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Old 11-29-2017, 01:23 PM   #1
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3rd fuel tank

Hello All:

Has anyone researched and/or installed a 3rd fuel tank in their Seneca?

I'd really like to have another 50 gallons or so. The additional 350 lbs of fuel plus the tank and support brackets don't bother me. Sometimes it's not really the extra range as it is buying your fuel where you want to rather than where you have to.

I'm in the research phase right now. I'd like to replace the forward most storage area on the driver's side, next to the DEF tank. Possibly make another hole, near the DEF fill, to fill the 3rd tank.

Thoughts? Suggestions?
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Aux tank fill hole.jpg  
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Old 12-01-2017, 08:18 PM   #2
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I love the idea but the 37ts really has no practical place for the tank so it out for us. I suppose you could install a tank between the rails behind the rear axle and use a couple of transfer pumps. The main issue would be tank level management. Not sure how you would accomplish that. If you get somewhere with this please publish your results.


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Old 12-01-2017, 09:30 PM   #3
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You have given me food for thought, as my unit is in storage till the beginning of April then, we have to head down to Middlebury for a factory visit. When I get back I will look into this, as on the drivers side first compartment I store a fire extinguisher and three jeep style jerry cans to top up my tanks after running generator for the weekend or longer . I think there may be a way but will have to look at dimensions. Having a solid mounted tank would be a definite plus.
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Old 12-02-2017, 01:43 PM   #4
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The research continues.
I went to a truck salvage yard to see what they had that might work. Freightliner makes an 80 gallon tank that would fit on the passenger side once I moved the batteries and all the connections, cables, sliding battery tray, etc. I would have done that except that the longer tank would not clear the right front hydraulic jack. Ugh. Too bad as it would have looked exactly the same as it does right now. Use the same brackets and straps. Add 1 additional bracket & strap. Fill tube would fit through the fiberglass step are just like now. I can't think of a good way to properly relocate the jack.

It looks like I'm back to the first idea putting a 3rd tank in the left front cargo box area. This is a more expensive proposition as I will need to properly drill holes into a heat treated frame. Buy more brackets and straps. Figure out the plumbing and routing of fuel lines. Likely I'll just gravity feed from the new tank to the old. Make skid plates to protect the fittings and hose.

I also stopped at a truck repair shop to get an estimate if I had them do the work. 10-15 hours of labor. Misc steel for fabrication. Hoses, fittings and shut off valves. Rough estimate of $2500 for 40 extra gallons. Not a very good Cost vs. Benefit in my opinion.

Stay tuned. The saga continues.
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Old 12-02-2017, 01:49 PM   #5
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Side Note:

My four coach batteries are just sitting loose in the tray. Time to fab up some battery hold downs.
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Old 12-04-2017, 09:05 AM   #6
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I have the same pain when it comes to fueling, and I'm entirely tired of filling both sides and we're often way out in the wild. So, here's just a thought: The new Freightliner M2 has a 100 gal, single fill tank that sits between the frame rails. I'm considering getting one and "hard plumbing" it to my other two tanks near the bottom. (gravity equalizes and as long as the new tank is not "higher" than the fill necks of my existing tanks, I'm good.) This could give me 170 gallons of fuel in case I want to tackle the Alaska highway or Atlanta at rush hour going 100 miles at 1mph using 1 gallon a mile. Anyway, just a thought. I'm already over this silly "tank balancing" complicated system that uses pumps and vents and such. Even if I don't do the center tank right away, I'm going to yank both tanks, clean them out, weld in bungs and install a pipe between the two to auto-balance as I fill from either side. I've already determined those two tanks are of the same height at the filler necks and I've measured that there will be very little impact on ground clearance.

