You need air, fuel, compression, and spark for a cylinder to make power. When the problem is intermittent, a compression problem is very unlikely since mechanical things that break don't magically fix themselves. Chrysler has extremely little trouble with fuel injectors so that should be one of the last things to suspect. If there is a reason to suspect an injector, a common approach is to switch it with one from a different cylinder, then see if that other cylinder develops a misfire. If it does, replace the injector.
Air is common to all the cylinders. A problem there is rare and would cause different symptoms. That leaves spark as the main suspect.
The spark plug is only one part of the system. The ignition coil and spark plug wire also have to be considered. The coil is actually two coils in one "coil pack". Each coil fires two spark plugs at the same time. Typically when the coil fails, two plugs won't fire and the engine will have very little power. You can try switching two spark plug wires to see if a different cylinder sets a misfire code, but with all the running around to have fault codes read, this is one time it makes more sense to just "throw random parts at the problem". Normally that's the least efficient and most costly approach but when you have so few parts to guess at, AND since they are common maintenance items anyway, I'd suggest replacing all of the spark plugs and all of the spark plug wires. When one shoe is worn out, you buy two new shoes, right? When your sock gets a hole in it, you don't just buy one new sock. It's not practical to replace one spark plug, other than as a test, and leave in the others that are just as old and worn.
The cost of new spark plugs and wires is relatively low, and since the chance of solving the problem is so high, it makes more sense to start with that before paying for your mechanic's time. If the problem is still there, at least you will have saved your mechanic the time and expense of replacing those parts. He can move on right away to the more involved tests and diagnosis.
Friday, May 18th, 2012 AT 10:46 PM