'165 Minimum Air Question...

[Programmer]

Shannon--

Has this thing got "early EGR" ??

Lyndon.

----- Original Message -----
From: Shannen Durphey <Shannen at grolen.com>
To: <gmecm at diy-efi.org>
Sent: Wednesday, February 14, 2001 10:01 PM
Subject: Re: '165 Minimum Air Question...


> Funny how something comes up on the list, then shows up in the shop next
day.
>
> I've got a 4.3l tbi engine with a mild GM marine cam installed.  Other
than the cam
> and an open element air cleaner, it appears to be stock.  The truck won't
idle,
> hunts, chuggles at low road speed, has no power, and when cold it runs
like a carby
> car with the choke removed.  It likes to die when coming to a stop, and
shifting
> from neutral to drive or reverse causes large rpm changes.
>
> Initially, looking at the IAC counts at idle (warm) shows 50-60.  But
watching MAP
> on the road I see that any amount of throttle opening sends MAP volts
right to the
> top of the scale.  That's a good sign that vacuum is really weak.  I can
hear an
> exhaust leak around the catalytic converter, and a definite sign of a bad
converter
> is poor vacuum.  So I downshift into second for some more driving.  Now I
see that
> opening and closing the throttle at 30-40mph causes the MAP signal to
change very
> rapidly.  This is not typical of a plugged exhaust system.  So I head back
to the
> shop for a good visual inspection.
>
> First find.  Throttle plates are completely closed.  The throttle stop
screw is not
> even touching the throttle arm.  Fuel is forming a small pool on top of
the
> throttle plates while the engine idles.  I even consider playing with the
TPS to
> see if acceleration enrichment will fill up the tb bores.  : )  Then I
find that
> the timing is retarded by about 8 degrees.  Well, that certainly explains
the lack
> of vacuum, power, response...  Then I check the oil.  Sure enough, smells
like fuel
> due to excessive fueling and misfires at low rpm.
>
> After adjusting the throttle plate opening, setting timing to factory
specs and
> changing oil, the IAC counts ended up around 19-20 with a stable idle.
But the cam
> changes the power curve, factory chip tries to idle around 600 when fully
warmed
> up, vacuum is off by about 2", so the truck runs rich at idle.  BL #'s
managed to
> get as low as 90!!  Other end of the spectrum, around 3500 rpm, light to
medium
> load, BL is up by 10-12 numbers.  By the time the ecm gets to trying to
switch
> between stall saver/ and an over rich 600 RPM idle speed, IAC count was
back up to
> a fluctuating 30-35.
>
> I'm going to make sure the fuel pressure is near the top end of the
factory specs.
> If it's low now, I can reduce the amount of work I'll need do at the top
of the VE
> tables by increasing it.  After that it's time to go into the chip.  I
really
> expect that the IAC counts will end up a fair amount below 50 when this
job is
> done.  That's the way it looks now, but things can change while I'm
working.  The
> numbers certainly aren't set in stone.
>
> Hopefully this gives an idea of how the aspects of tuning should be
considered
> together, rather than separately.  It's kinda like the old proverb about 3
blind
> men describing an elephant.
> Shannen
>
>
> Shannen Durphey wrote:
> >
> > It really depends on your car.  If you're running a completely stock
engine with
> > factory equipment and reasonable wear, then yes, it's high.  If you're
running a
> > cam with 320/320 duration and 104 degrees overlap, then no, you've
actually done a
> > good job of keeping the IAC counts down.  The number is a guideline that
I use when
> > tuning, when doing tune-ups, and when adjusting minimum air on a mild
car with a
> > working calibration.
> >
> > I have never seen an IAC count of 256 displayed on a scan tool.  Even
with a faulty
> > IAC being commanded full open, I've never seen this count displayed.  If
it was up
> > around 100 for a mild engine, I'd be thinking about other ways to get
the numbers
> > lower, and I'd be looking at the throttle plates, intake, etc for carbon
and
> > garbage buildup.  Opening throttle reduces vacuum to things like brakes
and pcv, so
> > it's not a bad idea to get the rpm as high as you can while keeping
throttle at a
> > minimum.  If ya gotta, ya gotta, but
> > if ya don't...
> >
> > But it's really a guideline to be used in conjunction with the other
factors which
> > affect idle speeds.
> >
> > Shannen
> >
> > Bob Wooten wrote:
> > >
> > > With the car sitting in @ the starting line waiting for the other guy
to
> > > dry hop my car was sitting @ 201 coolant temp, 160 MAT, & a IAC count
of
> > > 93.  would you consider this high?  i checked a number of runs (street
> > > mostly) & it appears from looking @ the scaling that this is a 256
count
> > > full scale & I don't think that i ever saw it get higher than 130 or
so (a
> > > peak not average).  this leads me to think that it has a way to go
before
> > > it runs out of room. the amount of flow through the IAC is non linear
in
> > > relation to the count though is it not?  maybe 93 is a ton?
> > >
> > > Thanks
> > > BW
> > >
> --------------------------------------------------------------------------
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