As always, if this plan causes something to flip upside down and kill us all, three things are important: 1. I didn't recommend it. 2. I forbade it. 3. Make sure it does not happen on any property that I own. :-)

Let us know if you find a solution you like. I will do the same.
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Old 12-04-2017, 09:09 AM   #7
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Harbor Freight sells an interesting roll of velcro with hook on one side, loop on another. I use it for lots of things around the motorhome and campsite and even my shop, and $6.99 for 100 feet is nice. Because of the double-sided properties, you can wrap "around and around" to quadruple your strength if you need it for certain situations.


https://www.harborfreight.com/3-4-qu...rap-96215.html
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Old 12-07-2017, 05:01 PM   #8
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Raybo I hope your not suggesting to use the harbor freight velcro to secure our new 100 gallon tank. Only kidding. That stuff is great.

On another note you say they make a 100 gallon tank now, is there not another freight liner tank that will replace our current ones with more capacity? There are two bungs on the lower side of my tanks that could be used for balancing I would imagine.
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Old 12-07-2017, 05:35 PM   #9
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Quote:
Originally Posted by brtnsnwbrds10 View Post
Raybo I hope your not suggesting to use the harbor freight velcro to secure our new 100 gallon tank. Only kidding. That stuff is great.

On another note you say they make a 100 gallon tank now, is there not another freight liner tank that will replace our current ones with more capacity? There are two bungs on the lower side of my tanks that could be used for balancing I would imagine.
It's my understanding that the new Freightliner RV chassis uses a single, 100 gallon tank located between the rear frame rails.
There is a factory 80 gallon that would work GREAT...if the leveling jack wasn't in the way. I have until Easter Jeep Safari to figure it out. I want to do it, but only in a professional workmanlike manner.

Keep the ideas and comments coming.
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Old 12-07-2017, 05:47 PM   #10
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Quote:
Originally Posted by RayBo View Post
Harbor Freight sells an interesting roll of velcro with hook on one side, loop on another. I use it for lots of things around the motorhome and campsite and even my shop, and $6.99 for 100 feet is nice. Because of the double-sided properties, you can wrap "around and around" to quadruple your strength if you need it for certain situations.


https://www.harborfreight.com/3-4-qu...rap-96215.html
Ha. :-) No, the velcro was only to hold down the batteries and it works fine.

Replacing the current tanks seems tough if you've laid under there on a creeper for any time. Seems to me that the new FL chassis, with the 100 gal tank can be a "teacher" for those of us that want more capacity and a single fill point. Our current tanks (Mine is 2012 Seneca FK) hold 74 gallons, but because of the equalizing system, you can actually only use about 45 gallons before you start starving the engine. Not sure of all the details, but seems to me that the crossover fittings are about 25-30 gallons "above" the bottoms of the "joined" tanks.

What I want to do (and am working on) is to put a 100 gal tank between the rails, like the new FL does today, and then tie that to the existing tanks at the lowest point, making sure that my 100 gal tank does not have a fill-neck height above those of the existing tanks. (Or, I could hermetically seal them as long as I don't go above the vent ports with my overall fill-level.)

Gravity will resolve the rest and the existing "pump based" equalization system will be disabled by me.

I figure if I can install a fuel range extender on the Jet Ranger, I can probably work out something suitable on the Seneca, so I'm probably going to get it done.

As I get it worked out, I'll post again, and will include pictures.
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Old 12-07-2017, 06:06 PM   #11
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Ha. :-)Gravity will resolve the rest and the existing "pump based" equalization system will be disabled by me.
There is no equalization pump, our M2's use a passive equalization system on the fuel return side. Some info attached.
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File Type: pdf Fuel transfer system.pdf (17.4 KB, 40 views)
File Type: pdf Fuel transfer system 2.pdf (143.1 KB, 22 views)
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Old 12-07-2017, 07:44 PM   #12
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So are we drawing fuel from both tanks simultaneously or from one tank and the fuel equalizes? And I know it's even asked before but how much usable fuel do we have in the tanks? I am allowing for 20% unusable or about 60 gallons available. Is that a reasonable assumption?
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Old 12-07-2017, 08:48 PM   #13
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So are we drawing fuel from both tanks simultaneously or from one tank and the fuel equalizes? And I know it's even asked before but how much usable fuel do we have in the tanks? I am allowing for 20% unusable or about 60 gallons available. Is that a reasonable assumption?
We do draw and return fuel to both tanks simultaneously. I am not totally clear on how it "knows" how to equalize the tank levels, but in time while running it will. I say this because there have been times I have only filled one tank at some fuel stops and in time it will equalize the levels. Also the generator on my Seneca only draws (and returns) from the driver's side tank and long parked genny runs will take that tank down. But after some driving miles the tanks again equalize.

