diesel tank theory

[Ron & Carol]

I too am an owner of a "slow to fill" and "leaks diesel fuel when filling" RV. I have reviewed several posts on this topic and still do not understand the relationship between the 3 "orifices" in the diesel tank.
orific #1: fill tube
orific #2: vent tube running up to fill tube top
orific #3: overflow tube

Its the overflow tube that has me puzzled. I unconnected my vent tube at the top of the fill tube and blew into the vent tube back toward the tank (I was hoping to push any fuel blocking the vent tube back into the tank) - I could not feel any air comming up the fill tube as I expected - but I did push diesel fuel out of the overflow tube - I did not expect that to happen. I am level and have about 3/4 of a full tank.

I need someone to explain to me how these 3 orifices work together in a properly working system. And why we need an overflow tube. (It seems to me that the overflow tube would act like a vent)

Another question is how can I get fuel into the vent tube to start with?

Another question is what is fuel foaming and how does that affect a properly working tank system.

Thanks in advance. I am dry camping and having satellite issues also - might not get back on until Monday.

[Ron & Carol]

I too am an owner of a "slow to fill" and "leaks diesel fuel when filling" RV. I have reviewed several posts on this topic and still do not understand the relationship between the 3 "orifices" in the diesel tank.
orific #1: fill tube
orific #2: vent tube running up to fill tube top
orific #3: overflow tube

Its the overflow tube that has me puzzled. I unconnected my vent tube at the top of the fill tube and blew into the vent tube back toward the tank (I was hoping to push any fuel blocking the vent tube back into the tank) - I could not feel any air comming up the fill tube as I expected - but I did push diesel fuel out of the overflow tube - I did not expect that to happen. I am level and have about 3/4 of a full tank.

I need someone to explain to me how these 3 orifices work together in a properly working system. And why we need an overflow tube. (It seems to me that the overflow tube would act like a vent)

Another question is how can I get fuel into the vent tube to start with?

Another question is what is fuel foaming and how does that affect a properly working tank system.

Thanks in advance. I am dry camping and having satellite issues also - might not get back on until Monday.

[Eddie Foy]

I suspect that the overflow tube dips down into the tank so that it is below the fuel level. Vent may be plugged up.



<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Ron & Carol:
I too am an owner of a "slow to fill" and "leaks diesel fuel when filling" RV. I have reviewed several posts on this topic and still do not understand the relationship between the 3 "orifices" in the diesel tank.
orific #1: fill tube
orific #2: vent tube running up to fill tube top
orific #3: overflow tube

Its the overflow tube that has me puzzled. I unconnected my vent tube at the top of the fill tube and blew into the vent tube back toward the tank (I was hoping to push any fuel blocking the vent tube back into the tank) - I could not feel any air comming up the fill tube as I expected - but I did push diesel fuel out of the overflow tube - I did not expect that to happen. I am level and have about 3/4 of a full tank.

I need someone to explain to me how these 3 orifices work together in a properly working system. And why we need an overflow tube. (It seems to me that the overflow tube would act like a vent)

Another question is how can I get fuel into the vent tube to start with?

Another question is what is fuel foaming and how does that affect a properly working tank system.

Thanks in advance. I am dry camping and having satellite issues also - might not get back on until Monday. </div></BLOCKQUOTE>

[Gary RVRoamer]

You got me - I didn't even know there was an "overflow tube" in the filler neck. The only thing I know as an "overflow tube" is the return line that sends excess fuel back from the engine feed line back to the tank. Diesels pump more fuel than is needed for combustion and the excess cools the injectors and then is cycled back to the tank. Most diesel fuel system literature I have read refers to this return as the "overflow tube".

Could it be just a line to allow fuel to flow back to the tank after it foams/gushes up the fuel filler during fueling? That would make it an overflow drain and its purpose would be to avoid backsplash and fuel spilling on the ground or into the vent tube.

Fuel gets in the vent line from the upper end when fuel splashes back hard enough to reach the end of the filler where the vent opens into it. Not hard to do with high pressure truck pumps.

Foaming seems to be the nature of the beast. Supposedly all hydrocarbon fuels foam to some degree, but diesel seems to be much worse than gas. There are anti-foaming additives available for diesel fuel and perhaps some fuel blends have more or less than others. Or maybe some brands have none at all? I don't know...

[chasfm11]

I'm interested in this topic, too, though my symptoms are slightly different than Ron's. I've done very little to deal with my problems other than to put a strap clamp around one part of the filler hose (between the inlet and the wall and the metal pipe connection to the the tank) to keep if from creating a kink and a resulting flat point in that hose. All the clamp does is force it to stay round.

Mine fills very slowly, kicking off even the very slow RV pumps at Flying Js multiple times. I have to manually hold and very lightly squeeze the satellite pumps (my filler is only on the passenger side) when I use the truck lanes. On the slower RV lane pumps, I can generally set the automatic feed after I've put in 20-30 on a 1/2 empty tank but usually not before then. Even the slowest speed on the satellite pumps kicks the automatic setting off in less than a second.

I've considered completely disassembling the filler plumbing, looking for an obstruction. I've always assumed that the problem was caused by poor engineering on the vent and that there was little that I could easily do about that.