what's the vacuum line going to the fuel pressure regulator do? - L98 motor

[Chiung Tien]

there's a vacuum line going from the intake plenum to the fuel pressure
regulator and also splitting to the A.I.R. system. My AIR system is not
there any more and I was wondering what would happen if I just plugged the
plenum and F.P.R. vacuum ports. Actually, I have plugged them and haven't
noticed anything, and was wondering if there is any harm in having done so.

[sudd...@networkone.net]

Chiung,

The vacuum line going to the fuel pressure regulator is what determines
the pressure the regulator works at. Without vacuum the fuel pressure
regulator restricts fuel from returning to the tank, thereby increasing
the pressure to the injectors. With vacuum applied the pressure
regulator lets more fuel return to the tank. If you disconnect the line
the fuel pressure will go up and your gas mileage will go way down. The
best way to see if the regulator is working properly is to screw in a
fuel pressure gauge to the fuel rail, test port, and read the gauge.
Check out the limits for your year model, then pull the vacuum line and
plug it. Your pressure should go up when the vacuum line is removed.
If it doesn't there is a problem with the vacuum source to your
regulator, or the regulator is bad. If you want to get technical you
can use a vacuum pump and apply vacuum to the regulator, by hand, and
check the readings at a given vacuum. It bears mention that the car has
to be running to make these readings. Why is the air pump gone from the
car? Did you change to one of those short pulley systems that
eliminates the air pump? Anyway, if you do not have smog to contend
with it might be ok. Without the air pump the computer will not be
reading the appropriate air fuel mixture out the exhaust. Also, the
catalytic converter will not function properly as it is not receiving
air to help light off the exhaust mixture. Just a thought...

cu

Von

[UpChuck]

The engine vacuum is used to modulate the fuel flow. When you go to wide
open throttle, the vacuum drops and the fuel pressure is raised, thus
enrichening the mixture.
The AIR pump is mostly effective at idle, I've read in this group of cars
passing the smog test without the air pump, I guess some under the hood
exams aren't very tough.