Triton318

I'm trying to understand the operation of a FPR. I've done some research, and I think I understand what they do and how they work. In most of my reading, there is mention of a vacuum source/hose on the regulator (that uses the intake manifold) and a return line to the tank.

"The fuel pressure regulator uses a diaphragm and spring combination within its housing with a vacuum source on the top side of the diaphragm to counteract the spring pressure when high demand dictates that higher fuel pressure is necessary."

What I don't understand is a regulator that is housed in the fuel tank, as in the 1996 XJs. I can see where a return line is not needed, because the fuel just leaves the FPR right into the tank. But what about the vacuum? Where does that come from?

firebane

Fuel pressure regulator is on the fuel rail and I'm not sure what the switch over year is but I think it was 95. So your 96 should not have one on the FPR and you will not have a vacuum line for it. FPR is in the tank area instead.

And you are correct about the purpose of the regulator... its basically there to ensure proper amount of fuel is fed to the injectors/engine as to not flood it or cause damage.

DFlintstone

I think the vacuum to the regulator on the rail till 94 or so had to do with them wrestling with smog limits. The injector "pulse width" is normally what controls how much fuel is delivered. I think they rigged it up so at idle with high manifold vacuum the pressure would drop about 10#. (lower pressure presumably would squire less fuel while the injector is open). Later when they went to a constant, (higher) pressure the vacuum was no longer needed so they moved the regulator to the tank. That's my guess anyway.

jordan96xj

The one in tank is not manipulated by a vacuum source. It is mechanical.

firebane, the purpose of the regulator is to keep the fuel in the rail always at the same specified pressure. The injectors simply open for shorter or longer amount of time (controlled by the computer) which ultimately controls the amount of fuel that enters the combustion chamber.

PatHenry

Switch over was in the middle of the 96 model year. So an early year 96 (made in late 95 or possibly early calendar year 96) has the rail mounted vacuum style and the front mounted pump (the kind where you don't have to drop the tank to replace the pump) and the late year 96's (and export models) use the "one-off" pump assembly that's very similar to the 97+ where you have to drop the tank to get at the pump assembly.

​​​​​​​My 95 definitely has the rail mounted regulator with the vacuum line.

jordan96xj

I have a late year 96. It has many 97ish things on it (especially connectors/sensors). The tank still has the fuel assembly on the front.

PatHenry

Do you have the pressure regulator in the tank or on the rail?

jordan96xj

In the tank, as part of the fuel pump assembly.

DFlintstone

Maybe that's the "ungetable" 96 fuel pump that has popped up a couple of times. Wouldn't mind seeing a pic of one some day.

Triton318

Well, they're pretty gettable now. I should've taken a picture of the one that arrived at my house today. Which is now installed in the gas tank. Which you don't have to drop to do. I found several sources for this pump module. From 175 to 225.