Idles high after being in gear.

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BigRedOne

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Hello,
So I have a 1989 c3500 7.4L with a 3sp th400. It has 116k on it. It is set up as a wedge ramp single car hauler I use for personal vehicles. I am having a very unusual problem. It starts in park and idles perfect at 750rpm. If I go into gear it idles down to around 625rpm and then slowly starts rise up to about 1100-1200rpms. when I shift back to park it will flash up to 1700rpms then slowly over a minute settle at 1200rpms. If I shut it off and restart it. It idles perfect at 750 until shifted into gear again.
I have done these and replaced the following:
1. IAC and have done the relearn many times while diagnosing and disconnecting the batt.
2. TPS set to .55
3. rebuilt TBI
4. New base gaskets. (both of them) The BB has the base coolant chamber to help warm up the TBI in cold weather
5. I have done the base idle set procedure. I have set it at 500rpm and it still ends up at 1100-1200 I have also set it to 625rpm and it still settles at 1100-1200 RPMS.
6. Replaced the egr control module. The egr valve tests fine.
7. new fuel filter
8. Cap rotor plugs and ignition module. Wires are MSD less than a year old
9. Capped off IAC and it Idles fine at set base idle. So I don't believe there is a vacuum leak.
10. new pcv
I am down to thinking It has to be something in the ECU or Prom? Telling it that it should be at that Idle. I don't want to just throw more parts at it at this point. It runs great down the road but at a light it starts to get warmed up because you're fighting that high idle with the brakes. Also it revs up to 1700 when you stop and park and its a bit embarrassing at a car show. Everything I have listed that I have replaced was a neglected part from the previous owner. I've had the truck about 5 mos. So I didn't mind replacing those things and each one made the truck run smoother but never fixed the idle issue. I've several friends that are gm mechanics and they just scratch their heads like its possessed. lol
I have no problem spending the money to make it right just don't need to waste money on nonsense. I have checked everywhere I can think of for vacuum leaks and there is none.
So I'm down to ecu? Or PROM? Which one has the idle data? If the PROM carries the idle info, could it have got corrupted? If not could the ecu just have gone bad? Shorted?

Oh and also was using a OBD1 scanner today and it idles at 1000rpms in park or gear with the scanner hooked up? Still flashes to 1700rpms when going from gear to park.
you see the IAC counts changing and hear the idle change accordingly.
Unhook the scanner idles at 750 in park again. Weird...
Also the p/n switch shows its working in the scanner.

Any info would be greatly appreciated.
Thanx
CJ
 
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PlayingWithTBI

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Unhook the scanner idles at 750 in park again. Weird...
Your scanner probably has the 10K Ohm resistor which was used on older TBI and TPI engines. There s/b a setting or jumper which will remove that resistance. Fix that 1st then see what your IAC counts are. Is your high idle solenoid working properly? When warm, it should have 12V constantly at the pins.
 

Schurkey

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Yep. The scan tool is telling the computer to increase the idle speed (and perhaps some other items--ignition timing, maybe. Been so long I've forgotten what is up with that system.)

On my old MTG2500, there were choices when connecting the tool. C & D (Codes and Data) got you the increased idle, C & D (Road Test) got you normal performance.
 

Schurkey

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THANK YOU for that link.

I copied 'n' pasted the relevant text into a text document and saved it for posterior.

I'd forgotten most of that. The only thing I remembered was the flashing MIL denoting both rich/lean and open/closed loop.

God bless real scan tools, that made all that crap obsolete.
 

BigRedOne

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Your scanner probably has the 10K Ohm resistor which was used on older TBI and TPI engines. There s/b a setting or jumper which will remove that resistance. Fix that 1st then see what your IAC counts are. Is your high idle solenoid working properly? When warm, it should have 12V constantly at the pins.
Not sure what the "high idle solenoid" is. I have an IAC. and there is no "idle stop solenoid" if that is what you are referring to. Am I missing something else?
I borrowed the scanner so I will have to get it again and see if it has a setting or jumper to retest.
 
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