Gas Tank Switching Valve Conundrum

Carl Stouffer

Carl Stouffer

Well-known member
Jan 15, 2009
4,693
381
83
Tucson, Arizona 85718
I am in the process of re-plumbing my fuel hoses from the tanks to the front hard fuel line. Part of what I am doing is to move the lines, pumps,
filters, etc to the outside of the frame at Jim Kanomata's suggestion. I am also replacing the electric fuel pump (a solenoid type pull through pump)
with the GMC Net preferred Carter along with a cartridge type filter and a new tank selector valve.

My plan is (was?) to run the new fuel pump off of the auxiliary tank only, like I had my old one set up. The problem is that the tank selector valve
says to NOT pressurize the inlets. The only way I can think of to run it that way is to pressurize the inlet to the valve by putting the pump between
the tank and the valve. Since there are two inlets on the valve and only one outlet, I can't think of any other way to do it besides running the
electric pump all the time.

The old set-up I have had on the coach for years puts pressure on the inlet side of the valve, on the aux inlet, and it has worked fine. Can anybody
give me a reason this will not continue to work as it has in the past? Is there a danger to "pressurizing the inlet" or any reason I shouldn't do it,
other than the obvious, the warning on the instructions.

I'm already two thirds of the way through this project and don't want to have to start again.

Thanks
--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles,
Solar battery charging. GMCSJ and GMCMI member
 
Carl,

Since you've got instructions saying to not pressurize the inlet, I presume
you've bought a new selector valve?

If you have NOT bought that valve, I strongly suggest comparing the cost of
a new one (which is certainly advisable in view of the number of failures
seen lately) to the cost of a second electric pump. The difference is
probably not great and the advantages of having redundant electric pumps
should be more than worth that cost.

You've probably read this diatribe of mine too many times, but I keep
hoping to help some newby by repeating it: An electric pump from each
tank, selected with the dash's tank selector switch, tee'd together into
the existing forward fuel line is the most reliable system possible because
of its redundancy. The mechanical pump should be bypassed to ensure that
it doesn't leak the pressurized fuel into the crankcase. Aside from
incorporating a "surge tank" with a return line to the fuel tanks, this is
probably the most effective vapor lock preventive system available (without
a much more complex fuel cooling system).

JMHO,

Ken H.

> I am in the process of re-plumbing my fuel hoses from the tanks to the
> front hard fuel line. Part of what I am doing is to move the lines, pumps,
> filters, etc to the outside of the frame at Jim Kanomata's suggestion. I
> am also replacing the electric fuel pump (a solenoid type pull through pump)
> with the GMC Net preferred Carter along with a cartridge type filter and a
> new tank selector valve.
>
> My plan is (was?) to run the new fuel pump off of the auxiliary tank only,
> like I had my old one set up. The problem is that the tank selector valve
> says to NOT pressurize the inlets. The only way I can think of to run it
> that way is to pressurize the inlet to the valve by putting the pump between
> the tank and the valve. Since there are two inlets on the valve and only
> one outlet, I can't think of any other way to do it besides running the
> electric pump all the time.
>
> The old set-up I have had on the coach for years puts pressure on the
> inlet side of the valve, on the aux inlet, and it has worked fine. Can
> anybody
> give me a reason this will not continue to work as it has in the past? Is
> there a danger to "pressurizing the inlet" or any reason I shouldn't do it,
> other than the obvious, the warning on the instructions.
>
>
 
Thanks Ken,

Yes, I did buy a new selector valve from Applied, along with all the other components. Your advice is well taken, and that is probably what I should
do.

However, I probably won't :roll: At least not this time around. The new valve is identical to the old one, which worked perfectly upon testing it
on a battery. I only replaced it (again) because it has been recommended here ans elsewhere. I had the old pump, which proved to not be up to the
task, before the valve and it never presented a problem.

