Nightmare misfires!!
 

I have a 98 GMC Jimmy SLS 4x4 196xxx miles. This has been an ongoing issue for going on 2 years now, and I still can't resolve the issue. It started as a p0171 (system too lean, bank 1) code, and then turned into a p0300 (random/multiple misfires). I have changed the O2 sensors, the catalytic converter, fuel pump x2, the fuel filter, upper intake gaskets, the spider injector, the pcv valves, spark plugs, wires, distributer cap and rotor, and ignition coil. It's also has 2 differentials and a newer transmission put in.
It is really bad some days ie:cold rainy days, humid days. And then it runs like nothing is wrong with it some days. It misfires really bad from a stop through 1st and 2nd gear. Rpms at 60mph will be at 1650 and fluctuate, and after it misfires so many times it will kick the rpms up to 2000 and it won't do it as much. It has a rough idle, but rpm stay steady. Could be the motor mounts causing some of the shaking at idle, but not all of it. Another thing that it'll do is the rpms will go almost up to 3000, and then it feels almost as if the truck is in neutral. The rpms go up but it doesn't accelerate. Then when I pull over to the side of the road the entire truck is shaking, I turn it off, and the first time it rolls over, but sounds like a lawn mower out of fuel *clunk, clunk clunk*. The second time it rolled over everything was fine again. It's only done the 3 times, but the misfires are constant. Any insight would be great, because I've got way too much invested in this truck to sell it, but I can't figure out my next move.

what brand of wires and distributor cap have you used? AC Delco are the better parts usually no issues with these parts, anything to do with spark should be AC Delco, also were the engine codes cleared properly?

NGK IX iridium spark plugs, and then the duralast gold Cao, rotor, and wires

For starters change the plugs out with ac delco 41 993. Should come gapped at 60 thousands which is correct.

Ac delco cap d328a and rotor d465.

Plug wires autolite 35-6151.

Use some dielectric grease everywhere that is appropriate. Then clear the codes and see what codes return. Let us know the outcome.

Agree with CWB, except the wires, go with acdelco also
Saying its worse in wet weather means it is moisture causing it. Dampness under cap or wet wires. Have you looked under the hood in the dark for arcing? if not give it a try. It can be a light arc so night time is best

Oh come on Capn' says they are good!:icon_pirate:

I would say ACDelco wires would be the best bet. I replaced mine a few months ago with no issues since. They aren't really any more expensive than any other name brand. I would definitely avoid budget brands for sure.

really who am I to talk, I have MSD 8.5mm wires and accel super coil

Blaster coil myself...:icon_wink:

MAP Sensor
 

My problem was the exact same, i unplugged the MAP sensor and no issues at all, got a new one and runs fine. Map sensor is to the left of the spider injector plugin. It has three wires. Unplug it's wire connector and try to fire it up.