Why do I need a Fuel Pressure Regulator ?
Posted: Tue Jul 08, 2014 1:10 am
The AE86 GTS (and all fuel injected cars) has a Fuel Pressure Regulator (FPR).
On the GTS it is mounted on the firewall end of the injector fuel rail. Fuel enters
the FPR from the fuel rail via a bolted and O-ringed plug-in connection. Excess
fuel flows out of the FPR and back to the tank via a hosetail and low pressure
rubber hose connected to a hard line.
FSM figures are...
38-44psi no vacuum (engine off)
30-33psi with vacuum (engine idling)
Technically, the FPR is a vacuum compensated pressure sustaining valve, but
FPR is close enough. Its purpose is to maintain a constant pressure differential
between the fuel rail and the inlet manifold. Why ? The ECU pulses the injectors
for a measured fraction of a second, and adjusts the duration of the pulse (open
time) according to load and rpm. The ECU itself has no fuel pressure sensor, so
it relies on the fuel pressure differential across the injector remaining constant.
Now, one end of the fuel injector is in the fuel rail, and the other end is in the
intake manifold runner. A simple spring and diaphragm pressure regulator (no
vacuum compensation) can maintain constant pressure in the fuel rail. But what
happens if the manifold vacuum changes ? At WOT there is atmospheric pressure
(15psi) in the manifold. At idle there is a round 5psi in the manifold, and in both
cases there is around 40psi in the fuel rail.
So at WOT the pressure difference is 25psi and at idle it is 35psi, which would
mean that the injectors are flowing less fuel per second of open time at WOT
than they are at idle. This is where vacuum compensation comes into play. To
correct the situation, manifold vacuum is applied to the top of the diaphragm
to offset some of the force exerted by the spring. This allows the valve to open
more, and drops the pressure in the rail. Now the rail pressure at idle will be
25psi, the same as the WOT pressure.
By varying the rail pressure to maintain a constant differential across the fuel
injectors, the FPR allows the ECU to accurately control fuel delivery to the engine.
That is why you need one So if one day your idle gets really rough, the engine
blows black smoke and struggles to stay alive, but your WOT performance is
fine... check to see if the FPR vacuum line came unplugged.
Cheers... jondee86
On the GTS it is mounted on the firewall end of the injector fuel rail. Fuel enters
the FPR from the fuel rail via a bolted and O-ringed plug-in connection. Excess
fuel flows out of the FPR and back to the tank via a hosetail and low pressure
rubber hose connected to a hard line.
FSM figures are...
38-44psi no vacuum (engine off)
30-33psi with vacuum (engine idling)
Technically, the FPR is a vacuum compensated pressure sustaining valve, but
FPR is close enough. Its purpose is to maintain a constant pressure differential
between the fuel rail and the inlet manifold. Why ? The ECU pulses the injectors
for a measured fraction of a second, and adjusts the duration of the pulse (open
time) according to load and rpm. The ECU itself has no fuel pressure sensor, so
it relies on the fuel pressure differential across the injector remaining constant.
Now, one end of the fuel injector is in the fuel rail, and the other end is in the
intake manifold runner. A simple spring and diaphragm pressure regulator (no
vacuum compensation) can maintain constant pressure in the fuel rail. But what
happens if the manifold vacuum changes ? At WOT there is atmospheric pressure
(15psi) in the manifold. At idle there is a round 5psi in the manifold, and in both
cases there is around 40psi in the fuel rail.
So at WOT the pressure difference is 25psi and at idle it is 35psi, which would
mean that the injectors are flowing less fuel per second of open time at WOT
than they are at idle. This is where vacuum compensation comes into play. To
correct the situation, manifold vacuum is applied to the top of the diaphragm
to offset some of the force exerted by the spring. This allows the valve to open
more, and drops the pressure in the rail. Now the rail pressure at idle will be
25psi, the same as the WOT pressure.
By varying the rail pressure to maintain a constant differential across the fuel
injectors, the FPR allows the ECU to accurately control fuel delivery to the engine.
That is why you need one So if one day your idle gets really rough, the engine
blows black smoke and struggles to stay alive, but your WOT performance is
fine... check to see if the FPR vacuum line came unplugged.
Cheers... jondee86