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

[David Papworth]

Try disconnecting the flexible hose going from the turbo to
the intercooler. You may be seeing a phenomenon called
turbo bootstrapping. Engine speeds up, more exhaust
through turbo, higher pressure into throttle body, more
air into engine, Engine speeds up more, more exhaust
through turbo, etc.

This phenomenon is typically more prevalent at higher
output than idle, but it is an easy thing to try to see
if the varying idle speed stabilizes. If this makes a
difference, it could guide the direction for further
tuning.

Getting a stable idle speed in this application, with so
many interacting systems, is not an easy problem.

----- 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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