IAC modifications

[Mark Romans]

Hi Bruce,  I am very intrigued with your explanation of idle instability
etc.  I put the new GM Perf Parts "Hot" LT4 cam in my 87 Vette.  The cam has
218 deg int and 228 deg exh @ .050 thousandths lift, 112 deg lobe centers
and .525 lift with 1.6 rockers and I have had to raise the idle speed to 950
rpm's in an attempt to get the idle to be stable.  It pulls 17" of vacumn at
950 rpm's.  If I touch the clutch or turn the steering wheel or the cooling
fan kicks on it usually dies, even with the idle set at 950 rpm.   The iac
sometimes starts hunting and the idle goes up and down until the iac can't
catch it and it dies.   When I watch the iac counts it is as if the iac
isn't fast enough to catch it and keep it from stalling.  I have the stock
throttle body (TPI) with a Lingenfelter Supperram Intake.  It doesn't seem
to idle that rough, it's just like the iac is too slow.   I have set the
initial spark map up to 25 deg at idle maf range and that helped some too.
But I am kind of at my wits end.  Can you give a better description of what
you did to the iac pintle.  I have heard that there are about 3 different
pintle shapes from GM depending on the application.  I don't know any more
than that.
TIA.    Mark.
-----Original Message-----
From: bruce plecan <nacelp at bright.net>
To: diy_efi at esl.eng.ohio-state.edu <diy_efi at esl.eng.ohio-state.edu>
Date: Saturday, November 29, 1997 9:42 AM
Subject: IAC modifications


>After installing too big of cam in a GM TBI system.  I say too
>big, cause numerous chip programmers said it wouldn't work.
>At one point in time I had a huge problem with the idle speed
>being erratic, and hunting around.  With the scanner I could
>see the IAC varing.  So after much experimenting I had some
>new cones machined for the end of the AIC shaft, as I recall
>they are a 6-32 thread.  Had only about .015" taper for the
>lenght of the cone, and then used an electric drill, and
>file to get the taper right for cold starts, and hot idle.
>While the computer uses the IAC during cruise conditions, I
>never experienced any problems, with cruise conditions with
>the custom taper cones (pintles).  Final cold idle was 1,100
>rpm, and hot idle 650-700, and electric rad. fan has only min.
>effect now on idle speed.  Also, bought a stepper motor
>controller kit, at an electronics store, slowed it down, and
>put a GM connector on it, so I could run the pintle in+out
>on the bench, rather then having to go thru the IAC routine.
>Had a spare TB housing so I could set it, on the bench.
>