I guess I could return the new valve and buy another pump. Do you have a wiring diagram for the set-up you are advocating?

--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles,
Solar battery charging. GMCSJ and GMCMI member
 
Lots of people have been pressurizing those at carb pressures (5 PSI) on GMCs with no problems.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
> Lots of people have been pressurizing those at carb pressures (5 PSI) on GMCs with no problems.

I'm one of them. I replaced a bad switching valve early on and added a pump between the tank and the valve later, not ever seeing that warning
before. It just made me a little concerned.

I also have a brand new (also Carter) mechanical pump on my brand new engine, so I'm not really worried about a diaphragm rupture pumping gas into my
crankcase.

--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction Arm Disc Brakes, Quadrabags, 3.70 LSD final drive, Lenzi knuckles/hubs, Dodge Truck 16" X 8" front wheels, Rear American Eagles,
Solar battery charging. GMCSJ and GMCMI member
 
Carl,

If I were you I'd "I guess I could return the new valve and buy another pump."

Regards,
Rob M.
The Pedantic Mechanic
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
USA '77 Kingsley - TZE 267V100808

-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of Carl Stouffer
Sent: Sunday, July 1, 2018 8:20 PM
To: gmclist
Subject: Re: [GMCnet] Gas Tank Switching Valve Conundrum

Thanks Ken,

Yes, I did buy a new selector valve from Applied, along with all the other components. Your advice is well taken, and that is
probably what I should do.

However, I probably won't :roll: At least not this time around. The new valve is identical to the old one, which worked
perfectly upon testing it on a battery. I only replaced it (again) because it has been recommended here ans elsewhere. I had the
old pump, which proved to not be up to the task, before the valve and it never presented a problem.

I guess I could return the new valve and buy another pump. Do you have a wiring diagram for the set-up you are advocating?

--
Carl Stouffer
 
Thanks Ken,

Yes, I did buy a new selector valve from Applied, along with all the other components. Your advice is well taken, and that is probably what I should
do.

However, I probably won't :roll: At least not this time around. The new valve is identical to the old one, which worked perfectly upon testing it
on a battery. I only replaced it (again) because it has been recommended here ans elsewhere. I had the old pump, which proved to not be up to the
task, before the valve and it never presented a problem.

I guess I could return the new valve and buy another pump. Do you have a wiring diagram for the set-up you are advocating?

--
Carl Stouffer
'75 ex Palm Beach
Tucson, AZ.
Chuck Aulgur Reaction
.........
 
I did what Ken suggested. Then there's a relay for the pumps. That the dash switch controls. Love it being outside of the frame. I mounted the components on a 1/4"aluminum plate. About 6"X14" Then mounted the plate to the underside of the floor. Steel line out of the two tanks. Then a steel line from the pumps to the front. The Carter 4070 is a great pump for a carb. Bob Dunahugh
 
> Thanks Ken,
>
> Do you have a wiring diagram for the set-up you are advocating?

just thinking about it briefly, it seems a little complicated if you want both to have the low oil pressure cut off also. probably need four relays to
do it all with one switch? but thats the way I would go for sure (two e.pumps, no m.pump)
 
Here's what that setup looks like, using Mr. Gasket pumps which have built - in check valves and Wix filters on the pump inputs:

http://www.gmcmhphotos.com/photos/g6198-cane-9-creek-rv-park-heflin-2c-al.html

I pulled the input lines off the selector and rputed them uncut to the pump inputs; and pulled the output line and routed it uncut to the tee on the
pumps' output and capped the selector to keep dirt and crud out of it. The fuel line fitting where the hard line to the carb used to fit the
mechanical pump is a standard which means the hard line is unchanged. Result: You can convert back to OEM in thirty minutes beside the road if you
had to. 25 of which would be replacing the mechanical pump.

--johnny
--
76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"I forgive them all, save those who hurt the dogs. They must answer to me in hell" - ol Andy, paraphrased
 
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