This is like asking your doctor over the phone what to do about your pain. First he has to determine if the cause is a stomach ache, a hang nail, or you cut your foot off with a chainsaw. It's impossible to tell what the problem is without performing diagnostic tests and observations, or worse, just blindly throwing parts at it until something solves the problem.
As the engine rotates, a sensor develops pulses at regular, evenly-spaced intervals. When a misfire occurs, the crankshaft slows down a very tiny bit. That's why you feel it. The engine computer detects the extra time between pulses and it knows which cylinder should have just fired. What it doesn't know is why the cylinder didn't fire.
The first thing everyone jumps at first is spark plugs because that was about the only thing that could cause a misfire on older cars. Now we also have fuel injectors, the individual computer circuits that drive them, individual ignition coils and their computer-driven circuits. To really add to the confusion, some vehicles, Ford trucks in particular, develop a problem where individual tubes that introduce burned exhaust gas into the cylinders become plugged with carbon. That causes those cylinders to operate very nicely when all they have to burn is fresh air and clean gas. A measured amount of exhaust gas must go into the engine as part of the emissions system. When five passages are plugged with carbon, all of the exhaust must go to the one working cylinder. That dilutes the fuel and air so much, it can't fire the fuel. The five cylinders with a problem run fine. The one working cylinder has the misfire.
Chrysler's exhaust gas recirculation (EGR) system is simpler and much more reliable, but if the valve sticks open, any cylinder can be affected.
There are some simple tricks you can do to find the cause of the misfire but it only works when the diagnostic fault code specifies the cylinder or you can feel the misfire occurring. If you can feel the steady misfire, unplug one ignition coil at a time while the engine is running, then listen and feel for the response. If the engine becomes more rough when a coil is unplugged, that cylinder is working. When no change occurs when unplugging a coil, that cylinder is dead.
When you can't feel the misfire, you'll have to rely on the fault code to identify the problem cylinder. Remove that coil which attaches to the spark plug and switch it with one of the other ones. Now you can run the engine and again unplug the coils one at a time. If the same cylinder is dead, the coil wasn't the problem. If a different cylinder became the dead one, the coil is the cause. The same can be done by switching injectors, but it takes a little longer.
Wednesday, March 3rd, 2010 AT 8:18 PM