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Old 12-01-2015, 08:49 PM   #1
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Auxiliary fuel tank

I am adding a RDS fuel tank to my 2012 F350 with a 6.7. I have hooked up the tank, spliced into the filler neck. The roller ball in the fill line is as close to vertical as I can get. I have the rollover vent in the tank which is open. The problem is that when I close the cap on the truth and let it free flow the main tank builds up pressure and will not keep draining. If I open the cap, pressure is released, and it will drain until I put the cap back on then pressure builds up and it quits draining. Is this normal or do I need to change something? Is there supposed to be pressure in the tank?
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Old 12-01-2015, 09:48 PM   #2
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I have had an '09 and now a '10 450 with a similar hookup. I never have any pressure buildup, but maybe '11-up Fords are different (with the DEF tank). I do not know what you mean about the rollover vent in the tank?? as mine does not appear to have one. Over 200000 miles with the same tank and only a few minor glitches.
Which RDS tank model do you have--I'd like to take a look at it.
If there was no pressure buildup in the truck tank before you added the RDS, then something in the hookup is apparently the cause. If the pressure builds up in the truck tank, it ought to be releasing thru the RDS, as mine has a vent cap on it. Maybe RDS has had to redesign the tanks since 2009 when I got mine, so I wouldn't know what is different, if anything.
Seems like I have read of other aux tanks on the newer Fords, and the only item usually mentioned is a problem with the DTE/MPG readings from a confused computer.
Joe
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Old 12-01-2015, 10:04 PM   #3
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I do not know what you mean about the rollover vent in the tank??
Joe
What I mean by rollover vent is the same fitting that came in the instal kit. It is a fitting with a ball inside that while in the filler hose shut the fuel line off the the Aux tank when the main tank is full, and in the Aux tank incase of a rollover would shut the vent off from draining.

There was pressure in the main tank before I installed the Aux tank. So the pressure is not new. I just don't know if it is normal for the truck or If I have a vent line blocked. Or if I need to run another line from the main tank to vent.

The tank I have is the RDS 71804, 37 gallon tank.
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Old 12-02-2015, 06:08 AM   #4
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Mine is a home concoction, 36 gal. It will build slight pressure up when the aux fuel pump is running but the truck is not. When the truck is running, it has less pressure building up when I'm transfering diesel. I have a togle switch with a small light that reminds me the pump is on.
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Old 12-02-2015, 07:19 AM   #5
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Sounds like everything is normal. I don't think that is called a rollover valve. I think it is supposed to cut off the flow of the aux tank when the main tank is full. The RDS tank is vented at the top. My fill ball doesn't work right and my main tank stays full all the way to the cap. If I take my cap off on the main tank diesel will flow out, so I don't take it off....ever, until the aux tank is empty and the main tank uses some diesel. This doesn't seem to hurt anything and if it did I would just use the cutoff valve occasionally to keep the main tank full. I guess I do have pressure on the main tank but it is OK.
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Old 12-02-2015, 08:18 AM   #6
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Is that roll over internal ball check valve free? These also act as a tank vent in their normal vertical position. Do you also have a pressure build up in the aux tank? If so, then look at the R.O. vent. My '11 does not have any main tank pressure build up - or at least none that I've encountered in the many times I've filled it up.

This is the R.O./vent that I'm using on my street rod. Yours should be similar.



Also, a question - did you have any problems adding the gravity fill to your truck's fill tube as just this past Saturday I was looking at Tractor Supply's 36 gallon offering (it's physically bigger then what I want)?
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Old 12-02-2015, 08:19 AM   #7
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Mine is a home concoction, 36 gal. It will build slight pressure up when the aux fuel pump is running but the truck is not. When the truck is running, it has less pressure building up when I'm transfering diesel. I have a togle switch with a small light that reminds me the pump is on.

