Fuel Pump Questions

304065 · 02-13-2002, 10:37 AM

Ladies and Gentlemen:

Does anyone know why there are THREE hose connections to the fuel pump? I know what they are, S (supply, from tank) D (druck, or pressure to the filter console) and R (return, which is an OUTLET from the pump to the tee fitting). I have the fuel system diagram from the factory manual, but here's what I don't understand-- what is the purpose of the return outlet on the pump? Does it take excess fuel that cannot be forced down the pressure line and send it up the tee, where it then flows against the pressure in the return line from the console and then back to the tank?

Why did they design the pump this way, why not just a feed line to the fuel filter and a return line to the tank? I think it has something to do with the pump being a "roller" type as opposed to a centrifugal type but I would love any clarification on this.

I'm thinking about converting to -8AN lines from my soon-to-be-installed fuel cell all the way to the MFI pump and have found a Mallory pump that has an AN return fitting but would love to know the theory of operation here before I go second-guessing the factory. My guess is that if the braided cloth hose was good enough for RSR's it should be good enough for my 155HP "E" but I am a safety freak . . .

T Bird · 02-13-2002, 05:31 PM

John
When pressure in the delivery line of the MFI fuel pump reaches 2 bar, a pressure relief valve opens in the pump and the excess fuel is dumped back to the tank through the return line. Typical operating pressure for the pump in an MFI car is 1 bar. I believe that the pressure relief valve and the return line keep pressure within operating parameters and return un-needed fuel to the tank.

By way of comparison, the pressure relief valve for the Bosch roller cell fuel pump designed for carbs operates at .3 bar and the pressure relief valve opens between .2 and .3 bar.

Years ago when I replaced the MFI fuel pump on my car with the Bosch roller cell to go with my then-new Webers, I tested the fuel pump by connecting short lengths of hose to the return, suction and delivery fittings on the pump. I placed the suction line in a coffee can full of gas and the delivery and return lines in empty cans. I energized the pump and before all the gas was sucked up and pumped into the "delivery" can, I pinched the delivery hose to restrict flow. Fuel immediately began to discharge from the return line. That's how it works.

304065 · 02-15-2002, 04:03 PM

Hey, T-Bird, thanks. That was EXACTLY what I was looking for.

Knowing that's how it works, I can now re-plumb with aeroquip all the way back.

Then again, 40' of Aeroquip @ $5.00/ft ($200), 8 hose fittings @ $7.00/ea ($56), aftermarket fuel pump with -8AN fittings ($250), adjustable fuel pressure regulator ($75), fuel pressure gauge for adjustment/monitoring ($30), 10 cushion clamps @ $1.00/ea ($10) and it looks like my fuel system mod will run a cool $621 before freight.

vs. $0 for the factory setup, which worked for the factory race cars. . . maybe there's nothing wrong with those cloth braided rubber lines in the first place. . .

thanks again, knowing myself I probably WILL do it.