749 O/L idle help needed.....

[David Papworth]

There are a ton of adjustment factors, but
the basic fuel injection control equation is:

IPW = (magic unit resolving Constant) * Displacement * RPM * MAP * VE

As your RPM fluctuates, the IPW will track it in direct proportion.

Thus if you get 1.25 ms at 750 RPM, at 650 we would
expect 650/750 * 1.25 = 1.083 ms, near enough to what
you are seeing.

Setting your VE table "flat" means that you are asking for the mixture
to stay constant, which means the PCM is doing exactly
what you programmed it to do by varying IPW in proportion to RPM.

The problem is, the VE table does nothing to establish a consistent idle
speed.
(You can de-stabilize it by causing the idle mixture to be too lean across
the board).
Perhaps you have a vaccuum leak?.

It isn't clear why the IAC control logic isn't doing a better job of holding
the idle
steady, unless the system is disabled or underdamped. Starting with a
slightly
richer mixture, and allowing the IAC to work harder to control the idle
speed,
seems like the "correct" way to get a smooth idle.

If you want to play with the VE tables for idle smoothness, first try
boosting them
across the board to see if a richer mixture idles smoother. Then
try setting up a negative feedback correction for mixture -- have the VE
increase
at lower RPM and reduce at higher RPM around the desired idle speed. This
will cause the
PCM to enrichen as the RPM drops and lean as it rises. This would set up a
negative feedback
loop which attempts to drive the RPM towards the center point. Try this
first with small
adjustments, and only for a small region around idle. I have no experience
to
say that this actually would work in practice, but that is what theory would
suggest. Some sort of TPS or MAP driven disable or compensation for this
skewing of the table would be desirable to avoid negative consequences to
off-idle driveability.

Good luck with your experiments

Dave Papworth

----- Original Message -----
From: "Barry Tisdale" <btisdale at cybersol.com>
To: <gmecm at diy-efi.org>
Sent: Sunday, October 29, 2000 12:29 PM
Subject: 749 O/L idle help needed.....


> Gentlemen:
>
> Trying to get a *solid* idle in my Sy -
>
> Oxy sensor set @ 0 time (therefore open loop always)
>
> Timing table set flat in the idle area - this is working - no timing
> variations
>
> A/F closed throttle table set flat in idle region; VE vs RPM table set
flat
> in idle area.
>
> Still, the BPWs fluctuate from, say, 1.09ms to 1.25ms.  IAC shows no more
> than 50 +/- 1, timing = no variation, idle speed from 650 to 750 (target
is
> 725).  Idle is best @ 1.25ms here, something trying to lean it out.
>
> MAT =~ 90F, coolant @ 160F - want steady warm idle first, will work on
cold
> later.  Manifold vac steady @ 16" +/- 1/2 (mild cam).
>
> *Something* is causing the BPWs to vary, almost like its still in C/L.
>
> Any suggestions?  Obviously, I'm overlooking something here, but I don't
> know what.
>
> Thanks - Barry
> --------------------------------------------------------------------------
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