So I have been having this wierd miss at idle for a long time on my 94 GMC Jimmy 4x4 VIN:W. Fuel pressure great. Over the last year i replaced the spider inj, complete distributor, cap, rotor, plugs, wires, tested map, iac, tps, and coil. Fuel pressure holds great

Pretty much the story is if you let my truck idle it does great for about 5 seconds then will miss every couple seconds. Sometimes makes the engine stumble then catch back up to itself. Can casue a minor lunge if stopped at a light. Temp doesn't matter. If I put very light throttle to it (5%) while idleing it revs up then comes back down and missed real bad. Anything under 25% throttle it will do the same thing but the miss gets less and less with more throttle. This is not noticeable while driving down the road. getting 17-18 mph.

My most recent discovery was when i set timing (at 0). With the Tan/Black plug unplugged it doesnt skip and the idle stays pretty steady, alot more steady than normal. Then plug it back in and start it up and it starts with the same crap again. I watched the timing advance, it will advance untill it skips and pulls back. And keeps repeating that. It misses when the timing mark is almost at the very top. IDK how advanced that is.

Pretty much looking for any pointers. I'm thinking about checking the knock sensor, IDK how maybe someone can help with that too. The truck has 222,000 miles on what i'm assuming is the original engine. Most things are still stamped GM. I'm looking for another computer at a junkyard to try just to rule that out.

 

I had a similar issue with a bravada I had, turned out the fuel hose that enters the back of the upper plenum was cracked inside the intake, it would spray extra fuel causing erratic idle in every cycle except wot. Might be worth checking, I never put a timmg light on mine that I remeber, but its guess.

 

thank you for the input but my fuel pressure doesn't leakdown and as of last time the upper intake was off there was no indication of a leak there. If only it were so simple. I parked the truck in exchange for my Celica for a while to save some gas $$$. So my progress has slowed and I also have a v8 swap to put my attention towards!

 

So, I changed the o2 sensor with another sensor that came off my v8 swap project (its not using it anyway). The old O2 sensor was all grey and ashy looking, I'm thinking its running lean. I'm gonna double check fuel pressure while running and go from there. Vacuum pressure seems ok but i'm gonna check with another gauge.

 

found out my "miss" is more than a miss, it actually sucks air in the exhasut pipe. I'm thinking it has a burnt exhaust valve from running it so long with my leaking fuel regulator in the past. Possible worn valve spring but idk, after my v8 project i'm just gonna pull the heads and take a look.