DFI, Batch Fire, and other myths

Clarence L.Snyder clare.snyder.on.ca at ibm.net
Thu Jan 21 23:40:53 GMT 1999


Kurek, Larry wrote:
> 
> Ok then....explain how this works to me....
> 
> The carbon on the back of the intake absorbs fuel, so it results in a lean
> condition? I can see this happening on startup...but what happens when the
> carbon is saturated? It WON'T absorb any more fuel, so how does it still
> affect fuel ratios? I doubt it is a matter of absorbing fuel when rich, then
> releasing it when leaner...and I doubt it would have a significant impact
> anyhow.
> 
> I think this is one of those urban myths...or is it one of those mechanic
> myth things? :)
> 
> Larry
> 
> >
> > On Thu, 21 Jan 1999, Shannen Durphey wrote:
> >
> > > The sometimes less than accurate GM classes I've attended teach that
> > > injection should occur when the intake valve is closed,and be
> > > completed immediately prior to intake valve opening.  This
> > would make
> > > use of the heat in the head of the valve help keep fuel
> > from pooling.
> > > This was mentioned earlier.  This might be GM's approach to sefi.
> > > According to a service bulletin, the 3.8l engine suffers from carbon
> > > accumulation on the back of the intake valve which significantly
> > > reduces performance.  I've seen the effects. It's very similar to
> > > losing the accelerator pump on a carb'ed engine, and the exhaust is
> > > excessively rich.  My thought was the carbon is insulating
> > the valve,
> > > preventing vaporization.
> > > Shannen
> > >
> > The way I heard it [at a Mopar school] is that the carbon
> > accumulation on
> > the back of the intake valve acts like a sponge, absorbing
> > enough gasoline
> > that the A/F mixture is upset. IIRC, only at low and medium
> > cruise power.
> >
> >
I don't know if it acts as a sponge or what, but the presence / absence
of carbon on the intake valves makes a SIGNIFICANT difference in
driveability. The 3.0 litre V6 in my aerostar had a BAD stumble off idle
when partly warmed up - to the point that on a cold day if I fired it up
and drove to the corner down the street it would invariably stall when
pulling away unless I floored it. Tried everything. Finally used a
"motor-vac" on it, which does an excellent job of decarbonizing - and
it's been perfect since.



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