I've began trying to fix a very weird problem with my truck. Its a 318 99 4x4 Dakota with a 46re swap. This truck has been amazing for me the past 3 or so years I've owned it, especially after swapping the transmission around 2 years ago. The truck recently started misfiring violently at speeds above 60 mph. The truck ONLY misfires above speeds of 60, or if I give it WOT at any speed. The problem is NOT an RPM issue, as if I rev it to any RPM in park/neutral it does not misfire. It began doing this around a week ago, and the only code I get is a P0203 Injector Circuit A Code, and a generic cylinder 1 misfire. I tested the resistance of cylinder 3 injector, and it was well within spec. I replaced it anyway, as I figured it was a good place to start, and I had multiple spares. This didn't change anything and my issue still persists. I also did a basic tune-up, as it was in need of one. I replaced all 8 plugs (checked gap on all), wires, dist cap and rotor. This also had no affect on the misfire and the injector code. What really confuses me is that when the vehicle begins to violently misfire at speed/load, I can audibly hear plenty of fuel exploding in the cat, which really makes me doubt that the issue is the injector not working/loss of fuel pressure. My main guess right now is the PCM, but I don't want to throw parts at my 20 year old truck that I am not dependent on. I swapped the cylinder 3 and cylinder 1 injector connectors to see if the PCM was the issue, and oddly, when the plugs are swapped, it will not give me an injector circuit code. It will only give me a cylinder 1 misfire code, but the truck still violently misfires either way. This is why I'm still not sold on the issue being PCM/wiring related. When I read about others on this forum having issues with the P0203 it is always accompanied by other injector circuit codes, never just the P0203. So for now I'm at a bit of a standstill and I'm not sure where to continue the troubleshooting. Sorry for how long this post has drawn out, there is just a lot of information that I feel is pertinent. My next bet would be to hook oscilloscope up to injector to see what's going on during these misfires, but any day I don't have to use an oscilloscope is a good day so I'd like to avoid extensive electrical investigations.. This will be cross posted on dakota-duragno and I will update this post in the event I receive helpful info/resolve the issue.

 

When you have weird problems like this take it to a shop that uses high end scanners to look at whats going on.

Could your cat need replacing or clogged muffler?

Crawl under the truck and look for things out of place a good visual inspection can catch things.

 

Muffler decided it didn’t want to be a part of my truck on the highway a couple of years ago. As far as I know the cat is the original and I will probably end up taking a look at it. The truck has had intermittent misfires as long as I have owned it so the cat certainly could be damaged by this point. I have a 5.9 for it I’m wanting to swap pretty soon and plan on doing the exhaust also but I’d like to find the cause of this issue before I start that. I know that if the issue is the PCM, then I’d be getting a new one anyway for the swap. Thanks for insight 98DAKAZ.

 

Fixed the issue. Hooked oscilloscope up to injector 3 and ran tests while the vehicle was running. Figured out that in higher vibration situations the power coming from the green and orange wire to the injector plug experienced intermittent loss of power. I soldered a new plug on and the issue has ceased.