[Diy_efi] idle control 1227747

Thu Mar 9 06:39:28 UTC 2006

> David: Ok on all. Yes I am just playing with the table you mentioned.
> >From what I understand that is suppose to be the FAST IDLE table, then
> when temp gets to a particular point the TARGET idle table is suppose to
> be invoked, but its not really clear how it all works.

  I'm not sure how the actual code in the ECM does it; but from playing with
the parameters in real-time, it's apparent they both interact and are in
effect at any temperature. If there were a magic temperature where the
operation switched from one to the other table, there should be a setting
somewhere to change it.  I looked and looked for this and never could find
it.
  It seems like the ECM uses the Counts value at startup and once the engine
idles down to "close to" the Target RPM value, it will "lock in" and begin
to regulate to that value. But the Counts have to be low enough for it to
get "in the ballpark" before it can lock in.
  With my boat, I had to put some really nasty numbers in there to make it
idle down even though it never gets above 140 or 150?F. It has "22" in all
positions but the 4 coldest; and it will idle down and lock in at 550 RPM no
problem. (with crash-box stern drive this is important)

> No matter how I
> play with the idle adjust screw the ECM wants to attempt to keep it
> running up a little over 1000 RPM. I can fool with the idle stop and
> drive the IAC counts down to zero, but when it is at 0 this simply means
> it is out of range. The IAC steps should really never get to 0, because
> once they do the ECM can no longer control the idle anymore. I am trying
> to get the IAC counts down to maybe 10 or so at desired idle. From what I
> read the lower the better at idle, but never 0. I will play with the
> table you mentioned
> thank you
> Darryl...

You're right - once it reaches 0 it isn't regulating anymore. If it ever
reaches 0, the only ill effect could be a slightly high idle.  If it is
regulating at a high count position (due to a low minimum airflow) the
symptoms can be as subtle as taking 2 or 3 tries to crank or as dangerous as
stalling at road speed when you take your foot off the gas! Five or 10 is a
good happy medium for base idle (no load) conditions.
Later,
David

> On Wed, 8 Mar 2006 21:20:48 -0800 "David Allen" <davida1 at hiwaay.net>
> writes:
> >   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
> > 2000.
> >   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.
> > David
> >
> >
> >
> > > 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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