had truck since new 278,00+miles retired truck put plow on it 2+ months ago,ran fine then. put anew battery in it a few days ago and it would not idle? had to keep my foot into it to keep it running.very!rich on gas ,burn your eyes.tuned up, replace pick up coil ect... did not work. had a used TBI on the shelf and out it on,seems to work fine except that when you put it into gear with your foot on the brake the idle creeps up and up until IAC is at max.i put it into park the idle stays at 1800+rpm will not come down untill i shut the truck off and restart it then is ok??? i replace the IAC and still the same?? and ideas?
thanks john

In general, if the ECM is closing the IAC and the idle is still high, there is a vacuum leak somewhere.

Running rich can be fuel pressure (9 - 13 PSI for TBI) or a bad Engine Coolant Temp Sensor.

Also check the MAP sensor. It should hold a vacuum and make sure the vacuum hose to it is in good shape. On my 90 GMC K1500 (I bought it new in Nov 89), the rubber end of the plastic vacuum hose to the MAP was bad and would not hold a vacuum causing a rough idle.

How does the Injector spray pattern look? It should be a nice cone shape without dripping.

//2000CAYukon

spray pattern is good on injectors.fuel pressure good .coolant temp 190-198. the thing about a vacuum leak it runs fine untill i put it in gear then the IAC opens slowly. when i put into park take foot off brake the IAC stays and idle does not come down.going to recheck for vacuum leak,just do not understand no high idle untill i put it in gear.i tried just holding my foot on the brake and that does not make the idle go up??

That is puzzling. Sounds like you have a scan tool. What does it show for throttle position when this happens?

Normally, opening the IAC will increase the idle (kind of like a controlled vacuum leak). If you unplug the TPS (it will set the check engine light), does the idle increase when in gear?

//2000CAYukon

going to do more checking tonight,will check TPS votage also,have snap on solus.seems to me voltage stays the same unless i press throttle?..not totaly sure. could it be possible ECM??

Yes the ECM could be bad. I would make sure that the TPS and MAP sensors are good first. For the TBI engines, there are only really 4 sensors that the ECM uses to control the engine: MAP, TPS, Knock and 02.

Also a good idea to check the ECM and 02 grounds at the intake manifold.

//2000CAYukon

Also check to make sure that a vacuum hose isn't cracked and only leaking once flexed from the rocking of the engine when going into gear.

new issue,came home from the garage..sad mechanic by trade ..mainly deisel/heavy equipment, truck in the garage for 4 days now. it would not start?? just a quick check put some gas in carb..crank engine..no start?? tells me no spark,went wiggle and check connections several times and it started????running rough then kinda smooths out it will idle got better iac and tps seems to be fine now the knock sensor says yes on scanner..i diconnect sensor still says yes and no check engine light?? changed ecm and prom chips still the same..o2 sensor says rich most of the time..going back to check wiring now.will keep postesd
any ideas? thanks

got a new delima..little more detail. originaly ..i have of my own a mt2500 snap on scanner dated to 1997. at work we have snap on solus dated to 2006 i had my scanner on it and it did all i said in early tread but knock sensor said no knock?? tried several times idle went up only when put into gear and brake on. i hook up solus and imediatly when started the knock sensor says yes... but no idle increase when put into gear with brake on??? but in both cases it still idles ..a litle rough but not bad. got to call snap on on this tomorrow about issues. but right now what still baffles me is no start tonight? will try again tomorrow. wierd all was ok the last time i run the truck..

I have not used a MT2500; however, it should show you knock counts. Perhaps you can post the values that you are seeing for INT, BLM, O2, KNOCK, TPS and MAP.

Also, on my 90, I had a rough idle that turned out to be a bad seal around the PCV valve.

//2000CAYukon

snap on mt2500 is just older verision , solus is snap on newr version of scanner .has more features than 2500 . am looking for a schematic on ecm wiring to IAC,TPS,02,Knock sensors circuit . going to call snap on to find out why both scanners have different readings

snap on mt2500 is just older verision , solus is snap on newr version of scanner .has more features than 2500 . am looking for a schematic on ecm wiring to IAC,TPS,02,Knock sensors circuit . going to call snap on to find out why both scanners have different readings

Check this site , It looks like the schematics your looking for:
http://www.autozone.com/az/cds/en_us/0900823d/80/08/79/51/0900823d80087951/repairInfoPages.htm

Hope it helps

found out something...!! thanks for link to wiring scematic!! found a new item..to do with circuit....and i think it solved my problem. the ESC which i had a spare on the shelf. seems the knock sensor goes through the ESC (electronic spark control)before the ECM to control timing,if it sences spark knock it retards timing...ect... you read earlier treads.. i replaced ESC and it seems all is fine!!!!! great!!!!!!! you fill in the rest..at napa this is a $100+ item..boy i am glad to save old parts...and most of all web sites like this!!!!!!!!!!!!!!!!!!!!
thanks again