'165 Minimum Air Question...

[Shannen Durphey]

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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