Maddies_syn

Hello all, I have a 91 G20 conversion van with a 5.7L that's been having some problems. It has a bad misfire that causes it to buck and stutter going down the road, the idle is way high to the point where I have to fight the idle when braking or sitting at a light, and when at a stop in gear it will almost die then try to surge forward or sometimes it actually will die. I find that if I drive around in drive instead of overdrive it's not nearly as bad.
It died completely going down the freeway last week after it started missing way worse and backfiring through the throttle body. I took it to a shop and they got it running again with a cap and rotor but the original issues are still there.
my first thought was plugs and wires but the ones in there are close to new. I also had a bad oxygen sensor which I just replaced today but with no change (check engine light isn't on anymore though). And there's the new cap/rotor the shop put in.
I have a sneaking suspicion that something may be up with the computer, maybe getting bad info from a, or multiple sensors, but I don't really know. I do know the temp gauge never moves more than a few mm from cold but I think the computer gets it's info from a different sensor.
Does anyone have any ideas what to try next? This van is my home so I really need to get it running right! Any suggestions would be greatly appreciated!

mountainmanjoe

Sounds like a vacuum leak.

Maddies_syn

I went and checked all the lines as best I could and I don't see/hear any leaks but I would like to replace the brittle hard lines anyway tho.

Some updated info. The problem isn't consistent. On a cold morning it runs pretty good. Still wants to die at stops but the misfire is almost non existing when it's cold. Same thing when the engine is cold. But as it warms up or the weather warms it gets progressively worse. I specifically notice that it's at it's worse during low rpm and moderate to high load (is as soon as it shifts into overdrive at around 35mph the misfire gets so bad that the van starts "bucking" down the road)
It does seem like the O2 sensor made an improvement. Especially with the idle speed and there's no more CEL. But I do notice that the misfires are accompanied by more backfires through the carb. It does it pretty much every severe miss now.
so maybe I'm wrong but the fact that it drives fine cold makes me think something computer related. Running decent in open loop but as soon as it hits closed loop it starts acting up. Idk if that's what's happening but it seems plausible. I don't know what to do about it though. I'm thinking about trying plugs and wires anyway even though the ones on there look pretty damn new. If that doesn't work I'm really at a loss

William Kisselstein

Have we checked the timing? It's kind of a pain to get a light aimed at the pointer on these.

Unfortunately it's OBD-1 so it's harder to get any data out of. Did you check for codes?