Author Topic: Saddle fuel tank problem  (Read 5232 times)

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Offline BlueFlame

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Saddle fuel tank problem
« on: August 11, 2015, 12:43:22 PM »
I have a Super C on a Freightliner truck (not motorhome) chassis.  The truck has saddle tanks (one on each side below the driver and passenger doors).  Until today I've not noticed any difference between the two tanks.  Today as I arrived at my destination the engine seemed to be idling rough.  Then I had problems starting it (had to crank much longer than usual).  My first suspicion was fuel, which should not have been low as I had over 20 gallons shown as remaining (I use Silverleaf VMSPc).  I checked to make sure the filters were open and I didn't have any water in the settling bowl.  All checked out OK.  Then I looked into the tanks themselves.  Using a flashlight I could see into the tanks, and using a dipstick I could measure the fuel depth.  What I found surprised me.  The driver tank was almost empty, to the point the open end of the intake line was becoming exposed to the air (thus the rough idle and poor starting, it was sucking air).  The passenger tank had over 8 inches of fuel in it.  I had been told the tanks were connected and the fuel levels would remain the same, and I always fill then both to the top.


So, my questions.  Was I misinformed?  Are saddle tanks really independent and I should not expect them to remain at a consistent level?  What would cause the fuel to be used at a different rate from the two tanks when I have both the tank valves fully open?  Any ideas on how I can account for this difference so I know before I run into a problem, since the fuel gauge and VMSPc only report a total value remaining, not a tank specific value?  Any other solutions/ideas?


Thanks for any help, input, or ideas.



Offline Brett Wolfe

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Re: Saddle fuel tank problem
« Reply #1 on: August 11, 2015, 03:00:30 PM »
Blue Flame,

Welcome to the Diesel RV Club.

Hopefully, someone else will be familiar with the Freightliner truck/Super C tank configuration. We do have some other Super C's in the club.

If not, since this is on a Freightliner truck chassis, I would assume that any Freightliner dealer would actually be more familiar with your set up than that on most RV's!

Offline Ken Carpenter

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Re: Saddle fuel tank problem
« Reply #2 on: August 11, 2015, 03:25:25 PM »
BlueFlame,

Another option will be to call the Freightliner Help Desk 800-385-4357.  If they don't know the answer they will be able to help you get the answer.
2005 Winnebago Journey 39K, C7 350HP
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Offline David.e.atherton

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Re: Saddle fuel tank problem
« Reply #3 on: August 11, 2015, 09:32:15 PM »
Hi, Ken and Brett, with saddle tanks on each side of a truck in question above to
get and keep same level of fuel. What needs to be done to correct problem and seems
funny it slipped by owner. the two fuel tanks need to have a cross-over line from connection at the fitting at the bottom end of each tank or fitting on the bottom of each tank and 1/2 or 5/8
cross-over hose connectec to each tank with a shut off valve at end of each fuel tank. The
fuel pickup needs to be only in one fuel tank. pay attention to any tractor trailer or stright
truck at truckstop. If owner says there is a cross-over line in place than the cross-over line
is plugged. Dave

Offline David Deming

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Re: Saddle fuel tank problem
« Reply #4 on: August 12, 2015, 05:33:58 AM »
  If I remember correctly the requirement for truck manufacturers was changed sometime in the 1990's so as to have what would be "Top Draw Tanks" only thus eliminating the bottom crossover which was always a problem getting hit by debris (tire caps) and broken loose dumping fuel onto the highways.  On top draw tank systems there is a T fitting with NO check valves (most time centered between the two tanks) and the fuel is pulled from which ever tank is higher (fuel level wise).  The original poster told of exposed pickup on left side with lots remaining on right side.  This can and does happen some due to the crown of the road (for water drainage on our highways) and if parked even for short periods of time off level right to left as the fuel will siphon to the lower side even with the engine idling (like during campground check in).  Once the siphon is broken with air the only way to fix the problem is put fuel into the side that the level is below the pickup and re-prime the system.  Some one will ask about the return line which has a T fitting also with two very lightly sprung check valves going to each tank.
 
  How do I know all this.  I have a 1998 Peterbilt (with top draw 57 gal. tanks)  I ran the fuel low just below1/8 on gauge (like the poster) I parked on an angle (left to right) with right side low as this was the only place at the event I was at.  Came out started the truck and was about 3/4 mile down the road and it quit. Was towed in.  All that was required was to added fuel to the left tank, pump the primer and all was good again.  I started investigating the fuel tank system design and discovered the above to be true from several different sources.  I started watching fuel amounts during fill ups and found that most times the left side takes 5 to 7 gals more to fill than the right side (since I am not a "Left Lane Squatter") this make sense.

  As for the dash gauge my truck the tank sending unit is in the right tank. Some friends with a Ford F650 Supercruser theirs is on the left tank but still only one tank has a sender unit. Another friend with a 1997 FLD120 the fuel sending unit is in the left tank only.
When I was investigating the the fuel system design I also asked about the fuel gauge differences.  Some claim it does not matter which side the sending unit is in.  Others claim it is in the left as that one is always lower due again to the crown of the roads.  One truck manufacturer and I do not remember which one had a switch and two sending units so you could have one dash gauge and use the switch to read the fuel level in either tank.  This was similar to some older Ford pickups (late 1960's and early 1970's) with two tanks.  My opinion is the sender should be in the left tank and if I had a choice that would be where I would have it located due to the road crown. 

  The motorhome guys do not have this problem as they most only have one tank 30 feet from the engine.

I hope this is of some help.
Dave (not the retired Cat guy)

Offline David.e.atherton

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Re: Saddle fuel tank problem
« Reply #5 on: August 12, 2015, 06:59:57 AM »
Good morning Dave, Good point to bring up. last week had service call on a frieghtliner pulling
40 foot King of Road Trailer. Cat 3126 B engine oil line to turbo problem. Could not help but notice
the fuel tank cross over line between the two fuel tanks This was a 2000 Freightliner. At camp
ground here in Quartzsite there is freightliner with the same setup on fuel tanks, pulling a Teton
fifth wheel trailer. I really did not give this any real thought but what you said makes a real lot of
sense, Dave Atherton Retired Cat Mechanic