Should it run with IAC unplugged? 04 gt

Hudson N

Member
Aug 7, 2018
50
3
8
Queen creek Arizona
I've been having surging issues for a couple of months now. I replaced my IAC and checked for vacume leaks and tested my TPS. The surge is only slight. Maybe 25-50 rpm when its warm and on a real cold start it surges quite a bit for about a minute before smoothing out. I've done a reset and I've never messed with the throttle set screw. But I heard somewhere that the car should die when I unplug the IAC and mine doesnt. Seems normal since the IAC should just add air and the set screw should allow just enough for an idle. So do I have a vacume leak somewhere? Only vacume line I haven't checked is to the charcoal canister in the passenger fender. Can I just cap that line off and see what happens? Thanks and sorry for multiple questions in one post.
 
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wmburns

SN Certified Technician
Aug 14, 2009
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But I heard somewhere that the car should die when I unplug the IAC and mine doesnt.
^^^^THIS says that your IAC isn't really doing anything.

IF your idea is to set the idle set screw just high enough to allow the motor to idle, THEN there won't be any adjustment left to allow the IAC to do it's job.

Troubleshoot IAC idle problems 1996-2004

Don't overlook the role that a loose or worn butterfly linkage can play in an idle surge problem. Or an incorrectly sized butterfly air bleed hole can play in a surge situation.

Do you have an ODB2 scanner that is capable of monitoring operational PID's? It would be helpful to "know" what the IAC duty percentage is.

Yes you can cap off the EVAP line to rule out a vacuum leak in the EVAP system.
 

Hudson N

Member
Aug 7, 2018
50
3
8
Queen creek Arizona
^^^^THIS says that your IAC isn't really doing anything.

IF your idea is to set the idle set screw just high enough to allow the motor to idle, THEN there won't be any adjustment left to allow the IAC to do it's job.

Troubleshoot IAC idle problems 1996-2004



Don't overlook the role that a loose or worn butterfly linkage can play in an idle surge problem. Or an incorrectly sized butterfly air bleed hole can play in a surge situation.

Do you have an ODB2 scanner that is capable of monitoring operational PID's? It would be helpful to "know" what the IAC duty percentage is.

Yes you can cap off the EVAP line to rule out a vacuum leak in the EVAP system.
When I unplug the IAC my rpms drop to less than 500. My IAC is operating. I figured that it should idle with it unplugged just wanted to make sure. My TB linkage is fine.
I dont have a scanner capable of monitoring the IAC's duty cycle. Any way to test the IAC with a multimeter?
 

Hudson N

Member
Aug 7, 2018
50
3
8
Queen creek Arizona
Another issue I'm having that's in the checklist is my charging system. My battery light is on, but my alternator is putting out charge, it charges at about 13.7v. Is that adequate or is my alternator not putting out enough. My battery is brand new. All my grounds look good as well, and my car passed the voltage drop test.
 

wmburns

SN Certified Technician
Aug 14, 2009
5,811
499
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Houston Texas
My battery light is on, but my alternator is putting out charge, it charges at about 13.7v. Is that adequate or is my alternator not putting out enough.
I actually have some recent real world experience with what happens when an alternator charges at the low end of "acceptable".

In my case I found batteries going bad after 6-16 months. I looked at the charge voltage and found it on the low end of "acceptable". I did an alternator rebuild which got the voltage above 14 volts and no more short lived batteries. I believe what was happening is the battery was slowly being discharged and never got enough of an "equalizing" charge to clean off the plates.

Howto perform charging system voltage drop test

I think you may have more than one problem going on. 13.7 volts (if accurate) is enough to turn off the battery charge warning light.
 

Hudson N

Member
Aug 7, 2018
50
3
8
Queen creek Arizona
I actually have some recent real world experience with what happens when an alternator charges at the low end of "acceptable".

In my case I found batteries going bad after 6-16 months. I looked at the charge voltage and found it on the low end of "acceptable". I did an alternator rebuild which got the voltage above 14 volts and no more short lived batteries. I believe what was happening is the battery was slowly being discharged and never got enough of an "equalizing" charge to clean off the plates.

Howto perform charging system voltage drop test

I think you may have more than one problem going on. 13.7 volts (if accurate) is enough to turn off the battery charge warning light.
I had an autozone test it and they told me it's a bad voltage regulator. I dont understand how that could be if the alternator is charging at 13.7. Would you reccomend that I go ahead and change the alternator? I dont want to spend 260 dollars on a new alternator just to find that there was an issue somewhere else.
 

wmburns

SN Certified Technician
Aug 14, 2009
5,811
499
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Houston Texas
I had an autozone test it and they told me it's a bad voltage regulator. I dont understand how that could be if the alternator is charging at 13.7. Would you reccomend that I go ahead and change the alternator? I dont want to spend 260 dollars on a new alternator just to find that there was an issue somewhere else.
If you want to know for certain then remove the alternator from the car and have it bench tested. The alternator is stupid easy to remove on the GT.

If the cost of a new (or rebuilt) alternator is too high for your taste then rebuild it yourself. There's information inside the link on how to do it yourself.
 
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