Idles high after being in gear.

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BigRedOne

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That's only for 94 - 95 7.4L which used smaller injectors with the higher pressure. You s/b good with 11 PSI
Haynes states 88-93 7.4l only. 26-32psi. It just seems weird that they would single out the 7.4L. But I have never seen it anywhere else. just in case they figured something out no one else did. Do you think lower pressure would cause the IAC to keep higher counts? I would think it would be the opposite. You?
 
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BigRedOne

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So I got my 6 Pack PROM installed today. (seems to actually run smoother)BUT NO CHANGE!! Uhg. kind of at my witts end. What could possibly be telling the IAC to not lower itself or attempt to regain idle? My brain hurts. Got a few more things to check tomorrow. I'm sure at this point I'm wasting my time but I need to check off some boxes before I start shopping for some dynamite to stick in this TBI crap.
 

PlayingWithTBI

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Haynes states 88-93 7.4l only. 26-32psi. It just seems weird that they would single out the 7.4L. But I have never seen it anywhere else
Don't believe everything you read in Haynes. 3 years ago I bought a 28 LB FPR spring from a 94 7.4L to run my 5.7L at that pressure. Here's a chart from GearHead-EFI showing part numbers and colors

You must be registered for see images attach
 

Sabinoerc

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So I got my 6 Pack PROM installed today. (seems to actually run smoother)BUT NO CHANGE!! Uhg. kind of at my witts end. What could possibly be telling the IAC to not lower itself or attempt to regain idle? My brain hurts. Got a few more things to check tomorrow. I'm sure at this point I'm wasting my time but I need to check off some boxes before I start shopping for some dynamite to stick in this TBI crap.
Well sorry it didn’t help. It seems you either have to live with it, get a different engine, or, if you want to fix it - we’re back to getting to looking at data logs.
If you want to work on your own vehicles, I personally think it gets harder post TBI as computer systems get more complex and manufacturers create proprietary systems for which you have to become a hacker to get into in order to figure out whats wrong (or just take it to the dealer). Eg. I have a 2006 BMW which is much harder to get into the data from engine than the TBI.
good luck on whatever!
 

George T

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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
I have a 1988 C3500 with a 7.4L TH400 and I had the same trouble. It was the coolant temp sensor. I hope this helps.
 

Keeper

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Bad data from coolant temp sensor?

Had same thought last week. It has been replaced. No change.
Piggybacking on George T post above.

No offense intended, because this may come across wrong for someone who obviously knows his stuff... but "it has been replaced" means the sensor feeding the ecm, not the one feeding the dash gauge, right?

Idling voodoo issues seem to always be either vacuum, or coolant temp sensor, in my admittedly limited experience.
 

BigRedOne

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Piggybacking on George T post above.

No offense intended, because this may come across wrong for someone who obviously knows his stuff... but "it has been replaced" means the sensor feeding the ecm, not the one feeding the dash gauge, right?

Idling voodoo issues seem to always be either vacuum, or coolant temp sensor, in my admittedly limited experience.
Thanx for your input. Yes I replaced the ECU coolant temp sender to no result. Actually have replaced pretty much everything to no result. Getting pretty frustrated. But at this point I can't give up. THought I would but I don't have it in me to quit I guess. lol
 

BigRedOne

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Don't believe everything you read in Haynes. 3 years ago I bought a 28 LB FPR spring from a 94 7.4L to run my 5.7L at that pressure. Here's a chart from GearHead-EFI showing part numbers and colors

You must be registered for see images attach
So playing. Where do I find this color code? The injectors I have seem to maybe have been replaced at one time. I have a part # from the top of the injector of 7024 and also RIN-554. it also has 2 red posts. I don't know if you can help me id these or not. Thanx if you can.
 

PlayingWithTBI

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I have a part # from the top of the injector of 7024 and also RIN-554. it also has 2 red posts. I don't know if you can help me id these or not. Thanx if you can.
I can't help you there. You could send them in to be cleaned and flow tested.

 
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