Fuel return

About inboard or outboard motors.

Moderator: Bill Edmundson

TomB
Posts: 615
Joined: Wed Jan 18, 2017 4:07 pm
Location: Holland, MI

Re: Fuel return

Post by TomB » Mon Mar 18, 2019 7:31 pm

Mr Hot Rod wrote:
Mon Mar 18, 2019 2:24 pm
I'm not too keen on pumping fuel at high pressure in the filler tube ...

Refer to Denon Osterman's Plumbing twin tanks for EFI ? thread.
Thanks Paul...another homework assignment! High pressure in the fill tube...I imagined (what you do when not smart enough to assume) that the return would be a few psi, hmmm.

I took a little detour today to the PCM Owners manual looking for: where fuel leaving the tank goes, where return fuel comes from and why, what is the pressure, any other neat stuff I might stumble over to help me understand the fuel system.

The fuel drawn from the tank is pumped into a fuel cell under low pressure (the idea is to always provide fuel and eliminate slosh factors), within the cell gas flows through a filter to a high pressure pump and is pumped under high pressure to the fuel rail. Pumping is at rate to meet or exceed WOT. At idle, the pumps are over doing it and fuel from the rail is returned to the fuel cell where it figuratively overflows back to the tank. Pressure on the return line would be near zero at WOT and near high pressure pump output less return line pressure drop at idle. Hmmm.

Seems like the trick for the EFI return is to short circuit the return somehow bringing the overflow directly to the low pressure pump inlet. With Paul's help, I see several threads working the idea. Someone was looking for a little tank. I see Denon and I are going down the same track. Could it be as simple as a fuel filter/water separator like www.marineengineparts.com/racor-660r-ra ... -separator ? Plumbing looks ok. Is the max pressure 7 psi to low? I don't know if the filter/separator goes before or after the low pressure pump. Is there a need for a back flow preventer? Too much pressure on the pump inlet?

A fuel cooler maybe?

Tom

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