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Fuel Pump Return/Vent Line?

We have an 1999 RV6a which I didn't build, an O-320 A3B, and a stock fuel installation as far as I can tell.

The cockpit side walls and the rear side of the firewall have stick-on thermal insulation, so it's a little difficult to follow the lines.

Anyway, there appears to be a 1/4" line returning from the engine-driven fuel pump, through the firewall, along the left side of the cockpit just under the instrument panel. Is this a fuel-return line? A vent for the fuel pump?

I ask because it has a valve in the line. 3 positions. The rearward position drops the fuel pressure to zero very quickly, yet IT IS NOT the main fuel feed to the gascolator.

The system works fine with the valve in the tranverse position [see photo].

fuel-pump-vent-valve.jpg


The top of the photo is forward.

And during a recent flight the engine ran fine with the valve in the forward position, but after shutdown it burped half a cup of gas onto the ground from the left main tank vent.

Is this familiar to anyone?

Thanks,

Dave
 
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A little more info please

Not all that strange but more info is needed to make a determination on what purpose this "return line" serves.

1. I assume there is a T on the outflow side of the mechanical fuel pump??
The 1/4" line returning to the valve in the picture and the 3/8" line leading to the carb bowl??

I see a T in the outflow line, can I assume the that each of those are returning to left and right fuel tanks?

Having the valve in the "transverse" position would close it.
Fore and aft pointing would allow fuel to pass through and return to the tank, assuming that is the set up here.

Return lines were popular with those who use low octane mogas, it is one component of the fuel system to help keep cool fuel in the system.
However, what is not normal is the entire 1/4" line opening up to the return.
Usually a restricting orifice is installed so as to return a small amount only.
It should not affect your fuel pressure as you have indicated.

The fact that your plane "burped" a half a cup of fuel out the tank vent tells me that this return line is possibly tied into the vent line. (Bad Idea)
A return line should be plumbed separately into a dedicated fitting to the tanks.
A lot of guessing here but I think I am close.
 
I have a fuel return on my 6. In my case, I have a third hole at the fuel pressure sensor block. In this third hole, I insert a -4 AN fitting that has the internals filled with a plug and a new hole drilled with a #60 bit. A -4 line goes from there to a open/close valve in the cockpit and then on to a fitting in the left fuel tank. This design is similar to the AFP purge valve and is used to evacuate hot fuel from the circuit and replace with cold fuel to avoid vapor issues; typically used to purge fuel after heat soaking on the ground.

To be done poperly, the fuel that is returned should be coming from the servo or spider and not the fuel pump. In the latter case, there is still a lot of hot fuel that is not being flushed out. There is very little fuel between the firewall and the pump; most is downstream. The servo has a port in it for taking off a fuel pressure sensor line and when I had a carb, I had a tee at the carb input. Second, you want some kind of a restrictor to limit the flow through the return. As you noticed, if you don;t do this, the return can rob all of the fuel and make mistakes or a failure of the valve cause the engine to stop running. My restrictor only allows about 12 GPH to return. The pump produces 40+ GPH and the engine only needs about 15. Therefore, any issues in the return cannot keep the engine from getting the fuel that it needs. Downside is that I need to run it longer to flush out the hot fuel.

Larry
 
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Thanks, Guys.

With your input, and a little more digging, yes that's what I think it is: a "flush" fuel circuit to prevent vapour-lock on the ground in hot weather with a recently-flown engine.

To use it, before start select the Main Fuel to either tank. Open the return valve to the rear position. Turn the electric fuel pump ON. Fuel will be pushed through the system and into the left tank, replacing hot fuel in the engine compartment lines with cooler fuel from the main tank selected.

I don't like it and don't think it's necessary here (Canada). We do burn Shell 91 mogas (no ethanol) with no problem, but only in cruise. We keep one tank full of 100LL which we take off and land on, and cruise on the 91. So there is no increased risk of vapour lock.

This winter I may remove it entirely.
 
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