1991 chevy 1500 5.7 tbi erratic idle

[tpulliam79]

I just purchased this truck and have put new plugs and wires, new iac, new coolant temperature sensor, new 02 , cleaned throttle body,. I am having issues with a jumpy idle especially in park or after sitting at a stop for a moment, no codes and it doesn't die, it did get better after iac and coolant temperature sensor. any ideas of what else could cause this?

[mnstrjim]

Mass air flow sensor, clogged catalytic converters, thing aint breathin right. May check your altenator voltage also. It could be low at idle and higher when reved up, causing mis or week fire to the plugs. Where did you buy it, was there a dramatic change in alltitude?

[gr8shot]

I bought a 454 (in my sig) thats doing a similar thing. I haven't figured it out yet either. I started unplugging sensors to see what would happen. I unhooked the electrical connection to the map sensor (back pass. side above intake manifold, black square box with vac. line and 3 prong electrical connection) and it's running pretty good. It would also run good with the coolant temp sensor unplugged. I haven't had much time to fool with it, but I've also got charging issues, so I'm hoping that it might go away when I solve those.

If you do figure it out, please let everyone here know. This seems to be happening to several people on here, and no amount of throwing parts seems to fix it.

PS, no mass airflow sensor on TBI motors, so thats a no-go.

[cancritter]

have a close look at the tbi throtleshaft for any play...if theres play it can throw your tps "throtle position sensor" off..if you havent replaced the TPS would do so as they wear and your trucks got a few years on it and dought prievious owner did...in anycase is one of them things to watch for with tbi's

[gr8shot]

I'll give it a shot this weekend. Worst case, it doesn't fix it and I take it back and get my money back... I've been reading another thread where the guy replaced his ICM and it fixed a similar problem, but he's burning up ICM's so there's some wiring problems there. It's also worth a shot!!!

[SUPERDAD]

Verify fuel pressure and base ignition timing. These are 2 things that are not controlled by the PCM. Have found several TBI idle problems that were caused by these being out of spec.

[tpulliam79]

well my friend owns a shop and he is going to hook it up and check out some readings and such. so I should have some kind of answer later today. I will post the results as soon as I know. thanks for all the advice and im sure ill have more questions. thanks again

[bowtiedude]

Check your base timing. Check your fuel pressure. Check for throttle shaft play will also create a vacuum leak. Check your vacuum hoses as well.

[tpulliam79]

well thought we had it narrowed to a dirty injector so I hjad them cleaned and now they flow nicely. hooked a smoke machine up. no vacuum leaks. the timing was advaNCED 3 DEGREES BUT HE TOLD ME THAT WOULDNT HURT ANYTHING... i DONT KNOW MUCH ABOUT TIMING , is 3 ok? and how to check throttle shaft? thanks again

[shadowmaker]

3* should be ok, I have mine advanced 4*.

EGR, coolant temp sensor, gasket under the throttle body, TPS sensor, intake manifold leak, the computer, MAP sensor.

I think if you have a TBI truck sooner or later you experience some kind of idle problem. At its a headache to figure out. I ve had one for a few yrs. Truck doesnt run bad, just annoying. I go thru spells of replacing stuff trying to find it but the truck is determined for me not to.

[tpulliam79]

well I am going to do some more work to the truck tomorrow and I want to seafroam it , is there a way to do it better than the brake booster? I read somewhere that I could pour it right down the tbi? is this true and if so how is it done the most effectively? I have never seafoamed before and want to make sure I am doing it the best way possible. thanks again

[tpulliam79]

about to put the seafoam in brake booster unless someone tells me putting it in the tbi is better , and how?

[NEUMANNZZ]

Just slowly pour it into the throttle body, its the best way for tbi. ensure engine is fully warmed up.
did you ohm out the pick up coil and coil.
go here for test:

http://www.autozone.com/az/cds/en_us...rInfoPages.htm

[tpulliam79]

putting a new coil on after the treatment. also pour in tbi while running? and when you say pour in you mean into the same spot the injectors spray in right? sorry I know dumb questions but this is my first tbi experience

[gr8shot]

any luck?

[tpulliam79]

well still no luck... new o2 sensor, icm , iac, tps , coil, cap, rotor ,plugs , wires , coolant temperature sensor, fuel filter, had injectors cleaned, mechanic was unsure... going to have fuel pressure tested in a day or two, I need to take a break from this for a day or two, and no vac leaks, and ran seafoam... I am stumped......the short block is from a 78 blazer but the heads are from the 91 shouldnt make a difference though. really seems like a fueling problem at this point hate to say it but I almost hope my fuel pump is the problem as I am out of ideas.

[tpulliam79]

whats the best way to clean the iac passages ? i need this in laymans terms as my brain is fried. was thinking take out the iac and spray some carb cleaner up there... good or bad idea? thanks also how to remove and clean egr. oh yeah how to test map sensor

[tpulliam79]

bump

[NEUMANNZZ]

just spray out the iac/passages with carb spray, engine off.
same with egr cleaning. to test egr. remove vacuum line and plug the line. run engine to see if theres an improvement.
what happens when you disconnect the est bypass wire?

[tpulliam79]

est bypass wire? is that the timing bypass wire?