I have put my borescope into the tanks and the engine draw lines do go within an inch or so from the tank bottom. So theoretically, it should be able to draw the tanks down to withing just a few gallons of empty. But if one tank level was lower than the other I suspect it would draw air there first and stall. I also saw the generator pickup tube and it is indeed several inches higher than the engine draw tube to keep one from emptying the tank with generator usage.

Normally my fill up cutoff is about 400 miles, I always try to keep a cushion to run the generator if I have to. But one time on a run to visit family I intentionally braved it and ran it lower than normal, eventually put 65 gallons back in on a 560 mile trip. Gauge buried on "E" and the low fuel warning light on for 100 miles, with the last miles on Chicago-area highways! But it didn't have any running issues. Scangauge was keeping track of gallons used but I was a bit nervous even though it showed I still had 9 gallons left.
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Old 12-08-2017, 02:24 AM   #14
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I was a bit nervous even though it showed I still had 9 gallons left.
Wow!!! Have you ever run out of fuel in a diesel? Itís a completely different experience than in a gas vehicle. ďBack in the dayĒ, if you ran out you had to remove the fuel filters & top them off w/ fuel then reinstall them. Repeat that process as needed. Often several times. If you were lucky your engine had a mechanical manual hand pump to help prime the filters & lines.
I havenít looked for such a hand pump on my Seneca.
My 2010 Dodge Dually w/ the 6.7 L Cummins states to turn the key on & off several times to let the electrical pump prime everything. That worked well when changing fuelters.
I donít intend to ever let mine get that low. Thatís why Iím so interested in more fuel capacity.
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Old 12-08-2017, 05:50 AM   #15
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There is no equalization pump, our M2's use a passive equalization system on the fuel return side. Some info attached.
Thanks for that info. So now, I'm not sure what is going on. I observe that if I fill my starboard tank and not the port, I hear a "whirring" sound (like a pump) and then the tanks equalize even when I'm not running the truck. (I shut the truck off when filling tanks.)

Anyway, I suppose it's possible the folks who sold it to me might have made a mod. (He was an aircraft A&P guy and I bought it at 7K miles from his widow, so I can't really ask him.) I'll have to get under there and see what is the story. I'll have to break out the borescope, because I can't get into a position to see what's on top of the Allison.

Regardless, whatever I have isn't working so great. I've done the vents, checked the lines for air entry points, etc., but I still just really don't like it. For me, a simple "bottom of the tanks" equalization pipe cures all these problems. The engine returns about 0.3 gallons a minute to the tanks, however just a 3/4" pipe can flow about 11 gpm at gravity pressure, so equalization would be quick and constant. Being forced to fill from both sides is just a poke in the eye for a MH. Big rig, ok, but for my wife, not so much.

I understand the point about the "unusable" 20 or so gallons. When I go over the steep pass in the Smokies at 8% grade, if I was 1/4 tank, I could imagine that the level of the fuel at that angle could have me sucking air and that would not be a happy day. So, the location of the fuel ports could really be why we only get 40 or so gallons out of 74. For me, I find no reason why there could not have been a fuel port at the bottom rear of the tanks (defended, of course) so that all the fuel would be usable. (Check that...wouldn't really work so well going downhill.)

