How To Diagnose Misfire Codes (GM 2.2L)

Testing a misfire code (P0300, P0301, P0302, P0303, P0304) isn't hard and I'll explain some of the testing tips and techniques that are performed to get the bottom of the misfire code and misfire condition.

The info in this article applies to the GM 2.2L 4 cylinder equipped Chevrolet Cavalier, or Pontiac Sunfire, or Chevrolet Sonoma, or GMC Sonoma.

What Is A Misfire Condition?

A misfire condition, in plain English, describes an engine that is not operating on all cylinders. Now, in the case of the GM 2.2L 4 cylinder engine, if one cylinder cuts out, the engine will only output 75% of it's power and this will cause some issues. If two or more engine cylinders are misfiring, your vehicle will not start. Here are some of the most common symptoms of a misfire condition on your GM 2.2L:

• The check engine light (CEL) will be on.
• One or more misfire codes (P0300, P0301, P0302, P0303, P0304) will be stored in the car or pickup's computer's memory.
• Misfire is present, but no misfire codes are registered in the computer's memory.
• Lack of power upon acceleration.
• Smell of unburned gasoline exiting the tail pipe.
• Rough idle and may stall.
• Cranks but does not start.
• Will not pass the emissions tests.
• Bad gas mileage.

What Causes A Misfire Condition?

As you may already know, each cylinder needs air, fuel, and spark to produce power. If any one of these components are missing, that engine cylinder will misfire. Therefore, the fault could lie in the ignition system (which is responsible for creating spark), or in the fuel system. Now, in most cases it's usually a lack of spark that's to blame for the misfire, but not always. Let's take a brief look at the different things that can cause your 2.2L to misfire.

Ignition System: As you're already aware, the ignition system on the GM 2.2L is composed of an ignition module, two ignition coils (that feed two cylinders with spark simultaneously), 4 spark plug wires, and 4 spark plugs. If any one of these goes bad (and they do), then you'll end up with a misfire condition. Now, all of these can be tested to see if they are the culprits behind the misfire.

Here are the most common ignition system problems causing a misfire:

1. Bad spark plugs.
   • Spark plugs that are carbon fouled due to the engine burning oil.
   • Broken spark plugs usually happens when they are installed or when the engine is washed.
2. Bad spark plug wires.
3. Bad ignition coil pack.
   • Each ignition coil pack fires spark to 2 cylinders simultaneously (Waste Spark) and what usually happens is that one ignition coil pack will stop firing spark from one tower or both.
4. Bad ignition control module
   • The ignition control module (ICM) sits under the 2 ignition coil packs.
   • The ignition control module can fail and stop activating one of the 2 ignition coil packs sitting atop it, causing 2 cylinders to go ‘dead’ for lack of spark.

Fuel System: Here are the most common fuel system component failures:

1. Bad fuel injectors.
   • This is the most common type of fuel system failure that will cause the engine to misfire since these guys don't last forever.
   • When a fuel injector 'fries' it usually doesn't leave you a bad fuel injector diagnostic trouble code but you'll definitely see a misfire code (P0300, P0301, P0302, P0303, or a P0304).
2. Fuel injectors not being fed with power or the activation signal.
   • The activation signal comes from the PCM (Powertrain Control Module = Fuel Injection Computer).
   • A lack of the power or activation signal usually points to either an open-circuit problem in the wiring or a bad computer.
3. Bad fuel pump.
   • When fuel pumps die, your vehicle will crank but not start, but sometimes it takes a long time for the fuel pump to die. In the meantime, it sends less than the ideal fuel pressure/volume to the fuel injectors. This results in a rough idle condition that can make you think it's an ignition system misfire.

Engine Mechanical Condition: An engine cylinder that is not producing enough compression, even if it has spark and fuel, will cause a misfire condition that will set a misfire code. Another thing that will cause a misfire (normally at idle) is vacuum leaks coming from a large vacuum hose or the intake manifold gasket.