Who is good at fuel pumps/fuel systems?

I have my Saab (1999 9-3) fuel pump out to fix some of the thrashing we did the day before the challenge. I have a question about the purpose of some stuff on the fuel pump assembly. The overall pump assembly hangs from the cap which is held on with a large ring - pretty standard. The high pressure and return line go through this cap. The level sender is attached to the inside assembly. On the assembly itself, there is a big cup that basically everything other than the level float sit in. There is only one hole in this 'swirl tank'. The pump sits inside a plastic mesh cup inside the swirl tank. All of it makes sense to me so far. Here's where I get stuck:

The output of the fuel pump goes up to the high pressure outlet in the cap, but there is a T fitting in the line, and the 3rd end of the T goes back into the bottom of the swirl tank through what I think is a one way valve, maybe a pressure valve. Seems to be normally closed (according to me blowing into the high pressure port on the cap and blocking the line to the fuel pump).

The thing in question is right by the bend in the float arm in the picture below.

What the heck is this thing and what does it do?

I'm guessing it is some sort of pressure relief valve that opens if there's a blockage in the line, to avoid stalling the fuel pump motor.

Might be a syphon deal to keep the pump reservoir full.

MadScientistMatt said:

I'm guessing it is some sort of pressure relief valve that opens if there's a blockage in the line, to avoid stalling the fuel pump motor.

I think it's also a first step pressure regulator.  That's what it looks like it is.  Which also prevents dead heading of the fuel system when injectors are shut off.

It may be there so that the pump can always be moving enough fuel to keep itself cool.  Maybe the pump is a little oversized for the lines, so that when the fuel pressure regulator opens up suddenly, it has reserve capacity. 

Ooh...maybe it closes off the output if the fuel pressure gets too low, assuming that the fuel line broke and preventing a large fire. 

Streetwiseguy said:

Might be a syphon deal to keep the pump reservoir full.

This is exactly what it is.  Bernoulli's Principle applies to all fluids.

Normally to drain a fuel tank, we will remove a fuel line and jumper the relay and just use the pump to pump the fuel out.  That does not work with this style fuel pump module, because with no fuel pressure, no fuel goes through that jet pump, so no fuel gets pulled in from the tank to the module, and the pump just empties the module and then sucks air.  This can happen no matter HOW much fuel is in the tank!

A lot of saddle tank equipped vehicles will also use one of these devices to pull fuel from one side to the other.  Sometimes it is fed by the outgoing fuel, sometimes it is fed by the return side.  That's how they work when there's only one pump in the system.

So, let me check my understanding: If fuel is pressurized from the pump out, then that puts pressure suction on the line, which would then put pressure on the valve in question and allow fuel to flow into the swirl tank (from the rest of the tank, not the pressure from the pump output)?

Venturi pump to keep the canister full. Excess fuel pressure is bled off and used to power the venturi pump.

Pressure to operate the venturi pump is at or slightly higher than the injector rail needs so the motor is not starved, high pressure pump is oversized to compensate for the volume the venturi pump uses.

Not a canoe this is a venturi pump example, commonly used for draining aquariums, basements, etc.

https://www.amazon.com/s/ref=nb_sb_ss_i_4_12?url=search-alias%3Daps&field-keywords=venturi+pump+garden+hose

The venturi pump lifts fuel to the canister when tank fuel level is low, canister acts as a reservoir/swirl pot to maintain a flooded inlet on the pressure pump.

Pressure pumps don't like to suck. Return fuel also goes to canister so it doesn't get pumped dry. Foot valve in venturi pump keeps canister full when shut off.

Now it makes sense, that venturi pump uses excess flow (maybe only operates above a certian pressure to not interfere with the fuel rail) to keep the swirl tak full when the rest of the tank level is below the top of the swirl pot.

Thanks all! I love learning!