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

[David Papworth]

One more correction:

At a constant throttle opening, MAP rises with decreasing RPM and falls with
increasing RPM. This would cause the normal fuel equation to compensate
in the appropriate direction to stabilize RPM.

However, it is possible that the compensation becomes either too much or too
late,
which creates an underdamped feedback loop that resonates at some frequency.

My previous comments about tweaking the idle MAP tables to force a constant
IPW despite changing MAP still apply. This creates a dead-zone about the
desired RPM where no corrections are applied. This may result in a more
steady RPM than one would get with a resonance in the control equation.


----- Original Message -----
From: "Barry Tisdale" <btisdale at cybersol.com>
To: <gmecm at diy-efi.org>
Sent: Sunday, October 29, 2000 6:25 PM
Subject: Re: 749 O/L idle help needed.....


> Thanks, David -
>
> I don't understand why the RPM makes any difference.  My understanding is
> that RPM does not enter into the BPW calculation; only things that affect
> the amount of air drawn in per cylinder "suck" (air density, basically -
> secondarily injector voltage, etc.) will change the BPW.  I see how the
> total fuel needs of the engine are reflected in RPM, but as RPM
corresponds
> 1:1 to injector events, there shouldn't be any variation in pulse widths
> depending on RPMs.
>
> On my benchtop ECU tests of the chip, varying the RPMs thru a normal idle
> range of 500-1000RPMs, the BPW does not change, as the VE tables are set
> alike over this range.
>
> However, varying the MAP, even just a bit, will change the BPW - again,
> even though the table is set to zero change between cells.  Everything
from
> 20kPa to 100kPa and 400-1000RPMs is set to the same VE.
>
> This fluctuation in BPW is seen in the vehicle as fluctuating RPMs @ idle.
> - +/- 25, but I'm after zero.
>
> Something is causing an increase in BPW with an increase in MAP - oxy
> sensor's out of the picture (O/L), so I'm missing something somewhere.
>
> Thanks again - Barry
>
>
>
> At 05:04 PM 10/29/2000 -0800, you wrote:
> >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).
>
> --------------------------------------------------------------------------
--
> To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
> in the body of a message (not the subject) to majordomo at lists.diy-efi.org
>

----------------------------------------------------------------------------
To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org