DFI, Batch Fire, and other myths

[Shannen Durphey]

I just found the bulletin.  It says the carbon deposits may have a
sponge like tendency and absorb fuel, affecting engine until it enters
closed loop. Also, carbon may act as insulator, preventing heat of
intake valve from aiding in fuel evaporation and causing cold weather
driveability problems such as stalling, hesitation, rough idle.  Most
likely, the "sponge effect" made for the hesitation that I felt.
Shannen

Jim Davies wrote:
> 
> 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.