fuel gauge rising by itself, could my aux tank be syphoning into my main tank?
 

I have a 45 gallon RDS transfer tank. Yesterday I mounted a pickup tube from the top of the tank and ran fuel lines to a 12v carter fuel pump. I ran a line from the pump and spliced it into my fuel filler neck.

I filled up the transfer tank and tried out the 12v pump. Works great. With the pump I filled my main tank to a little bit under the full mark on the fuel gauge. I drove around a lot yesterday and noticed the fuel gauge didn't go down but I didn't think much of it.

Today I drove around for 10 or so miles and now my fuel gauge is up past the full mark.

The 12v pump I put in is turned off, its loud and I would be able to hear it if it were running.
I opened the fuel door and pulled the cap off, with the truck idling or turned off I can't hear fuel pouring into the fill neck.

Is it possible that when under load my main tank could be creating a vacuum and syphoning fuel from my aux tank through my 12v pump?? I wouldn't think that would be possible but seems to be the only explanation for why my fuel gauge is rising by itself.

Here is how I have the aux tank and 12v pump plumbed into the main tank:

http://i121.photobucket.com/albums/o...ps7dbb4c77.png



Although pricy at $74 bucks, would this solve my problem?

http://www.dan-marc.com/79-afc161.html

They have another one:

http://www.dan-marc.com/79-afc11112.html

Which is $33 bucks but is only rated to 30 PSI. Would that be enough?

Also should I put it before or after my inline fuel pump?

I have a fully independent tank in the bed that has 12v switches on the on the fuel lines. When my auxiliary is full I have the same problem the fuel runs into my factory tank. Switches are hooked directly to the battery so they are hot all the time. The only thing that I can figure is that the switches are only rated for like 3 or 4 lbs fuel pressure and the fuel leaks by when tank is full.

Does your main tank ever overflow?

I dont understand how it would gravity feed into the main tank. My pick up tube is placed on the top of my aux tank. So the fuel would have to go UP then down....

I put the 12v pump system in because I specifically did not want a gravity feed system.

Should I install some kind of electronic check valve in the fuel line between the tanks?

Then I could hit a switch for the check valve and then turn the 12v pump on, and shut both back off when finished

Mine gravity flows because the suction is on the bottom of the tank. The only thing I can figure is when it isn't full it flows back through the two way switch I have connected to the fuel line. Once it get about 1/2 empty it will quit. Yes the factory tank has overflowed when I left my switch on.

I am aware the factory tank will overflow if I leave the 12v pump on.

However it is filling my stock tank with the pump OFF.

Somehow it must be siphoning the fuel. It was pretty hot today, another theory is my aux tank was heated up by the sun and instead of venting through my aux tank full over vent, the pressure inside the tank pushed the fuel through the inline pump and into my stock tank? I filled the tank with 41 gallons the day before.

Either way I will be ordering a 12v fuel shut off valve and install it. Going to wire it into the switch for the 12v pump so it should automatically open when I turn the pump on and close when the pump is off.

sounds like you will want to go ahead and vent the aux. tank then with the shut off in place.

I sell industrial pumps for a living..If fuel pumps are the same, well, they are not valves..Once a syphon is started, even with the pump off it may still allow fuel past.. example, our trucks will run with a bad fuel pump, just not well..

There's no real pressure in that fuel line before the pump, so pressure is not a factor. If fact, it it's really siphoning, there's a slight vacuum.

Take the lower-pressure of the two valves and put it between your filter and pump as your drawing shows. The filter will keep crud out of the valve, improving its reliability.

Should solve the problem of siphoning.

Wire it to that fuel pump's wiring, and when transferring fuel the valve will allow flow.

Pop

My money is on what Joey said, a pump isn't a valve. Once the siphon starts, it continues to draw.

Now, my aux tank gravity feeds the stick tank. I can fill up both tanks and leave the valve open and when fuel is drawn out of the main tank, it is automatically replaced by fuel in the aux tank. The only time this setup has ever leaked was when one of the clamps on the fill tube loosened up. I immediately replaced it with a t-bolt clamp and haven't had any trouble since. I also put a new fuel cap on at the time so it wouldn't leak there.

So I guess my question is, why all trouble with the pump? It will gravity feed faster than you can burn it, and will stop when it's full. (Provided all your clamps are tight and the cap is sealing like it should.

I have heard of gravity feed setups leaking when going over trucks or bumps. I will be offroad and or on gravel roads quite often.
I would also like to keep the aux tank full and only use that fuel for emergency situations, cycling it of course every 3 months or so.

And with the pump vs gravity feed I will have a better idea on how much fuel is actually left in the aux tank.

Ok, the what you need is an electric valve actuator. I'll see if I can find the post that talked about them.

Something like one of these. 12 Volt Fuel Shut Off Valves

This way you can control it from in the cab. Might take an extra switch, but it should solve the problem you have now.