[Gmecm] creepy idle

[Ryan Hess]

Maybe a bad ground on the CTS?  Does the  coolant temp jump at all when you shift into/out of park?  If the  commanded rpm changes, then either it thinks it's colder than it is, or  something is causing it to raise it (PS switch, AC switch, P/N  switch)... Or there's some erroneous value in your idle rpm table.

Steve Ravet <Steve.Ravet at arm.com> wrote:  My Blazer has developed a new problem over the last few weeks.  While
idling in gear the ECM commands the idle up and up, opening the IAC
further and further.  Fully warmed up the idle in neutral may be
commanded to 580 or 600, with an IAC count around 10.  This is normal.
Put it in gear and the commanded value creeps up:  600, 625, 650, and so
on until it stops at 737.  The IAC keeps opening in response to the
commanded idle, and the idle itself continues to rise.

At an IAC of about 80 the idle tops out around 700.  This takes about 30
seconds.  But the ECM insists on 737 so the IAC keeps opening up to 180
or so.  At this point the truck is straining against the brake.

If I shift to neutral the engine revs to over 2000, and generally the
commanded idle will then drop to 600 or so, and the IAC counts decrease,
slowly bringing the idle back to the lower commanded speed.  But, if I
stay in gear then nothing changes, the IAC stays open and the idle
always strains agains the brakes.  It feels like a carbed car does when
it's stuck on the fast idle cam.  With my foot off the brake the wide
open IAC will keep the car rolling at 40 mph or better on a level road.

On the scan tool I've observed the following things:

no trouble codes.
battery voltage is 12.3 to 12.8
coolant temp is 190-205
TPS voltage is around .43 with the throttle closed.
VSS is connected and works correctly.

Other things the manual suggests checking:
MAP -- haven't done this yet
rich/lean -- wasn't watching BLMs while driving but the manual says
rich/lean is less than 115/greater than 150, and I'm pretty sure it's
within that.  I've never seen the BLM that far from 128 except when my
cat got plugged up
leaky injectors -- Had leaky O-rings, but replaced them and rebuilt the
TB a couple weeks ago with no change.  Problem was present with leaky
injectors and is still present.
IAC -- I haven't been through the chart, don't have the IAC driver or
noid light, but I think the IAC is OK.  I cleaned it up while rebuilding
it, and it must be moving because I see the RPM changing along with the
reported IAC step count on the scan tool.
P/N switch -- this is broken, as I discovered while troubleshooting the
idle problem.  It reports drive regardless of the shifter setting.  I'll
fix it, but it doesn't seem like a likely source of the problem.  The
problem happens in drive, which is what the switch reports.  The switch
is only wrong in park, which is when the idle works correctly.

The manual has other suggestions like PCV and EGR which I don't think
apply because the idle is smooth, it's just being commanded too high by
the ECM.  So the question:

What else, besides the broken P/N switch, could be causing the ECM to
raise the idle so high?

Now that you've read all of this (and thanks), I should tell  you what
the engine is:

TBI 350 from a '91 Caprice, installed in an S-10 Blazer.  This swap is
going on 9 years old so the installation bugs were worked out a long
time ago.

thanks for any suggestions,

--steve


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