What exactly is the theory/use of the fuel return line anyway?
 

Also is the usage only on MFI?

most fuel pumps pump more volume than needed by the injectors, so extra fuel is bypassed through the return lines back to the tank. It is also on cis cars and almost any car with fuel injection for that matter

Why would Porsche then just manufacture a pump with the correct volume, as opposed to the extra line back to the tank?

I know on the CIS it can relieve excess pressure on the fuel system for a better air/fuel ratio under certain conditions.

At least I think thats one of the uses.... just read about it today in the Bosch book

The challenge is that the engine needs more or less fuel depending on the rpm and load. When the demand is high, more pressure is needed to deliver the fuel.. lower, lower.. One set pressure will only work for a certain rpm.

Because of this:

http://www.pelicanparts.com/catalog/...cell_pedal.jpg

And this:

http://www.pelicanparts.com/catalog/...reman_tach.jpg

Tom

OK., so I'm reading here there's a similarity between "volume" and "pressure" as used here, and the applications are on MFI and CIS?

in a fixed fuel line diameter volume increases with psi.
there is also an increase in friction in the line and injector, but it's negligible w/our relatively low volumes. The return line, even if not part of the orig system, can make sense in every fuel sys imo.

Also note that the extra fuel cycled around helps cool the pump.

George

What George said. Electric fuel pumps, for the most part, are fixed displacement pumps. The fluid pumped dissipates heat and without the loop, the pump dead-heads and overheats. The loop also allows for a constant pressure head to be available on demand.

Yes, there are a variety of reasons:

- Recirculating fuel helps to cool the pump
- The pump must maintain various pressures and flow rates. The system (CIS/MFI/Motronic) regulates the amount of fuel put into the cylinder by means of the injector. Excess is pumped back to the tank.
- Pressure can be very tightly regulated. Pressure would change drastically if the fuel flow changed (like with a carb pump). Carbs simply take as much fuel as they need, and then shut off the fuel bowls.

The system works a little bit like a turbo - uses what it needs and then sends the rest through a wastegate...

-Wayne

maybe that's why I had such a hard time getting fuel to the pump when connected it up after the clutch replacement...so I opened the fuel tank cap and pushed air into the tank until fuel flowed to the pump. Once the pump was primed and the lines connected it worked fine.