FL 70 dual fuel tanks, problems ???

john11139 · Dec 18, 2013

I just bought a 2002 Fl 70 Freightliner with a Cat 3126. Went to pick it up and noticed the fuel gauge read just a little above 1/4. (it has dual tanks) I pulled into a gas station and the only pump I could get to was the one on drivers side. I put about 25 gal in it to get home. When I got back in the cab I noticed the fuel gauge haden't moved any. I figured it would just eventually level out in the other tank. I have a GMC 6500 with dual tanks, and it you fill the right tank, gauge will still show low and will eventually fill the left and gauge will come up. On the way home I noticed the gauge kept going down and when I got home almost empty. Got to checking and see that the fuel sending unit is in the right tank. Checked the fuel levels in tanks with a stick and the left tank was almost full and right tank almost empty. Both fuel tanks feed to a T fitting that sits back behind the transmission. I don't see any connections or wires where this would switched manually. Would the line from left tank be plugged? How does it work on this system. I don't want to remove any to check it till I get some information. Afraid I might get air in the lines and then would have real problem getting it bleed.

guido4475 · Dec 18, 2013

There's a junction block where the return lines feed into it than go to the tanks called a GT splitter valve. Try that first. Also try changing the vent on top of each tank with an overflow hose coming out of it running down the backside of each tank.

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jelliott · Dec 18, 2013

I agree with Guido. Often times it is the return lines that are the problem more often than the intake. Was the tank that was full warm? That would indicate the fuel has been running through the engine and returning.

john11139 · Dec 18, 2013

No tank was not warm. Cold outside (32%) I did notice when I screwed the cap off fuel tank it sounded like it had air pressure built up in it. (should it have pressure) Do you think that junction block could have crud in it? If disconnect the lines and blow them out how do I prevent a air lock? Don't want to get into that type of problem. I assume from what you are saying that the return lines is what balances it out.

WanderngFool · Dec 18, 2013

This was in an International not an FL70 but I ran out of fuel once even though my rh tank was at least half full. This happened right after I had some transmission work done so I guessed that maybe the mechanics were bumping into fuel lines around the tranny. I tightened a few connections that didn't feel right and problem solved.

x06col · Dec 19, 2013

I once had an insect lay an egg or two in a vent line for one of my tanks and when the larvi grew large enough it blocked the venting action and fuel would not flow out of that tank. Worth a looksee at the vent hoses.

jelliott · Dec 24, 2013

I have heard of spiders clogging them up too.

john11139 · Dec 26, 2013

Problem solved. Thanks for suggestions. Took vent valve out, it was stuck and I think vent tub may have been partially plugged.

jelliott · Dec 26, 2013

Great. Nice to see an easy and most importantly an economic repair as well!

Paine · Aug 13, 2021

First off, I apologize for the bump of an old topic!

I am not new to diesels but I am new to the larger rigs and the 1999 FL70 in question.

I have a similar questions regarding the dual tanks without a crossover tube between the tanks, I assume the only way to actually fill both tanks is to fill them at the pump from the individual fill neck? Looks like the return will slowly move fuel from one tank to the other but that would only equalize the amount in each tank, not actually give you two full tanks.