I'm still interested in the new 100 gal "between the rails" and then tying it into the existing system. It's not that I mind stopping for fuel frequently, it's just that I really, really don't like to do so.

I'll post again when I get more info and will welcome any more thoughts in the mean time.
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Old 12-08-2017, 06:26 AM   #16
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3rd fuel tank

If I was doing this I would remove the two front tanks from supplying the engine and keep them for the generator. Then install the 100 gallon tank between the rails with a fuel pump. The fill line and venting can be done behind the bedroom cabinets. Kind of a pain to work over the transmission which is where the passive fuel lines are connected but there is a soft patch under the carpet that gives you access to this so it might not be as big an issue as it looks like at first glance. Then it would be a matter of getting the right size supply and return fuel lines as well as transfer pump.

Or two isolated systems use the new 100 gallon tank as a storage tank and install a transfer pump to one of the two existing tanks and hit the switch when you get to certain point. Issue with this is potential overflow of a tank if you turn the pump on but forget to turn it off. Maybe some sort of high level safety can be added.

One question that I have but have not investigated. What is the venting process in the existing tanks? I believe the vent is on the caps and if this is correct that is why the passive transfer is so slow. The air has to move in through a very small hole. This is why when filling I always fill 2-4 gallons less on the second tank I fill. I typically fill the smaller drivers side tank first then reposition and fill passenger tank. Some day I will experiment and remove the passenger cap first then fill the drivers side to see if I can get to 90% fill just filling from one side.


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Old 12-08-2017, 06:48 AM   #17
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Wow!!! Have you ever run out of fuel in a diesel? Itís a completely different experience than in a gas vehicle. ďBack in the dayĒ, if you ran out you had to remove the fuel filters & top them off w/ fuel then reinstall them. Repeat that process as needed. Often several times. If you were lucky your engine had a mechanical manual hand pump to help prime the filters & lines.
I havenít looked for such a hand pump on my Seneca.
My 2010 Dodge Dually w/ the 6.7 L Cummins states to turn the key on & off several times to let the electrical pump prime everything. That worked well when changing fuelters.
I donít intend to ever let mine get that low. Thatís why Iím so interested in more fuel capacity.
I have had diesels run out, some boats in the Navy with a Westerbeke diesel that you had to even loosen the injector fittings to bleed the air. And once we had a fire truck run out at a fire scene when the fuel return line vibrated loose and the pump operator didn't catch it before it emptied the tank and died. It took our department mechanics 1/2 hour to get it running after the repair.

My Seneca has a hand primer pump on my primary fuel filter bracket. Works well to refill the fuel filters after changing. I just pump it until it won't depress anymore and the engine cranks just a bit longer than normal then fires. And the next few cold starts also usually take a few seconds longer, but after that things are fine.
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Old 12-08-2017, 07:21 AM   #18
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Going off on a bit of a tangent of the above, the "Extendafill" fuel filler extensions claim to add about 7 gallon per tank capacity or 14 gallons total, presumably by allowing more of the air space above the filler to hold fuel. So conservatively, this adds 100 miles or so of additional range, which I would be happy with.

Any throughts on over filling stock tanks using this product? I wonder how much of this space is needed for expansion. I do like the concept and it looks like a very well made product. Thoughts?
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Old 12-08-2017, 08:27 AM   #19
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Normally my fill up cutoff is about 400 miles
We do the same thing. We always fill up around the 400 mile range. I travel with the wife, 2-kids and a dog. 400 miles is a good stopping point for the dog and I to empty our bladders and then fill up the tanks. I don't know, for us, if the extra 100 gallons would be worth it. I still am interested on seeing it when someone does it.
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Old 12-08-2017, 06:07 PM   #20
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I do indeed like the idea of the new 100 gallon tank between the rails. Except for the extra 700-800 lbs behind the rear axle.

400 miles and then stopping is fine. Filling up at that time could be optional.

Keep the thoughts and opinions coming.
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