92 454 Fire rescue truck, idle issues

Im working on a 1992 3500 gm with a 454 bbc. My problem is it will not idle down after warm up, it stays at a constant 1100rpm. I have checked for vacum leaks, another shop has replaced the egr valve, I cleaned the IAC motor, checked the timing, set the tps, and even checked the throttle linkage. Also the coolant temp sensor appears to be in range. There are no trouble codes, I'm startint to get a little discouraged, there isn't muck else that pops to mind that I can thinmk of. it is a fire rescue unit, but as far as engine work done to it, I don't think there is any beyond pcm calibration. If any one has any good Ideas please feel free to let me in on them, any help is appreciated.

Check for a tiny stone caught in the recess at the end of the throttle shaft. Seen it happen many times.

Had the same problem with my 1989 1500 5.7 Pick up . I had a bad TBI Gasket . They start to suck in and leak vacum at the rear . Now it idles real low .

The throttle shaft is clear and returning all the way, there is no play in it. I took off the TB and replaced the gasket. It still won't drop down to a proper idle. Has anyone heard of a PCM malfunction causing this? The idle does not fluctuate and it drives fine up the road. Also, is there any way to test the IAC motor? It may be seized, the truck only has 20,000 miles on it, it rarely gets used.(very low miles for a '92 lol ) I'm bangin my head against the wall, I don't like throwing parts and money at vehicles till they are fixed.

When you are looking down at the Idle air speed control motor . Have some one turn the key to the run But Do not start it . You should see it move to open . Some times if you have a bad ECM Ground . This can make the engine not run right . Also many times even with no codes a bad MAP sensor can do this . It will tell the engine it has a vacum leak . When it dose not .

I am having the same problem on a 89 3500 454tbi motor. I checked every were for vaccum leaks found none .I have changed the tps and iac and still nothing idles about 1100 rpm.I have put a snap-on scanner on it and all sensors look to be in the right parameters . I even ran the iac test with the scanner and it works fine.Any ideas would be appreciated. Does the spark conntrol on these have anything to do with idle cause I did notice it has ESC ignition and I dont know much about them .Also could it be a bad ECM I know Chevy had alot of trouble with there early systems? I am lost!
Thanks Brian

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Originally Posted by DennyW

My other thought on this is, is your thermostat actually getting up to the 195º it is supposed to. If it's running cooler than that, it will cause this also. :)

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This is true check on your Scanner were the engine temp is . If you have a bad temp sensor that is reading a lower temp then what the engine really is . This will tell the ECM to open up the Idle air valve more . Also unplug the ETS plug. check it to see if it's not all green . Again I did have a bad ECM ground on a 4.3 one time and it would also not idle down .But after I repaired it . It idled fine . If all fails the ECM may be bad .

If your running a pump has someone left the hand throttle turned up some? We have looked for days trying to trouble shoot one of our older gas pumpers and someone had left it turned up. Just a thought.

check here TBI Tuning (87-95 / OBD I) ECM/PCM - FSC Forum

I'm for checking the pump setting.

Stupid stuff like that tends to bite ya.

Once, when selling a lawn mower which had been utterly reliable for about 8 years, it wouldn't start. Quite embarassing really. In the process of jumping it off, realized the battery wasn't dead. Apparently, you need to press the brake while starting it. I'd been doing it without thinking about it for years, then forgot all about it.

Another time, I had been driving a parts van for several days, as an appliance technician. Then it just wouldn't start. I called up the boss, and he tells me to mash the brake while I turn the key. Bingo.