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

[Barry Tisdale]

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).

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