Fuel tanks

tom geiger

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Dec 31, 2006
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So kinda going thru my to do list on my motorhome. Being that the Covid numbers were clicking up around KC we decided to lay low around the house.
So thus all the projects on the motorhome.

The next project is to work on the fuel tank system. Never been confident on the gas gage. I had a mechanic drop the tanks and replace all the
hoses. At that time he told me about only one sending unit working, so he set it up to have the rear sender work. The instruction to me was that the
fuel gage 1/2 tank reading was actually be a 1/4 tank. Now I seem to be running out of gas at just above 1/2 tank so plan on replacing the tank
selector valve, but in re-reading the info online on the subject I ran across this.

I'm having senders go out more frquently and with the crappy action of the new senders available, I have been opening up the senders, cleaning up the
windings of the resister coil resetting the tension on the wiper and checking connections on the terminal to make the old senders work again. Have
been pretty successful in getting at least one of the 2 senders working. In reality you only need one sender (preferrably in the rear tank) to work.
Both senders read the same fuel level until the tank with the sender goes below 1/4 tank. I would never run a coach below 1/4 tank-- they are too
heavy to push! If you watch the odometer and not go below 1/4 tank as much as possible, one good sender will do ya. On fuel injected coaches, I toe
a crossover between the drains of the tanks to allow sharing of fuel right to the bottom which actually negates the need for 2 senders. If the FI
system dumps it's pressure into the fill rail, you could draw one tank
totally dry without the lower crossover line.

I am wondering having this lower crossover hose to combine both tanks would be my best solution? I am hoping this will be so I can get a reliable
fuel level reading and use most of my fuel capacity. Seems to be a simple and slick solution to all this. If so then not seeing a need for fuel
selector valve?

Thanks,
Tom
--
Tom Geiger
76 Eleganza II
KCMO
 
Tom,
I have EFI so running low on fuel is something I don't want to do. I also don't think of the tanks as Main & Aux, more like 2 tanks, front and rear.

From experience I find that once the fuel level gets below 1/2, more fuel tends to stay in the front (Aux) tank than in the rear (Main) tank. This is
proven in these diagrams on Gene's site:

http://bdub.net/gmcmotorhome.info/tank.html

Because of the forward position of the fill ports, fuel tends to move to the front tank when going down-hill and when braking and more significantly
when braking and going downhill. In the latter case I've seen 1/2 tank reading on level ground turn to Empty in rear (Main) and 3/4 reading on front
(Aux) tank.

My EFI fuel return goes into the fill tube at the front. I've also noted that when running at 1/4 tank and below, level ground on the front (Aux)
tank, the return fuel tends to run past the front and into the rear (main) tank. So my front tank gets pumped down fast and the rear fills up. So
when I get low I'm watching the gas gauge closely and changing back and forth wherever the gauge says the fuel is. So I can't imagine running on one
tank sensor. I need both to see where if the fuel is!

So my rule of thumb is once I get to 1/2 tank reading, I'm looking to fill up with fuel. That's about 200-250 miles of driving and 4 to 5 hours of
sitting. My butt is numb and my bladder is full! so I need a break too.

I know it bothers some that there is still 1/2 tank left that they "are not using". But that 1/2 tank is about 20 usable gallons which is about $50 in
fuel. The cost to fix it so you can reliably and consistently get that last $50 in fuel is going to cost you much more than $50. Its still there if
you need it, you just need to be more careful about watching your tank levels when going that low.

I did see a product which was like a mat kinda filter. It was laid out in the tank and held fuel like a sponge bringing it to the fuel pickup from
wherever it was in the tank. But it was expensive and the largest at about $400 each was still not big enough for our tanks.

You mentioned about replacing your fuel selector valve. If you have EFI, I would suggest instead installing dual fuel pumps. The cost of the second
fuel pump is about the same as the cost of the selector valve and it gives you redundancy in your fuel pumps.

http://www.gmcmhphotos.com/photos/g7210-dual-fuel-pumps.html

One last thing, the thought of that tube between the tank drains always makes me nervous for road debris hitting it. Make sure you have a study shield
to protect it from tire carcasses etc hitting it!.

--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
 
Bruce, thanks for the write up on this. Solid logic in it. It clears up the issue with actual behavior of the gas in the tanks in application which
is very good. I’ve been seriously considered going over to a EFI system but not at this point. I’m hoping to do something about my all electric
fridge first and then maybe next year look at the EFI systems. I was considering the small resevior tank system to avoid all the return line setup to
the gas tanks. I also was looking at a FI-Tech system but had read some breakdown failures from some which brings in to question of its
dependability.
But it sounds like joining the tanks with a lower flow hose is not ideal and restoring original fuel system is recommended in any setup. So I can
proceed in that direction.

Thanks Bruce,
Tom
--
Tom Geiger
76 Eleganza II
KCMO
 
Tom,
The addition of a surge tank is a good idea to get around the sloshing fuel. I believe Ken Henderson among others have made their own. Maybe he will
chime in on his experiences.

