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Old 05-30-2020, 04:31 AM   #1
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One fuel tank empty and the other is full

We went down to see the rocket launch in Titusville this week that didn't happen. On the trip down I noticed a new issue. My 2012 Seneca now completely empties the passenger side fuel tank and the drivers side tank stays full. The fuel gauge must be in the drivers tank because it hardly came off the full mark.

When I went to fill at a truck stop, the passenger side tank took 40 gallons while the driver side took 4 gallons. When I cracked the lid on the drivers side, I could hear it had a vacuum. I assumed a air vent for the driver side tank was stopped up, so when I filled them both up I left the driver side tank 1/2 turn from tight.

This fill up, I got half the mileage I usually get before the tank reads close to empty. I filled it up again. This time, the driver side tank took all the fuel and the passengers side tank took 2 gallons.

On the way home to Johnson City TN from Titusville, I filled up one time in Savannah. This time I tightened both caps as usual. Sure enough, the fuel gauge never came off full for about 250 miles. About the NC SC border on I-26 I pulled over and loosened the cap on the driver side tank some. By the time I crossed over into TN the drivers tank was almost completely empty and the fuel light was on. Not wanting to take a chance of running out of fuel, I filled the drivers side.

I know this freightliner fuel tank system is a POS and has never really read correctly. Any ideas on where to start checking for a blockage?
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Old 05-30-2020, 06:22 AM   #2
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Just a shot in the dark, but I'd check the cross tank transfer hose. With a unit as old as a 2012 you may have diesel algae built up enough in that hose to cause blockage. I have seen fuel lines on boats clogged with algae enough you could pull it out like a spaghetti noodle. If your drawing a vacuum, you've got blockage somewhere. Thinking of this, the vent must be in the passenger tank to allow the syphon and fuel transfer from passenger side to driver's side. Just throwing out ideas of how I would start investigating this.
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Old 05-30-2020, 08:32 AM   #3
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There is a 3/8 poly line coming off each tank that is open where they terminates. They come off a fitting on the top center of the tank and run down the inboard side of the tanks. Not sure if this is a overflow or vent or both. I would start there. The generator pulls and returns from the driver side tank but if the transfer lines are clear then it really should not have any effect.

As rgolding says the cross over seems like an area to look at. The lines come from each tank and run over the transmission where they tee together and run to the engine. One line appears to be 1/2 inch and the other 5/8. I would assume that the larger one is the supply and the smaller is return.
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Old 05-30-2020, 05:34 PM   #4
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Here is some info on our Equiflo fuel system as configured by Freightliner. Maybe it can help.

Jayco Seneca Freightliner M2 Two Separate Fuel Tanks and Equiflo, How the hell does this work? | RV Underground
Attached Files
File Type: pdf Fuel transfer system.pdf (17.4 KB, 22 views)
File Type: pdf Fuel transfer system 2.pdf (143.1 KB, 13 views)
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Old 05-31-2020, 10:45 AM   #5
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I think DOT did away with true cross over hoses some time ago. It is my understanding that these tanks are dual feed dual return. All hose should be the same size and length. Could be an air bubble or blockage of some sort. There is some equalization, but it appears something is not right with yours.
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Old 05-31-2020, 10:47 AM   #6
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Equalization: If full of fuel (bubble free), the lines will act as a siphon
to level the fuel in the two tanks, just like a crossover line. An
air bubble can break the siphon if trapped in an upward loop, and
stop flow from one tank.
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Old 05-31-2020, 01:14 PM   #7
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Here are some photos of the various lines and where they tie together over the transmission.

As mentioned this relies on siphon to balance a charged line. While a air bubble could cause similar issues I would think this unlikely as the union from each tank is at the high point over the transmission and the failures are occurring while the unit is under power.

The bottom of my driver side vent hose had been chewed on by something so I cut it to be sure there wasn't any issue.Click image for larger version

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Old 05-31-2020, 01:18 PM   #8
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I have driven down there twice to see space shots in years back that were cancelled. Sure is frustrating! But when they do go, it's so worth it.

We saw the last nightime shuttle launch, what an experience. No social distancing that morning!
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Old 06-02-2020, 07:42 AM   #9
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Quote:
Originally Posted by RVermont View Post
Here are some photos of the various lines and where they tie together over the transmission.

As mentioned this relies on siphon to balance a charged line. While a air bubble could cause similar issues I would think this unlikely as the union from each tank is at the high point over the transmission and the failures are occurring while the unit is under power.

The bottom of my driver side vent hose had been chewed on by something so I cut it to be sure there wasn't any issue.Attachment 59416Attachment 59417
You might want to put some screen or mesh over that vent hose. It will keep the critters out.
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Old 06-14-2020, 10:18 AM   #10
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UPDATE:

BOTH tank breather tubes had mud dobber crap in them. Take the plates off below the doors to pull them off to clean. It took 15 minutes.
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Old 06-14-2020, 10:29 AM   #11
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Quote:
Originally Posted by Clayduster71 View Post
UPDATE:

BOTH tank breather tubes had mud dobber crap in them. Take the plates off below the doors to pull them off to clean. It took 15 minutes.
Thanks for your update, and what you found to be the issue. It might help some other Seneca owners that get the same issue. Happy trails!
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