Bad IAC now? Won’t idle down with IAC unplugged and ALDL jumped

[Harold Crumbie]

So after a tbi replacement and doing the idle setup all was well for about 100 miles.

Then it started dropping waaaay low at idle sometimes then normal then low again. Basically had no brains about idling.

I decided to do the idle setup again. So I put in the ALDL jumper, disconnected the IAC connector and fired it up. 1500 rpm with the idle screw backed off. Won’t go lower. I’ve checked or replaced all the vacuum hoses.

I’m stumped again and really grouchy at this truck.

 

[67GMC]

Do you have access to a scan tool? I would be thinking about the O2 sensor. It could be trying to go into closed loop but doesn't like it and then goes back to open loop as it can't get the mixture right. I've got an MT2500 (the red brick). Had a similar issue. No SES light on but my truck was staying in open loop. Was better once it was warmed up but it ran crappy off the start.

 

[someotherguy]

Sounds like you disconnected the IAC with it all the way open. Once it's disconnected then jumping the ALDL terminals can't affect it (obviously) and you sound like you've successfully gone through the IAC relearn previously...

So either you've got your order of operations wrong, temporary brain fart ya know? Or the IAC is indeed bad.

Richard

 

[industrialfish]

The idle screw won't do anything if the IAC is open too far.

Just for clarification: You jump the pins, turn the key on (not starting it) wait 30 seconds, then unplug the IAC, turn key off, remove jumper, start truck, adjust idle, turn truck off, plug in IAC.

 

[PlayingWithTBI]

So either you've got your order of operations wrong, temporary brain fart ya know? Or the IAC is indeed bad.

^^^What he said. In the calibration, if you have the IAC connected and you jumper A & B while idling it will raise your idle to whatever the setting is, usually 1000+ RPM. Here's an example set @850

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[Harold Crumbie]

^^^What he said. In the calibration, if you have the IAC connected and you jumper A & B while idling it will raise your idle to whatever the setting is, usually 1000+ RPM. Here's an example set @850

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I really do need to get an ALDL connector and some software I'm all set for OBDII, but not the old ones.

 

[Harold Crumbie]

Well... I did this at lunch today

Just for clarification: You jump the pins, turn the key on (not starting it) wait 30 seconds, then unplug the IAC, turn key off, remove jumper, start truck, adjust idle, turn truck off, plug in IAC.

No change. I got a fresh base gasket to put in this evening, maybe it's bad and getting air from underneath?

 

[PlayingWithTBI]

Just for clarification: You jump the pins, turn the key on (not starting it) wait 30 seconds, then unplug the IAC, turn key off, remove jumper, start truck, adjust idle, turn truck off, plug in IAC.

That's correct.

It'd be nice if you could get some real time data to see the IAC steps and tell if it's actually working right before you decide to replace it. Yes, a vacuum leak can cause your issue too.

 

[Harold Crumbie]

Well. That was a waste of $9.19. Sigh.
So. When you do the process above does it command the iac to extend all the way out? Wondering if I stopped at that point and pulled the iac would it tell me anything?

 

[PlayingWithTBI]

Wondering if I stopped at that point and pulled the iac would it tell me anything?

How about unscrewing the IAC, keep it hooked up then short pins A&B, turn on the key, and see if it extends? I don't think you need an external ground according to this schematic for an 88

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