I just aquired a 91 1/2 ton with a 4.3 in it . It just started to idle high (1800-2000) I've changed the plugs,wires, cap and button, map and iac valve, i've also spray'd carb cleaner around checking for vac leaks but found none, also egr seems to be working fine. Any help with this problem would be great.

thanks  

I would actually pull TBI unit and check gasket between it and intake. These are known to fail. I actually had one go bad on my 89 several years ago.  

Thanks, I will try that.  

my 93 k1500 w/4.3 v6 actually did the same thing a few months ago and checked everything out and found that the idle air control valve (IAC),on the pass. side of tbi was bad...after 160K miles,changed it and ran it down the road so the plunger can set itself ,according to destructions and my idle went back to normal range while warming up and reg. idling.  

It could be IAC but I had same problem with a bad gasket. Mine looked and checked good from outside but when inspected it reveled that the gasket was eaten away internally. Furthermore, a short time after that I had to reset idle stop screw too because it seems that due to wear the throttle plates were bleeding more air at idle which was putting it beyond the range of the IAC to control at times. (cleaning and even replacing IAC did not fix it) After resetting it too, which allowed plates to close more, and together with gasket change, I have had no more idle problems and that was about 6 years ago too.  

well guys, i replaced the tbi gasket, old one looked o.k., i try'd messing with iac valve , adjusting it 1/2 turn in either direction. no luck.   now I get a kind of looping idle. Noticed temp only go's alittle passed 100 after running for a while, would that have something to do with it?  

well it could, if the trucks not getting up to temp then the comp will try and keep feeding it gas untill it thinks its up to temp...try the t stat see if its stuck or broke then.  

turning the iac valve one way or the other isnt going to fix it or change idle. you have to replace it cause the spring thats on the plunger is stickin and wont allow for proper idling.  

If you have not already done so, replace the ECT or engine coolant temperature sensor. There is two on the model, one in head and one near thermostat housing. You want to replace one near thermostat. They can go bad with age and cause engine to run rich. If engine is hot or cold, mixture will still be correct if this sensor is good. (one on head is for temp gauge)  

You could also try the intake manifold gasket.I recently had the same problem with the loping idle issue.the idle was very eratic until i drove it for a while and got it up to temp.I took mine to the dealer because i was getting lean bank codes.from the computer.they changed all the gasket's on the intake manifold and all is good now.just a suggestion.as sno-man said this is a very common problem,especially if you have vortec heads/intake.good luck  

well,it turned out to be the throttle position sensor guys, thanks again for all your help.