bmfIROC-Z

iTrader: (1)

i just purchased a 88 silverado 350 tbi and it runs awesome except @ idle , symptoms:when i start it the idle surges from 400-700rpm most of the time i have to feather the throttle to keep it running, if i idle it up to about 2k rpm for a couple minutes and slowly let off the throttle it will idle at about 700rpm,but if i just let the surge work itself out the the idle will go up to about 1200rpm, it wants to die at stop lights,ive went thru all the vaccum lines,pulled the throttle body off and cleaned it and replaced tb gasket,ive tried 2 different iac's, nothing has made a difference,like i said before anything above an idle and it runs awesome,no check engine light,great throttle response and acceleration,it just will not idle right and its kind of a pita,i dont have alot of money right now so i cant take it to the shop or replace a bunch of stuff hoping to fix it,so has anybody else had this problem? are there some steps i could follow to accurately diagnose this problem? and i was also wondering if i can adjust the tps and iac on a tbi engine like the tpi engines?this is my work truck so i need to get this fixed and any help would be very much appreciated,thanks in advance.

RFmaster

There are couple possibilities that might be causing idle instabilities.

1) dirty or sticky IAC
2) Leaky EGR valve
3) TB to intake manifold gasket leak.

//RF

bmfIROC-Z

iTrader: (1)

i just put in a 3rd(brand new) iac and nothing changed,i checked the tps voltage and it was .57v @ idle and 3.97v @ wot so i think its ok ,am i right?,ill check out the egr tomorrow,any other suggestions guys?

bmfIROC-Z

iTrader: (1)

any more suggestions?i really need to get this fixed,please help somebody.

RFmaster

I went back and re-read your original post. Since you have tried replacing gaskets and IAC we'll have to assume that they good. Couple of other possibilities.

Did you try to reset IAC after replacing it with a new unit? - see TGO HOWTO technical article. ECM has no clue were IAC pentacle is! It just commands number of steps and hopefully IAC reaches that relative position. So, resetting IAC is mandatory after replacement. Another possibility is that one out four control lines connecting ECM to IAC is broken - open circuit. Ths will prevent IAC from moving. Get old IAC and see if it moves (warning: pentacle may fly off rendering IAC inoperative)

A stuck open EGR circuit will cause rough idle. For this round I would get a block off plate just to eliminate EGR circuit out of the equation. Any good parts store should have it. Alternatively, remove EGR and clean thoroughly intake manifold passages and EGR valve stem. Carbon build up may prevent EGR from fully closing causing intake mixture dilution at idle - erratic idle, etc.

I'll assume that timing was set correctly, your ignition system is good shape, spark plug wires going to correct cylinders and sparks are not fouled-up.

//RF

bmfIROC-Z

iTrader: (1)

thanks man at least,one person was tryin to help,i decided to pull the engine rebuild it and put a carb on it cause tbi sucks.