[Gmecm] 93 CHEVY g20 VAN

[Phillip Kuhn]

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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