I have a new to me 1986 F150 with dual fuel tanks. The small front tank back feeds into the larger rear tank when the rear tank is lower than the front. The back feed is very slow, but consistent.

Question: is this a normal or expected thing, or is there a check valve that is malfunctioning? Or, as I only discovered this because I was checking out the rear tank which had not been used by previous owner, can I safely ignore the issue and jut run the fuel out of the front tank first?

Thoughts?

 

Do you have EFI?

 

No efi,
No fuel supply problems to carb, wouldn't have noticed it, but I took the bed off and had access to poke around all I wanted.

 

Does your truck have a return line or just a feed line? Your symptoms sound odd.
Normally on these trucks one tank feeding the other is related to the switching valve malfunctioning and the return fuel getting pumped into the other tank, regardless of what angle the truck is sitting at.

 

Yes, exactly as 82F100SWB said, it sounds like a Return line is returning fuel to the wrong tank, this can happen on models equipped with electric pumps and fuel return lines (generally, those with EFI).

 

I just read the way I phrased my problem, I was unclear. The problem occurs when the fuel level in the front tank is higher than the rear tank, I believe. I did not reverse the tank fuel levels and recheck. I will now, big DUH! On me for not thinking to do that.

I will have to see if there is a return line, I don't know for a fact one way or the other. Since it is a carbed engine, I didn't think to look for one. But 30 years and at least 3 previous owners could make for some interesting modifications.

FWIW the engine is a 351w with a 4180c carb. And I am new to the world of fixing up old trucks so point and laugh all ya want (laughing) I am clueless but teachable.

 

I corrected the engine from 351m to w , that is what I get for trying to post while running my Ag-Gator.

Regardless, I appreciate the input folks! I gathered from your questions that I need to investigate whether the beast had the efi removed at some point along with it being de-smogged.

Yes?

Not relevant to this thread, but to reassure you that your help and advice wont be tossed into a black-hole project - I have already
removed caked and baked oil from the entire front and bottom of the engine, and replaced the oil pan gasket (leak gone),
replaced the starter solenoid and starter,
bent and installed a choke rod (went from no choke to it works),
adjusted the timing (how cool is that the way the timing light works, and the way the engine changes when you turn the distributor!),
oh! And I "T'd" a manual oil pressure gauge into the oil pressure sensor port ran a line to the cab, and found out that while the dash gauge said zero oil pressure after warm up, the manual gauge says 60 psi @ 2000 rpm and 15 psi at idle. Yay.

Waging war on rust with Tabco panels and a MIG. Anybody want to see photos of the mechanical or cosmetic progress?

I will look tomorrow into the electric fuel pump / return line issue. Thanks for the insight!

 

What they are saying is there is no way the fuel can get from one tank to another unless you have one of the trucks that had a fuel return system. Or someone has done some weird plumbing on your truck.

 

I thought that is what they were saying, but thank you for the confirmation Franklin!

However, that (absent a fuel return system, fuel can not flow from one tank to the other) turns out to be untrue.

This is what I worked out today.

1. The beast does not have a fuel return line. It does have the vapor lines on top of the tanks, but they are not the source of the transfer.

The transfer happens through what is / should be the outflow line on the rear tank.

2. The fuel tank selector switch functions (switching from front to rear tank and back).

3. The condition happens only when the fuel level in the front tank is higher than the level in the rear tank, the front tank is selected, and it happens in both engine off and running conditions.

My working theory at this point is that the valve? That physically switches from one fuel line to the other does not actuate completely, allowing a small amount of fuel to back feed into the rear tank. This does not seem to starve the engine, or cause irregular fuel flow. I drove the beast home 2.5 hours at highway speeds off the front tank the day I bought it.

I am not looking for high performance or heavy towing duty from this truck, so at this point I am inclined to put degunking or replacing the physical switching mech somewhere lowish on the to do list.

Does that sound reasonable? Have I overlooked a condition that might create a boom and flames? That would be sub-optimal.

 

I guess fuel could theoretically cross through the valve as it's the only common component connecting everything together.

Could a siphon be possible combined with a sticking valve? Hmmm....

What engine is this?

 

If your theory is true and the switching valve is leaking, I see that causing problems if one of the tanks goes empty and the other is low as you are driving. It may start sucking air from the empty tank causing stalling problems or surging.

Too test your theory, I think I would pinch one of the lines to the tank off, and see if it stops the problem.

 

It is a 351w with fuel supply lines that are in part original, in part replacement. Fwiw: from what I can see any work that has been done is thoughtful and workmanlike if unusual (in the case of the push button starter work around for the malfunctioning key switch ignition activator).

 

I guess the primary question is, how many fuel lines do you have on your valve, 3 or 6? In the factory configuration, that valve is located above the tanks, thereby preventing a siphon.

 

I will try that franklin, just to see what happens. It would be interesting to know.

 

I will look tomorrow regarding the number of lines. I would look tonight, but it is dark, the beast is parked outside the shop, and I have been into the marker's mark which makes for less than accurate observations on mechanical systems and politics, so I avoid both in this situation. Report tomorrow, after coffee.