[Gmecm] 93 CHEVY g20 VAN

[WopOnTour]

Hmmmm interesting post Phillip
What GM vehicle with a returnless system has the PCM controlling the fuel 
pump voltage as you describe? I'm aware of the older 3800 SC that had either 
a FP resistor or a FP speed controller (2 speeds), but these were used on a 
conventional return-type systems and basically existed to accomplish the 
same goals as a returnless system- i.e. reducing the fuel volume being 
exposed to the heat of the engine via a trip to the fuel rail.
I am not aware of any GM "returnless" system that has the PCM influencing 
the FP voltage directly, other than turning ON the FP relay. (although I've 
heard some Mustangs do)
Correct me if I'm wrong here
WopOnTour

----- Original Message ----- 
From: "Phillip Kuhn" <pmkls1 at yahoo.com>
To: <gmecm at diy-efi.org>
Sent: Saturday, December 31, 2005 1:13 PM
Subject: Re: [Gmecm] 93 CHEVY g20 VAN


> Well you have the basic theory right but where you are
> misunderstood is this:
>
> The need for a vaccum source on a fuel pressure
> regulator is because when you suddenly load up an
> engine the need for fuel increases rapidly and the
> computer compensates for this by increasing fuel
> injector pulse width to supply more fuel.The regulator
> is designed to spike the fuel pressure in these high
> demand situations to maintian a good flow and
> atomization out of the injectors, it is also a source
> of enrichment as we all know that you can make an
> engine run richer by simply increasing the fuel
> pressure. The reason for the engine vaccum source on
> the regulator is the vaccum under normal load
> situations helps to pull against the spring pressure
> on the inside of the diaphragm. This actually works
> against the spring pressure helping the diaphragm to
> move and thus controlling the amount of fuel pressure.
> When you suddenly load up an engine the vaccum
> decreases rapidly and therefore there is no vaccum
> acting against the spring pressure on the diaphragm
> and the fuel pressure spikes. That is the reason for
> the vaccum source on most fuel pressure regulators.
>
> Example 1; If you install a fuel pressure gauge on a
> vehicle and start it watch as the fuel pressure will
> spike if you "goose" the engine.
>
> Example 2; If you turn on a garden hose and hold your
> thumb over the end it will spray out in a mist and
> travel x amount of feet. But, if you suddenly remove
> your thumb from the end it will go from a mist into a
> stream and will travel a significant amount of
> distance as it now has no pressure behind it.
>
> Now for returnless systems,
>
> This system was put in place for one simple reason:
> Emissions
> The powers that be decided that you can reduce
> evaporative emmissions by only supplying an engine
> with the exact amount of pressurized fuel that it
> needs therefore eliminating the constant cycle of fuel
> traveling the entire length of the vehicle and
> evaporating more quickly as it re-enters the tank.
> These systems actually use the pcm to regulate the
> amount of voltage to the fuel pump thus regulating
> it's output. These systems will actually spike the
> fuel pressure under high load conditions by increasing
> voltage to the pump and increasing it's output.
>
> I can't honestly remember the last time I worked on a
> TBI vehicle so I won't argue the presence of a vaccum
> source on the regulator. However, having worked at
> various GM dealers for the past 9 years I will say
> that I would be inclined to suggest a faulty regulator
> or IAT sensor as the source of the problem or at least
> check those first given the symptoms described. Any
> other causes, while none are impossible, are less
> likely to be present again given the description of
> the symptoms.
>
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