove the aluminum cover that holds the plastic ignition coil cover (this is the cover that has the two ignition coils attached to it).
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 the engine while you observe the compression gauge.
Once the needle stops climbing on the gauge, have your helper stop cranking the engine.
Write the compression value down on a piece of paper. Include the number of the cylinder this reading belongs to.
Repeat steps 4 through 8 on the remaining cylinders.
Let's take a look at what your test results mean:
CASE 1: No compression in 2 or more cylinders. This test result tells you that the engine has serious internal problems.
The most common issues would be: Broken timing chain. Or a blown head gasket. Or the engine threw a rod.
CASE 2: Low compression in one or more cylinders. To a certain point, it's normal for the compression to vary a little between cylinders (as the engine accumulates thousands of miles).
But if these values vary too much, then you're gonna' have a bonafide misfire on your hands.
The next step is to do some math to find out if this low compression value is within a normal parameter or not. Go to: Interpreting Your Compression Test Results.