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

[Dave Zug]

Oh this is so cool -

a perfectly articulated question and a great, helpful answer - out in the
open!

Tell us how it works out and thanks for the brain-food!



----- Original Message -----
From: David Papworth <papworth at ichips.intel.com>
To: <gmecm at diy-efi.org>
Sent: Sunday, October 29, 2000 5:04 PM
Subject: Re: 749 O/L idle help needed.....


> 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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