[Diy_efi] idle control 1227747

David Allen davida1
Thu Mar 9 05:20:48 UTC 2006

  This can be confusing and I am not the expert but... I did have the same
problem and did finally figure it out!
  I'm assuming this is a newly injected engine converted from carb?
  If the engine had a vacuum leak or if the minimum airflow (idle stop screw
on TB) was too high, the ECM should show the IAC counts at "0" as it closed
the IAC fully trying to bring down the speed.
  Ideally you want the IAC nearly closed (0 steps) when idling at base idle
warned up. This way most of the air enters around the throttle blades and
you get better fuel mixture distribution.
  When adjusting my car, it would not idle down until it was almost
borderline overheated. Under normal conditions, the IAC counts would never
drop all the way to 0 even if I created a big air leak and sent the RPM to
  I had to reduce the values in "IAC steps vs. coolant temp" table. Using a
real-time tuning device I gradually reduced the table across the board until
the engine ran at the specified speed. If the "IAC steps Vs. coolant temp"
values are too high it will prevent the ECM from locking in on the desired
idle RPM constant. To get a high-compresson, roller cam 305 to idle down I
had to reduce the values by about 5 (if I remember correctly). In addition,
the main spark table can have the timing reduced around the idle region to
help smooth and slow the idle.
  Hope this helps, and hope someone with more insight can contribute.

> 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
> Darryl..
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