Gatornapper

That I cannot locate!

Been a non-professional mechanic for 50 years, have my own shop, and have once again run into a problem I cannot solve and need help. The problem is in the fuel system on my sweet '96 F-150 XLT 4x4, E40D tranny, factory Off-Road and Trailer-Towing package. Didn't know where to post the question as there is no "Fuel System" category.

I had to replace my front fuel tank 3 years ago. As the front sending unit for the Fuel Gauge wasn't working & I'm an electrical guy, I did a repair/rebuild on the sending unit, which worked on the bench, but quit again not long after my installation. But the outer overflow pipe to the tank will not seal. 2 attempts by myself, 1 by my back-up pro shop, have failed to seal the outer overflow fuel pipe. Because of the sending unit problem and the overflow problem - I do not use the front tank - until the day comes I have time to fix both. Been slammed for a year with no time to do these repairs. Main point is - I do not use the front tank.

When the engine is running (only), I get a slow fuel drip from the front bottom of the front fuel tank. But I cannot locate the source of the leak. It creates quite a dangerous pool of gas at a long stop-light, and I've got to deal with it before a disaster happens.

As most here know, each tank has its own fuel pump, gauge sending unit, fuel supply AND RETURN lines. The dash switch for both the fuel pumps and sending units works fine.

What I want to know, is when tanks are switched, WHAT SWITCHES THE FLOW OF FUEL IN THE RETURN LINES? Is there an electrically activated valve to switch the fuel return? If so, I cannot find it on the Electrical Schematic. For the fuel system to work properly, the return has to be switched with the tank, returning fuel to the same tank from which it is drawn.

I am wondering if the front tank isn't getting filled from a return of fuel to it. It seems to have fuel in it, but with a dead sending unit, I cannot tell. Taping the metal tank, it seems to sound full.

Any explanation of the fuel return system is greatly appreciated, as is any diagnostic help in finding the source of the leak - from the front tank I do not use......

PerryB

Are you sure it's a return system? The later gens. are not. Our hired man has a 93 (same system) and he has a problem with one tank slowly filling the other, but I think it's due to a faulty check valve in the inoperative pump.

Gatornapper

Please forgive me for not stating my initial diagnosis - which, I have just proven and established.
There is no fuel leak in the new front fuel tank. The only leak occurs when the front tank is over-filled, as the over-flow pipe is not sealing.
The front tank is being filled by the RETURN LINE THAT SHOULD BE GOING BACK TO THE REAR TANK.
When the tank fills, it overflows, and gas leaks to the ground.

I just proved and established this by driving 45 mile - on an empty front tank. Remember, the front tank sending unit does not work .....but now I'm determined to replace it soon. And I ran the front tank dry long ago, and do not use it.

Proof #2 is that now there is no dripping/leak from the front tank......the fuel is below the full point, below the over-flow pipe.

Now here is the $64,000 dollar question (for all of you over 65): Where is the defective valve that switches the return line flow between the front and rear tanks?

TIA!

Gatornapper

Perry -
You may have nailed it .....BUT, I am 90% sure there is a return line - there were two lines going into the tank I replaced, and the Haynes service manual refers to the return line system - but then, it does not state a year this changed.

Also wondering how a check-valve system could work....saw nothing about one in the Haynes manual - but then, we all know how terrible Haynes manuals are, don't we?

Thinking more, the check valve in each line to each tank would allow the fuel line to be pressurized but not fill the other tank - makes sense - another reason I need to replace the rear pump/gauge unit.....but then, exactly where is the check valve? In the pump/sending unit piece?

Give me an original Shop Manual - if I can get one......have one for every machine I own - cept for my '76 Porsche 914 and my '96 F-150....

Both my pumps work fine.

Might you mean pressure-relief valve, instead of check valve?

Thanks for the reply - Any additional info you have is greatly appreciated.

"Hired man..." So you own a farm or ranch I guess?

PS - I'm a bit jealous. Always wanted a Lariat - and you've got two.......

PerryB

If I understand the system correctly, there is no switching valve. You select whichever tank you desire, and fuel is prevented from flowing to the unused tank by a check valve that I THINK is in the pump assembly. To my knowledge there is no return on this system. A couple months ago I was attempting to figure out the system on the employee's truck. It was filling it's front tank when burning the rear. Then he told me the rear tank doesn't read on the gauge. At this point I told him he needs two new pump/sender assy's. and he say's "too expensive, forget it", so the exploration stopped at that point, but I'm pretty sure the system works as mentioned above. Where's the vapor-purge canister located? I wonder if it's filling with gas and expelling onto the tank?

--- Yes, rice farmer. Northern Sacramento valley.

Gatornapper

Perry -

Evap cannister under hood on right side.

Think you nailed it. Will replace pump/sending unit/valve assembly as soon as I can verify that the valve is there......

Guess the other line to the tank is for evap gas vapors........not a return......make sense anyway. Wish there was a one-line fuel system diagram like my Harley's have.......oh, there surely is one in the orginal factory service manual......

Have a great day Perry.

Ever read Victor Davis Hanson? Brilliant valley farmer out there, in your area I think. Historian who is political writer......

Gatornapper

There is a valve located at the aft tank, #9, but not sure what it does. Could be valve that switches tanks???? Anyone know? Graphic is blurry and not clear enough to read when you blow it up.......



Thanks to Steve83.....

Gatornapper

This detail indicates on 90-96 there is a return line from the Pressure Regulator.......

​​​​​​http://www.supermotors.net/clubs/sup...y/media/283795

Steve83

Click this & read the caption:


(phone app link)

For more info about the fuel system, pumps, & the known problem with the FDM's check valve, keep clicking "NEXT" until you've read the whole album. For info about repairing the fuel level sender, follow the relevant link in the caption of the pic showing the sender.

BTW
You don't need to e-mail the link to your thread out to anyone. The nature of a BBS is that everyone will read what he wants to read, in his own time. Be patient.

Gatornapper

Once again, Steve83 has come to my rescue.

Been planning on a new FDM anyway, and that will solve both my bad check-valve and sending unit issues, not that expensive.

In the meantime, I'll just run 100 miles on my full rear tank, then switch to front for about 80 - seems like that's the ratio of fuel being sent to the front tank - until I can replace the FDM - in the next week or so...... and I'll try not to run the front tank dry doing that........