[Diy_efi] idle control 1227747

Rick McLeod dunvegan
Thu Mar 9 16:43:16 UTC 2006

It sounds as if Darryl is not setting up the base idle correctly, the idle stop screw w/ the IAC disabled (cable removed) and the pintle completely extended to close the IAC air passage should have the idle stop screw setting idle speed to about 500 as stated by Bret, then the IAC can take over and it's function is to raise idle above mechanical and control idle at decel to prevent lift-stall tendency, IAC can't reduce idle below mechanical, only increase above mechanical, AFAIK from what I've read and experienced.
  One other item I've not seen mentioned, what is the VSS, as 

The idle control logic will not lower the engine speed whenever:            The vehicle speed is > 0 mph (hardcoded @ 0xF372), -or-          The tps% is greater than the value at: 0x5C6 (* 0.3906)

This means that the vehicle must be absolutely still for the idle control  logic to reduce engine speed. Else, the steps are held where they are. Of  course, the throttle follower and stall saver are still in effect.
  If you haven't read the IAC white paper http://diy-efi.org/gmecm/papers/747iac.txt then do so, it is very good, well written, and I think would anser some of the questions flying here.

Bret Levandowski <skishop69 at yahoo.com> wrote:
  Iac steps vs coolant is extra air for higher idle on cold start. At 56 deg C and higher you should have no more than 24 counts on the table. (this will actually vary motor to motor and should be slightly less for yours) The motor build/size dictates counts below that temp. This table will have some effect on idle. You want to use the target idle RPM vs coolant temp table. Set your desired idle in this table. If you're still idling too high and IAC counts are too high, check your ECM constants table and make sure that the % TPS to enable open loop idle is at least 1% higher than your ACTUAL TPS reading at idle, otherwise it's trying to idle in CL. ( which from what I've been told is a no - no) Make sure your 'base idle is set correct. Unplug and remove the IAC. Fully extend the pintle and re-install it in the TB (I'll save everyone the trouble...I'VE NEVER SEEN THIS DAMAGE AN IAC) I've had people chastise me for this before ;p  Anyhooo... Be sure to warm up the vehicle before
 starting this procedure. After installation, don't plug the IAC in. Start the vehicle and monitor the idle. It should be set around 500 rpm (depending on cam specs) with TPS anywhere from .4v to .7v. Shut it off, plug in IAC re-start and hopefully you're at target idle. If not....      Ski

Darryl M Gilbert <dgilbert78 at juno.com> wrote:   Hello: 1227747 355cu in SB Chevy. Trying to figure out how to get idle
down to the idle speeds in the target idle vs coolant table. Currently
ECM wants to keep the engine running approx 1100 RPM, with 80 IAC steps.
Temp is at 171 deg F. TPS reading .51 volts at idle, but Idle speed will
not come down to target idle table speeds?? Is there a switch that tells
the ECM when to invoke the target idle table?? I think IAC steps vs
Coolant is for Fast Idle when cold?/ Any help will be greatly appreciated
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