[Diy_efi] Out of the blue high idle

[Joe Lutz]

The answer to your question is yes.
If you unplug the connector, the IAC valve stays where it is.  So if it is 
too far open, then it will still be that way when you unplug it.  Try just 
unplugging the TPS and see if it idles down.  Check all your vacuum lines 
and your brake booster for vacuum leaks.  You can just pinch off the line 
going to the brake booster to see if that's the source of the leak.  Check 
the pcv valve and line.  Check your throttle valves and linkage to make sure 
the butterflies are closing all the way.
This may sound crazy but you can do this to see if the air is coming in 
through the IAC valve.  There is a small port in the top of the throttle 
body that feeds the IAC valve passage.  Use your finger to plug it to see if 
it idles down or dies.  If it does, then the IAC valve is either letting too 
much air in or not closing far enough to control idle speed.  Carbon can get 
in the IAC passage and cause the valve to not seat all the way.  Use carb 
cleaner to clean out the passage if that's the case.

Hope that helps,
Joe


From: TomQ <zippy at tomquinn.com>
>Reply-To: List for general do-it-yourself EFI talk <diy_efi at diy-efi.org>
>To: diy_efi at diy-efi.org
>Subject: RE: [Diy_efi] Out of the blue high idle
>Date: Thu, 03 Jul 2003 10:55:09 -0700
>
>I unplugged the connector of the IAC (the new one) and it still revs to
>high. Does the TBI only adjust the idle through the IAC?
>
>-TomQ
>
>-----Original Message-----
From: TomQ [mailto:zippy at tomquinn.com]
>Sent: Wednesday, July 02, 2003 6:31 PM
>To: 'List for general do-it-yourself EFI talk'
>Subject: RE: [Diy_efi] Out of the blue high idle
>
>
>Ok I replaced the TPS and the IAC and it didn't fix the problem. What was
>interesting is when I drove it to the store to pick up the parts the idle
>seems to have fixed itself after the motor warmed up a bit (say 15 minutes
>of driving). If I let it cool all the way down (say 2 hours) the problem
>comes back. More ideas? Any parameters I should check with ALDL?
>
>-TomQ
>
>
>When the TPS fails, the new carbon track becomes the "auto zero" point. You
>then end up with a high idle when the throttle is at rest. Replace your 
>TPS.
>
>Lyndon
>
> > Hi all,
> >
> > I have been running a GM '747 TBI system on my motor (4.2L I6) now for
>about
> > two years and it has worked great with no problems at all. All of a 
>sudden
>a
> > few days ago when I started it up it started to idle way up at around 
>2500
> > instead of 800. First thing I thought was maybe there is a leak in the
> > intake somewhere. I looked it over the intake trac but didn't see 
>anything
> > with holes or cracks in it. I did replace two intake caps that looked
> > questionable but that did not fix the problem. I am now getting an error
>21,
> > throttle position sensor too high. I am looking for advice on how I 
>should
> > start to tackle this problem.
> >
> > TomQ
>
>
>
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