Ready, Aim, Misfire

Engine misfires can cause performance problems as simple as rough idle and flat acceleration, as well as more serious problems like catalytic converter damage. The net result is overall poor performance, as well as a serious emissions problem. Correctly diagnosing the cause of an engine misfire is essential to restoring engine performance. And while the engine check light (MIL) will illuminate, under certain conditions, when a misfire occurs, the MIL won’t tell you how to solve the problem. All is not lost, though, because a good working knowledge of engine systems, plus some experience, are your best tools for diagnosing this type of problem.

What is a Misfire?

An engine misfire is the result of incomplete combustion, pure and simple. It can be combustion that occurs at the wrong time, such a pre-ignition due to a hot spot. It can occur in only one cylinder, or in several cylinders. Sometimes, an engine will idle smoothly, but will misfire under acceleration. Conversely, the engine will idle roughly, but accelerate smoothly. In reality, we can’t always say what causes a misfire specifically, so let’s just stick to the notion that it’s bad combustion.

In order to better understand bad combustion, let’s review what good combustion is. Under normal operating conditions, the air/fuel mixture is ignited by a spark from the spark plug. A flame front moves through the air/fuel mixture, burning the mixture to produce power in the cylinder. The timing of this spark is a command from the Powertrain Control Module (PCM) that is determined from the inputs of a variety sensors located throughout the engine. This event (the spark) has become very precise under all operating conditions due to the use of computer controls. The net result is consistent engine performance through all modes of operation.

When a misfire, or bad combustion occurs, the normal combustion process is interrupted, negatively affecting engine performance. So what can interrupt the combustion process? In simple terms, anything that changes a combustion parameter. Now that’s a pretty wide reaching answer, but just think about those things that can adversely affect combustion.

Pre-ignition (the beginning of combustion before the spark plug fires) certainly affects the combustion process. It results from hot spots in the combustion chamber. Also, the build-up of carbon deposits in the combustion chamber can effectively raise the compression ratio. The result is detonation, which is the igniting of the remaining air/fuel mixture ahead of the flame front. While we’re on the subject of things that can affect ignition, a bad spark plug wire (on those vehicles without coil-on-plug ignition) is a common cause of misfires. Also, a clogged fuel injector will limit fuel flow, causing a misfire.

There are a host of other conditions that cause misfires. We can get into a discussion on the valvetrain. Weak valve springs and burnt valves can cause misfires. And don’t forget vacuum leaks. As you can see, the answer to what causes a misfire is not specific, as misfires can be quite random. And that can make diagnosis tricky.

Diagnosing Misfires

As a starting point, talk to your customer and ask specifically about engine performance at idle and under acceleration. If the engine has a rough idle, this is a good indication that a misfire condition exists. While not the only cause of rough idle, misfiring is one of the major causes. Also, if the engine has flat power under acceleration, an engine misfire can be the cause in this scenario, too.

Next, ask your customer if the check engine light (MIL) is illuminated or is flashing while the vehicle is being driven. If your customer doesn’t know, or is not aware of the MIL being on, start the engine and see for yourself. If the MIL is flashing on and off, this indicates that the PCM has detected an active, severe engine misfire that can cause damage to the catalytic converter. This is a condition that must be corrected immediately. If the MIL is illuminated (steady on, but not flashing), it’s time to get out the DRBIII (or equivalent scan tool) and check for diagnostic trouble codes (DTCs).

Let’s review how the PCM can detect a misfire in order to set a DTC. On any OBDII vehicle, the PCM has a misfire counter, which works in the following manner: The PCM determines the twisting force of the engine, in terms of RPM, at the crankshaft or flywheel. This is based on the overall speed of the engine. If a misfire occurs, the amount of force from combustion in the cylinder decreases (it could be a lot or just a little bit). The PCM doesn’t actually measure this change in force, it just detects the decrease in engine speed (that particular piston travels a bit slower due to the decrease in force). This decrease in engine speed is classified as a misfire.

Not only does the PCM know that a misfire has occurred, it knows where. Thanks to the crankshaft position sensor, the PCM can detect the specific cylinder with the decreased piston speed. The conclusion is that a misfire has occurred, the MIL is illuminated and a DTC is set.

Using the DRBIII, or other scan tool, the basic trouble code for an engine misfire is P0300, which indicates a multiple cylinder misfire. If the code is P0301, the misfire is in cylinder #1, P0302 indicates cylinder #2 and so on. Now that we know where the misfire is occurring, we can find the cause and solve the problem.

Solving Misfire Problems

After confirming that a misfire exists, we can now find a fix. It might be a bit trickier if you have a multiple misfire DTC, but we’ll use the same process to pinpoint the cause. If you have access the diagnostic manual for the particular vehicle, a decision tree is listed. For those who do not have access to such a book, let’s review it in general terms. First, do a comprehensive visual check under the hood. Look for bad sensor connections, worn accessory drive belts, vacuum leaks, misaligned pulleys and any restrictions to the air induction system.

The next step is to check the ignition system (this is the source of many misfire problems). Using a spark plug tester, check the spark. A crisp, blue spark should be seen. Start the engine, turn down the lights in your work area and look for any arcing between a plug wire and ground. Check the spark plug for cracks in the insulation, which will cause a misfire.

As the spark plug wires age, the insulation can begin to crack. With such a high voltage passing through the wire, it will seek the path of least resistance. If there is a crack in the insulation, the current will jump through that opening to ground, causing a misfire. Figure 1 shows such a wire. Notice the black dot and the surrounding circular pattern.

Check fuel pressure. If it is above specifications, replace the fuel filter/regulator. Low fuel pressure might indicate a problem with the fuel line. Fuel injector performance should be checked, too, as bad injectors can cause misfires.

Finally, a compression test should be performed. Often, carbon build-up on the intake and exhaust valves will cause a compression leak, which can be detected by such a test. In severe cases, such leakage problems can burn valves, as shown in Figure 2. To correct this build-up problem, use Mopar® Combustion Chamber Cleaner, P/N 04318001AB.