Fuel Pump Return line AND fuel line tees

[john carroll]

It is my experience that any sort of a tee in the supply line to a pump will
eventually give you problems under some condition.  Certainly there are
situations where the risk is minimized.  If both of the supplies have plenty
of head or a feed pump to supply the high head pump with several times its
pumping capacity, don't stay up nights worrying about it.  This is
particularly true if the pumping system is not mission critical and the
equipment is cheap to replace.  Most fuel systems do not qualify.

The problems arise when one of the lines sucks its first gasp of air.
Despite the fuel available from the other line, air bubbles have been
introduced into the system, and will continue to be introduced as long as it
is easier to pump air from the empty line than fuel from the full one.

If one insists on using a header tank, be certain to vent the header tank to
the main tank, or alternativly, vent both to the atmosphere in order to
equalize the pressure.

At 08:26 PM 8/27/96 -0500, you wrote:
>At 06:02 PM 8/26/96 -0600, you wrote:
>>Dumping the bypassed fuel back into the inlet side of the pump is a bad
>>deal.  The sudden depresurization and turbulance encourages the lighter
>>fractions to bubble out of solution.  After a few passes through the
>>regulator and engine compartment the fuel warms up, compounding the problem.
>>A friend did this despite being warnined.  He is alive, his airplane is
>>bent.  Put it back into the tank.
>>
>>john carroll
>>jac at wave.sheridan.wy.us
>>
>>
>>
>>
>>At 03:13 PM 8/26/96 -0500, you wrote:
>>>>Matthew Lamari <mlamari at origin.ea.com> wrote:
>>>>
>>>>I was just thinking of your idea (quite good) for the return line to a pump
>>>>going to the back of the pump (thus saving further butchery of the fuel tank
>>>>area.)
>>>>
>>>>I was wondering, though, that, in the flow circuit, it is possible for the
>>>>pump to end up pulling fuel through the regulator.  I guess this would
>>>>depend on the flow resistance between the fork and the tank, and the pump
>>>>and the return fork. 
>>>>
>>>>Since it was your idea (don't want to take credit) I was wondering if you
>>>>could mention it to the group and see if anyone has tried it or shoots it
>>>>down for some reason.
>>>>
>>>>It sounds like a cool way to add such a thing; but I was wondering if it had
>>>>any hazards I should know about.
>>>>
>>>>thanks,
>>>>Matthew.
>>>>
>>>
>>>Okay, here goes:
>>>
>>>On the Holley EFI system (Matt, you could call Holley tech. support
>>>at 502-781-9741 about this), a return petrol line is required from
>>>the pressure reg which is located at the TBI.  The fuel pump runs
>>>continuously with the excess fed back to the tank.  I understand
>>>some other systems do this as well.
>>>
>>>Matt wanted to avoid installing a return line (I put mine into the
>>>filler neck).  I suggested perhaps putting a tee into the pump intake
>>>and feeding back to there.  Then, if no fuel being used by car,
>>>the fuel would circulate in loop from pump to TBI back to pump.  Only
>>>fuel drawn from tank would be actual amount used.  The way the return line
>>>system works, the fuel is constantly circulated through the tank,
>>>its filter sock, and whatever other filters are in line.
>>>
>>>Any comments?  Any reason why this wouldn't work?
>>>
>>>tom cloud
>>>
>>>
>>john carroll
>>jac at wave.sheridan.wy.us
>>
>
>I agree with what the rest of the group is saying, it seems logical.
>However, in Frank 'Choco' Munday's book, SBC TPFI, he recommends adding the
>tee to the pump suction line. Based on the quality of the book, you could
>probably get by with it. I have complete confidence that Choco has tested it.
>
>Remember, 'It's better to burn out, than fade away....'
>
>Good luck.
>
>GMD
>
>
john carroll
jac at wave.sheridan.wy.us