Mine is gravity flow and it will not flow when the truck is off. It builds up pressure in a couple of minutes and will not flow again until I open the cap to release pressure.
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Old 12-02-2015, 09:11 AM   #8
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Mine is gravity flow and it will not flow when the truck is off. It builds up pressure in a couple of minutes and will not flow again until I open the cap to release pressure.
I have a electric pump that transfers the fuel. My dodge holds bout 35 gal, so when it is down to 1/4 tank it takes about 25 gals to fill back up. I do have a light thats on when the pump is on, (believe me I have forgot that I have the pump on) but I only put in about 25 gals in the aux tank. This way I will not overflow the main tank. I push the fuel thru a filter, to the main tank. It takes about 1/2 hour to transfer 25 gals.
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Old 12-02-2015, 10:27 AM   #9
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Can you use a vented cap ? That may reduce the pressure.
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Old 12-02-2015, 11:19 AM   #10
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Is that roll over internal ball check valve free? These also act as a tank vent in their normal vertical position. Do you also have a pressure build up in the aux tank? If so, then look at the R.O. vent. My '11 does not have any main tank pressure build up - or at least none that I've encountered in the many times I've filled it up.

This is the R.O./vent that I'm using on my street rod. Yours should be similar.



Also, a question - did you have any problems adding the gravity fill to your truck's fill tube as just this past Saturday I was looking at Tractor Supply's 36 gallon offering (it's physically bigger then what I want)?

No I just cut the rubber hose. I didn't have a saw to cut the metal.
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Old 12-05-2015, 09:31 AM   #11
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cholt,
Your problem has my interest, as we probably have very similar setups except for the truck year. A) Are you SURE the 'pressure' you notice is pressure out, or vacuum in? Reason I ask, is I always had to open the fill cap to let air in when the tank was draining into the truck--it apparently is vented outward, not inward. As soon as I put a second fill neck (just a 45d 2" street ell) to more easily reach in with the pump hose, my vacuum problems went away, as the threaded plug on the ell is not air-tight. This is sounding like you need a vent to let air IN, not let air out.
I was having to stop once or twice per aux tank fillup to let air in--the tank would quit draining and the truck would start using from the main tank. Stopping to open the OEM fill cap would result in a 'hiss' of air moving INTO the tank, not out.
Never happens now that I have the second fill neck.
Hope this will help you.
Joe
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Old 12-05-2015, 11:20 PM   #12
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cholt,
As soon as I put a second fill neck (just a 45d 2" street ell) to more easily reach in with the pump hose, my vacuum problems went away, as the threaded plug on the ell is not air-tight.
Joe

To answer your question no I am not sure that it is pressure coming out and not a vacuum going in. I just hear the swish and assumed that it was pressure building up in the tank but you very well could be right in it could be vacuumed building up in the tank. What I don't understand is how you fix the problem do you have any pictures of it?
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Old 12-06-2015, 10:18 AM   #13
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RDS aux tank

Here are a couple of pics of my tank--hope they explain what I have done.
My truck is a 450 and hard to reach the vented cap, so I added the 2nd fill neck and at the same time, the drain/'swish' problem vanished. I didn't plan it that way, just worked out.
My setup has worked on my last 350, an '09 450 and now the '10 450. I have only had 2 'hiccups' with the setup--forgetting to replace the truck tank cap when filling the aux tank (fuel WILL run out of the neck) and filling to very top of the aux tank on a hot day (the cooler fuel heated up and diesel WILL expand, and leaked out of the new fill neck). Both those times made a pretty ugly mess.
Outside of this, the tank has served me well--using GasBuddy app, I look ahead for fuel prices and locate good price to fill. This will almost always save up to 20c/gal and I have found differences of up to 70c/gal. Over 300000 miles, any savings really add up. This was really helpful on our Alaska trip in 2012.
Joe
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Old 12-16-2015, 05:28 PM   #14
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I too have the same tank set up as wingnut60 and my last trip out with the fiver I also had the problem of fuel coming out of the main truck filler until I opened the cap and broke the cyphon from aux.tank to truck tank it was cold outside if that has anything to do with it? Hasn't done it since and that was after a fuel fill up.
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