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Fuel tanks [message #358684]

Wed, 16 September 2020 06:48

tgeiger

Messages: 518
Registered: February 2006
Location: kansas city

Karma: -1

Senior Member

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

[Updated on: Wed, 16 September 2020 06:51]

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Re: Fuel tanks [message #358686 is a reply to message #358684]

Wed, 16 September 2020 07:42

RF_Burns

Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada

Karma: 3

Senior Member

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://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

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Re: Fuel tanks [message #358687 is a reply to message #358684]

Wed, 16 September 2020 08:36

tgeiger

Messages: 518
Registered: February 2006
Location: kansas city

Karma: -1

Senior Member

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

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Re: Fuel tanks [message #358689 is a reply to message #358684]

Wed, 16 September 2020 11:02

RF_Burns

Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada

Karma: 3

Senior Member

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.

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Re: Fuel tanks [message #358717 is a reply to message #358684]

Thu, 17 September 2020 05:05

tgeiger

Messages: 518
Registered: February 2006
Location: kansas city

Karma: -1

Senior Member

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

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Re: Fuel tanks [message #358718 is a reply to message #358684]

Thu, 17 September 2020 06:28

RF_Burns

Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada

Karma: 3

Senior Member

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.

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Re: Fuel tanks [message #358719 is a reply to message #358684]

Thu, 17 September 2020 06:47

RF_Burns

Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada

Karma: 3

Senior Member

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

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Re: Fuel tanks [message #363045 is a reply to message #358718]

Thu, 25 March 2021 15:53

74_Coach

Messages: 71
Registered: July 2019
Location: Arroyo Grande, CA

Karma: 0

Member

RF_Burns wrote on Thu, 17 September 2020 04:28

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.

This is the diagram I used to fix screwed up fuel tank venting while installing 2 new fuel tank(s) plus I followed Matt's advice of placing the vent lines as high as possible without droops. I drove the beast down to the gas station yesterday to "fill-er" up and it was the first time since purchasing this turd that fuel did not come barfing out at me through the neck. The gas pump also stopped pumping fuel when it got a signal that the tank was full. Imagine that - fuel tanks completely full!!!

Ed Clerkin | 1974 GMC Canyon Lands 26’
Central Coast CA

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Re: Fuel tanks [message #363093 is a reply to message #358684]

Sat, 27 March 2021 16:29

tgeiger

Messages: 518
Registered: February 2006
Location: kansas city

Karma: -1

Senior Member

Perfect info guys, much appreciated. Will be digging into this soon. These pointers sure help!

TG

Tom Geiger 76 Eleganza II KCMO

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