So I have a 99 1500 sport with the 5.9L and the check engine light keeps coming on saying the number 4 and 5 cylinders are misfiring and thatI have multiple cylinder misfires. I am assuming that since only the 4 and 5 came up that those are the only ones. I recently purchased the truck so I didn't know when the last time the plugs, wires, cap and rotor had been replaced soI started there. The truck is also up on 38.5s so it has been "used" to say the least but its in pretty good condition still just needs a little love. Anyway took the plugs out which wasn't easy they were stuck in there and they weren't all the same most were bosch platinum 4s and I had one champion that came out ofI believe 5 that was just done half the little arm was gone rusted to hell and back so after seeing that I was sure that this would fix it. Started it up and it ran better than before. Before it would chug quite a bit at idle still does slightly but not too bad. It does have true dual exhaust so I am assuming that is why it still chugs a little but I am supposed to get an x pipe put in tomorrow to hopefullyfix that. But it sat there running for about 10 minutesI reved it a few times sounded good, exhaust was a little more clean and smooth sounding. Took it out to go wash it at the local power wash and did just fine. Drove back with my roommate so he could drop off his Jeep after washing it also and then we made a trip out to autozone. On the way out of our apartment complex the check engine light came back on. When we got to autozone they scanned the truck again and the same three codes came up again. Cleared them again drove back and hasn't come back on again but I am expecting it to. Any suggestions on where I should look next? I bought a spark tester to test the wires to see if maybe they don't have good spark.

 

Bosch plugs run to hot in our engines, I've read about and seen them cause all sorts of problems in Magnum V8s. Good thing you removed them. If the plugs were mismatched, in my opinion, you should check every single part of the engine that you can. You should never mix and match with different kinds of plugs. If the previous owner did that, he either didn't know anything about engines or didn't care enough about the truck. Either one is cause for concern.

You should probably just get new wires, cap, rotor, etc. and see if that fixes it. Make sure and route the wires correctly.

Search the forums here (use the "Search" link at the top of the page) for electrical/engine problems and their solutions. Those threads are all over the place. Find a thread by someone who has had a similar problem to yours, and see what they did.

Let us know how it turns out.