DFI, Batch Fire, and other myths

[Jim Davies]

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.