71 911E MFI fuel pressure questions

[Corsair26]

Hi all, After a long wait the MFI for my '71 911E 2.2ltr has returned from Germany and is re-installed. It looks like new.
However the engine ran poorly, cant be tuned and the prime suspect after a lot of head scratching and testing is the small regulator / overflow valve on the front (facing the right side of the car) of the fuel filter console.
Yes all other "demandments" have been ticked.
Im now waiting for a new one to arrive shortly from Porsche.
Frankly it looks like a normal hose adapter but apparently something important lurks within.

I assume it maintains a constant pressure in the fuel system? including the MFI, that allows fuel to flow through the MFI and any fuel not used passes on back to the fuel filter canister where it again returns through this little guy to the fuel tank in front while pressure is kept constant. Or is it just a non return check valve?

I cant take my console (the top of the filter housing) off at the moment so I wonder what the channels /pathways inside of it looks like?
And why did they use this method?
Im sure it has a very good reason.
But why not just return the unused fuel directly back to the fuel tank via an inline regulator? Why send it through the filter console/canister again?

Ive seen threads on here showing the fuel system graphic in the manual is incorrect and I can see that it is.
Fuel should come in the the right hand side(to the front of the car) through the filter, out the bottom to the MFI, from the MFI back into the console on the left (to the rear of the car) and out through the little regulator back to the fuel tank. Would the engine even run if it was the other way around?


1. So! does anyone know the pressure value that this regulator is supposed to have? (and at what pressure does it hold?) Or what does it really do?
2. What's going on inside the fuel filter canister console?
3. Why did they make it this way? Because Im sure its for a very good reason that I just cant see - one that does not seem to be in any document i have found.
4. Has anyone replaced this little guy, the engine hums to perfection and found the world is a brighter place?

I bow to greater knowledge.

Thanks in advance.

[stownsen914]

The return fitting in the fuel filter console has a simple spring loaded "check valve" in it that limits flow back to the tank. This keeps the upstream fuel pressure that the MFI pump on the engine sees at the required 1 atm pressure (14.7 psi). I've heard different things about the criticality of this pressure value, but my own experience was that my non-stock MFI 2.7L ran very poorly when I found the pressure to be only 10 psi. Getting the pressure to 15 psi fixed it. In my case I used a generic fuel pressure regulator tapped into the return line to override the stock check valve. I wasn't concerned about originality in my application, so this worked for me. I'd assume a replacement check valve should do the trick.

As for why Porsche arranged the fuel system this way, I recall reading a thread here once that the design of the fuel filter console encourages continuity of fuel flow to the MFI pump.

[Uwon]

^^^Following…..stownsan914 thank you for explaining how the console works as I think many of us have been scratching our heads for years.
Johan

[Corsair26]

Originally Posted by stownsen914

The return fitting in the fuel filter console has a simple spring loaded "check valve" in it that limits flow back to the tank. This keeps the upstream fuel pressure that the MFI pump on the engine sees at the required 1 atm pressure (14.7 psi). I've heard different things about the criticality of this pressure value, but my own experience was that my non-stock MFI 2.7L ran very poorly when I found the pressure to be only 10 psi. Getting the pressure to 15 psi fixed it. In my case I used a generic fuel pressure regulator tapped into the return line to override the stock check valve. I wasn't concerned about originality in my application, so this worked for me. I'd assume a replacement check valve should do the trick.

As for why Porsche arranged the fuel system this way, I recall reading a thread here once that the design of the fuel filter console encourages continuity of fuel flow to the MFI pump.

Great thanks. I was thinking that was the case. Stable and constant fuel pressure at 1ATM +- .2ish at the MFI pump appears to critical. I'll let you know how the new valve works (or not). Cheers

[mhurwitz]

I happen to have mine apart

[Corsair26]

Thats fantastic. Thanks for that pic.