Preset Adjustable Fuel Pressure Regulator?

[JasinC19]

I have an adjustable FPR installed.

I'm getting ready for the engine's first start (wow...) and want to make sure the FPR is as close to correct as can be.

I have a fuel pressure gauge installed on the schrader valve on the feed line, but wouldn't that just measure the pressure coming off of my awesome new Walbro pump? Once the car is running would the gauge then read correctly?

Is there a way to adjust it with a vacuum gun or something?

Thanks,
Jason

 

[Mustang5L5]

Cycle the key to on, the gauge should momentarily peg at the correct setting and then drop.

Just get it in the ballpark, start the engine and adjust from there


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[JasinC19]

So it kind of measures backpressure then?

 

[Mustang5L5]

Well it doesn't measure anything. The regulator drops the fuel pressure from the pump down to the correct range inside the fuel rail.

When you hook a gauge up to the fuel rail, you are reading the pressure after the regulator.

When you cycle the key, the pump rubs and the rail pressurizes. You will see the fuel rail pressure. Adjust the reg until you get around 39psi when the pump cycles

Then start the car and adjust from there


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[JasinC19]

I guess the way I think of it is there is a lot of pressure coming from the pump. The FPR is like a valve that is partially open. The pressure after the fuel passes through the FPR is 39 psi (once adjusted).

It would make more sense to me if the gauge was placed in the stream AFTER the FPR, but it isn't.

So while I believe you, I am still confused.

Maybe I just have no clue how an AFPR works!

 

[pissedoff92]

If im reading this right the guage should be after the fpr

 

[JasinC19]

I've been reading that the line with the schrader valve is the feed line, which gets hooked up to the connection closest to your radiator.

The return line (at least on my bbk kit) is the line coming off of the regulator mount.

 

[Blown88GT]

I guess the way I think of it is there is a lot of pressure coming from the pump. The FPR is like a valve that is partially open. The pressure after the fuel passes through the FPR is 39 psi (once adjusted).

It would make more sense to me if the gauge was placed in the stream AFTER the FPR, but it isn't.

So while I believe you, I am still confused.

Maybe I just have no clue how an AFPR works!

I've been reading that the line with the schrader valve is the feed line, which gets hooked up to the connection closest to your radiator.

The return line (at least on my bbk kit) is the line coming off of the regulator mount.

Since fuel is a non-compressible fluid, the pressure at the pump or lines is the same. The return line could be different depending on how much fuel the injectors are using. The Shrader Valve (gauge) is in the line that feeds the injectors. The FPR can be anywhere in the feed line downstream of the injectors, but the closer to them the better. It's function is to maintain a constant pressure drop across the injectors.

If you "deadhead" the running pump (pinch off the return line), the pressure will increase to whatever the pump is capable of.

 

[Chuckman]

It would make more sense to me if the gauge was placed in the stream AFTER the FPR, but it isn't.

it is, the schrader valve (where you put your gauge) is on the return line to the tank. fuel goes tank->fpr->rail->scrader->tank.

 

[JasinC19]

it is, the schrader valve (where you put your gauge) is on the return line to the tank. fuel goes tank->fpr->rail->scrader->tank.

U sure?


STEP 8 Remove the black plastic cap on the fuel feed line near the front end of the passenger side valve
cover (near oil filler cap). While holding a rag over the valve to avoid fuel spray, carefully release any fuel
system pressure by depressing the Schrader valve using a small screwdriver or pick.

 

[Noobz347]

Gentleman,

I believe that some of you are thinking of a non-return style fuel system...


Non-Return Style:

Return Style Fuel System used on Fox Bodies:

On the return style fuel system, the last thing in the circuit is the fuel pressure regulator before unused fuel returns to the tank. The regulator DOES squeeze off the escape of fuel in the return line to regulate pressure at the fuel rails.

The non-return style regulates pressure before the rails and ramps up pump volume to meet throttle demands.

The BOTTOM pic is how the return style Fox Body fuel system is arranged.

 

[JasinC19]

Not the best pic, but you can see why it's misleading just by looking at it. The feed line (short one on the right of the picture towards front of the engine) looks like it would send the fuel right past the FPR.

So half the injectors would get all the pressure and half would get the adjusted.

I guess as long as I have the feed line (with schrader valve) hooked up to the short hose and return line hooked up to long hose, I should be ok right?
View attachment 152548

 

[Noobz347]

Correct... the feed line goes to the fuel rail and the return line comes off of the regulator.

...and you're right. The feed line DOES go past the regulator. The regulator is adjusting fuel pressure on the return line, not the feed line. Once fuel pressure in the rails gets to the preset pressure, it releases and allows unused fuel to return to the tank. The way it stays balanced (rail to rail) is that the pump provides enough volume at pressure so that both rails equalize before the the regulator allows fuel to escape.