Something's Missing

Engine misfire diagnostics can be a tough task. But here are some tips to help with the job.

Engine misfire is a problem that can result from a number of different conditions. Diagnosing the cause can pose some significant challenges. Misfire can take a variety of forms, but, in many cases, it might not be that obvious to the driver of the vehicle. A slightly rough idle, or an illuminated malfunction indicator lamp (MIL) might be the early signs of a problem. The long term consequences of engine misfire, however, can be significant. These include damaged engine parts and serious driveability issues.

Is the Engine Misfiring?

As mentioned, the driver might notice that the engine is a bit rough, but this condition doesn’t necessarily mean that the engine is misfiring. And the idle condition might be something that’s just ignored as long as the vehicle is operating acceptably in all other modes. It should be mentioned, though, a rough idle is the main symptom for this problem. The illuminated MIL is probably what will bring the vehicle to the repair shop for a closer look.

Fortunately, onboard diagnostics often give technicians a good head start in diagnosing engine misfire problems. If the MIL is illuminated, that means that there is a Diagnostic Trouble Code (DTC) stored in the powertrain control module (PCM). That code can reveal if a misfire is occurring.

If the DTC is P0300, that indicates a multiple cylinder misfire. If the code is P0301, that indicates the misfire is in cylinder #1, P0302 indicates the misfire is in cylinder #2, etc, for however many cylinders there are in the engine. Now we know that the engine is misfiring and the rough idle is not be caused by some other condition, such as a dirty throttle body (see the How To article in this issue to remedy that situation).

You might be asking yourself how does the PCM know that the engine is misfiring? Simple. It has a misfire counter. In any vehicle that is equipped with OBDII, any ignition pulse that is not normal is classified as a misfire. This abnormal ignition might be caused by a hot spot in the cylinder or combustion chamber, dirty intake or fuel injector or a cracked spark plug. Regardless of the cause, it can be detected simply by the PCM.

The PCM monitors engine speed and can determine 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 measure this change in force, but it can detect it by a decrease in engine speed (that particular piston travels a bit slower due to the decrease in force). And with a crankshaft position sensor installed on the engine, the PCM knows in which cylinder the decreased speed occurred. The conclusion is that a misfire occurred and a DTC is set.

Verifying, Diagnosing and Solving the Problem

Even with a P0300 DTC, technicians should verify that the engine is, indeed, misfiring. And that's easy to do with the DRBIII® scan tool. Start the vehicle, go to the OBDII monitor, and then to the misfire counter. The misfire counter counts the number of misfires every 200 ignition pulses. After the number 200 is reached, the counter resets to zero. So if misfires are being counted, then you have verified the problem.

Now, let’s find out where the misfire is occurring. If you have a specific cylinder code, such as P0303, you can go right to that cylinder. If it’s multiple cylinders, it’ll be a little tougher to find the problem cylinders.

The next step is performing a visual inspection of the entire ignition system. As we’ll see, the problem is found most often in this area. Start the engine and turn down the lights in your work area. You're looking for any arcing between a spark plug wire and ground. This can occur anywhere from the coil (if it is not a coil-on-plug design) to the spark plug.

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. Figure 1 shows such a wire. Notice the black dot and the surrounding circular pattern. This condition will cause a misfire.

Also look for carbon tracks (thin black lines) on the body of the ignition coil (if the case is not black in color) and on the plastic insulator of engines with plug-in-head designs. Arcing can burn right through these insulators.

To complete your visual inspection, remove each spark plug, one at a time, looking for cracks in the insulation. This crack will appear as a line of carbon, that if cleaned away, is a thin black line. Figure 2 illustrates such a crack.

After the visual inspection is done, you can perform some specific tests to pinpoint the problem. If you have a code that indicates a misfire in a particular cylinder and you suspect the spark plug is bad, switch the plug with the plug in an adjacent cylinder. If the plug is bad, the DTC will indicate the misfire in that adjacent cylinder. Swapping plugs is always a good method to pinpoint a misfire.

Another test that you should perform is a standard compression test on each cylinder. Often, carbon build-up on the intake and exhaust valves will leak compression. This condition can be detected with the compression test. If you notice a significant drop in one or more cylinders, it's a good bet that carbon build up is the problem (Figure 3 shows an exhaust valve that has burned as a result of misfiring in such an engine).

To correct the carbon build-up problem, use Mopar Combustion Chamber Cleaner, P/N 04318001AB.

With experience you will find that the majority of engine misfire problems can be traced to bad ignition wires and damaged spark plugs. Cracked coil packs are also a common culprit as is carbon build up in the combustion chamber. Finally, in some engines that use heat shields for the fuel injectors, engine misfires have occurred if those heat shields are removed. The injector overheats and malfunctions, causing the misfire.