1986 K10 5.7 missing @ idle

[Jays86]

Hey everyone, I'm looking for some advice on where to start with my new truck. I'm almost wishing I picked up a pre-smog truck, but it is what it is, and I need to at least fix what I have. I have an '86 K10 Silverado with a 5.7, and I'm having a missing issue at idle and it seems to be running hot at idle, especially with the a/c on. The truck has already passed smog, and by looking at the numbers, it looks like it passed fairly easy. I've worked on cars for over 20 years, so I'm fairly mechanically inclined, just not as knowledgeable with Chevy's as I am with others. The only non original part I see on the engine is a mechanical advance MSD distributor, which must have been overlooked during the smog. The engine runs rough at idle and the idle fluctuates a bit. The idle was quite high the other day, 900+ rpm's in park, and would fluctuate from 900-980. Then all of a sudden the check engine light came on, the idle smoothed out perfectly, but then went back to hunting up and down again. I did check the #1 plug, and it's definitely lean! I have a huge stack of receipts showing that literally everything on this truck has been rebuilt. It hasn't been driven much, and with all the paperwork and odometer readings, there's about 10,000 miles on the engine. Where should I start? Ditch the MSD for a factory HEI? Have the carb rebuilt including rebushed throttle shafts to make sure there's no leaks there? Could the ECM be part of the issue? I actually had no idea this year of truck even had an ECM! I also read something about this truck may have an electronically controlled mixture? Is this true? I'm also assuming that means there is no way to manually adjust the carb? Sorry for the rambling, it's not as easy for me to type out questions, as it is to talk to someone over the phone to ask questions lol. Any help is greatly appreciated!

 

[Jarhead79]

Is it tbi? An issue I had with my 92 tbi was a hunting idle and what sounded like a miss. I replaced. My aic, tps, temp sending unit, and numerous other things, when none of that fixed the issue, I was exploring the tbi itself. Noticed when my idle dropped one injector would not spray as well as the other one, after almost a year of hunting things down and changing parts, turned out it was just a bad injector is all. If you have a carb, I'd check your timing and adjustments, especially if it's a quadrajet. I had a lot of tinkering I had to do to get my 79 350 times well and adjusted. As far as running hot, check your hoses and heater core, make sure you don't have air in the system, and that you're fluid isn't all nasty, and your thermostat is good.

 

[Jays86]

No, it's the last year for the carb. The carb was supposedly rebuilt, but it wasn't done properly. I took it to a local guy who fully rebuilt it, added a new float, and rebushed the throttle shafts. Runs awesome now, but it falls on it's face a bit off idle with the A/C on. I have to look into that next.

 

[1987 GMC Jimmy]

Well, I have the same year carb, but it's the computerized E4ME not like the truck one (not computerized). I know mine has a dashpot or idle stop solenoid which, to the best of my knowledge, is supposed to help with run on, and it's supposed to kick the idle up a little when you cut the air on. I haven't seen an '86 truck with a Quadrajet in like ten years so I don't remember if it had one or not. It looks like this and bolts into the carb body right in front of the throttle arm. Maybe see if it's missing?

 

[Jays86]

Well, I have the same year carb, but it's the computerized E4ME not like the truck one (not computerized). I know mine has a dashpot or idle stop solenoid which, to the best of my knowledge, is supposed to help with run on, and it's supposed to kick the idle up a little when you cut the air on. I haven't seen an '86 truck with a Quadrajet in like ten years so I don't remember if it had one or not. It looks like this and bolts into the carb body right in front of the throttle arm. Maybe see if it's missing?

Awesome, thanks man! I'll look today to see if it's there, it seems to run perfect since having it rebuilt the right way, it just has that off idle stumble when the ac is on. It's been brutally hot here lately, 100+ for the past week, so I'm not going to run the truck without the ac on! lol