I made one using a spin-on fuel filter as the tank. I never got time to make it work so I finally took it off. I still have it sitting on a shelf
here.
http://www.gmcmhphotos.com/photos/mechanical/p27747-close-up.html

Pipe extended down to pick up fuel from bottom of filter tank.
http://www.gmcmhphotos.com/photos/mechanical/p27745-short-nipple-threads-in-on-the-outlet-side.html

The first version of the FiTech surge tank ( I can't remember what they called it.. Command center maybe?) had the high-pressure fuel pump and the
fuel regulator inside the tank. This meant the regulator bypassed the excess fuel right back into the surge tank. The fuel just went round and round
in the tank heated by the pump and the action of being pumped till it overheated the fuel in the surge tank. I've read a few accounts in various auto
forums about how hot the tank would get.

They have a new version now, but I have not investigated it. I would certainly return the fuel to the main tanks to prevent heating the fuel.

Whatever EFI system you decide on, be sure the electronics are housed separately so you can mount the module inside rather than in the hot engine
compartment.

--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
 
Being a lifelong boater (100ton Captain) and a pilot (10,000 hrs plus, over
60 years) including over 4,000 hours in helicopters, Fuel is high up my
priority list. Owned GMC's since 1979, over 250,000 miles in them. And I
have always used the system in a GMC as the way I feel was intended- run the
main tank until it (almost) runs out, or does run out, switch to aux and
find a station with the approx 6 gallons you have left. Yes, take in
consideration the ups and downs of the road...bla blabla. I also know my mpg
and get fuel before I pass the 300 +- mile mark.

If you want to REALLY know, buy an instrument up to the task. I never owned
a personal helicopter that I didn't install a fuel flow gauge. Most of my
personal aircraft had them as well. They include a transducer in the fuel
line that tells you how much fuel you are using and more importantly, how
much fuel you have used. Then do the math. Fairly cheap (see fueltron for
example).

Dean the Resourceful Norse Hanson

75 Avion

Fremont, Ca.
 
Dean, love that idea. Very important gage when no tires on the ground. When I run out of gas I come to a rolling stop, not so much I would imagine for
anyone flying. A little bit more involved to get wheels safely on the ground. Great suggestion, I always thought it would be nice to apply a number
on gallons of gas used and left in the tanks.

Thank you,
TG
--
Tom Geiger
76 Eleganza II
KCMO
 
The GMC fuel system is a riddle locked up in an inegma. It really must have
been designed? by committee. Nothing about it makes sense. There are
several ways that it can be improved upon.
One, might be to eliminate the common manifold between tanks. Install
a separate fuel inlet for each tank. That way the fuel that you put in the
tank stays in the tank until it is consumed.
A second method might be to remove both tanks, and replace them with
one large tank of equal volume. That would eliminate a whole bunch of
trouble. Tank switching valve, extra fuel lines, electrical switches and
wires, etc.
Fuel flow meter, not a bad idea, I guess. Might work well for some
guys.
Jim Hupy
Salem, Oregon

On Wed, Sep 16, 2020, 10:30 AM tom geiger via Gmclist <

> Dean, love that idea. Very important gage when no tires on the ground.
> When I run out of gas I come to a rolling stop, not so much I would imagine
> for
> anyone flying. A little bit more involved to get wheels safely on the
> ground. Great suggestion, I always thought it would be nice to apply a
> number
> on gallons of gas used and left in the tanks.
>
> Thank you,
> TG
> --
> Tom Geiger
> 76 Eleganza II
> KCMO
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
The EBL system will display a calculated gallons of fuel used, but it is calculated, not measured. I'm guessing to measure it you would need to
measure the fuel delivered by the pump, less the fuel returned to the tank.

But these are also '70s era fuel fill systems. So did you get 35 or 40 or 50 gallons in your tanks? I'm remembering the fill behind the rear licence
plate. Usually got a fuel bath trying to get a full tank of gas.

http://www.gmcmhphotos.com/photos/misc/p58240-10mpg-2c-it-can-be-done.html

--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
 
One more question on the fuel tanks. Anyone have a good diagram of the fuel hose and wiring of the senders? Just need a good understanding of what
it should be when I lower the tanks. I have no idea what my mechanic did when he worked on them.

Thanks,
TG
--
Tom Geiger
76 Eleganza II
KCMO
 
There is a diagram of the fuel lines in the service manuals and there is a single page copy of it somewhere on the Internet, but I could not find it
this morning.

Over at gmcmhphotos.com if you search for "fuel tanks" and "fuel lines" you will come up with close to 200 photos other have posted about taking their
tanks down.

On the back of the dash fuel selector switch you will see 2 wires on one side and 3 on the other side. The 2 wire side is the fuel selector valve
power. The 3 wire side is the gauge side. The center wire goes to the gauge. The wire opposite the valve power wire will be the Aux fuel sender since
the valve gets power in the Aux position (I think its the top wire). The other wire (bottom?) will be the main tank fuel level sensor.
--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
 
Here is a hand drawn diagram of the fuel lines.

http://www.gmcmhphotos.com/photos/fuel-system/p51345-gmc-fuel-tank-diagram.html

And here is some advice posted by Matt Colie a few years ago
Jon,
It is not all that confusing when you are under there. (Says a guy that has had his tanks down too many times.)

Three lines of advice.
1 - If the plugs won't loosen, get a pump and pull the fuel out and into a 50gal drum. Get the tanks empty.
2 - Take the fill pipe down first and put it back in last.
3 - Get a set of cargo straps from Hazard Fright to lower and then lift the tanks. If you try to do it with any kind of jack, the jack has to be
where you want to be. (Straps 7~10$ find a coupon)

While you are there, replace every bit of rubber you can reach.

Matt
--
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that