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KDB
03-02-2001, 03:29 PM
HI, Would anyone have any ideas on the following crazy problem. I have a '78 Chevy K10 with dual tanks both of which have been used regularly for the 7 odd years I’ve had the truck.

The return line from the pump goes into a plastic 'T' with hoses going from the 'T' to each fuel tank. Several months ago, the return hose started leaking where it connects to the 'T'. I found it was soft & deteriorated. Also, there were no clamps on the hoses attached to the ‘T’ (I thought this was strange!)

So, off to the parts store I go with a piece of the old hose, and the plastic ‘T’. I replaced all three hoses, the ‘T’, and installed clamps at the ‘T’. Problem solved right. Wrong!

Now if I attempt to use the driver’s side tank, the fuel seems to be ‘Returned’ to the passenger side tank! I can start with a full drivers tank, an empty passenger tank, drive 10-15 miles, and the driver’s tank will be empty/passenger tank will be full. Using the passenger tank only works fine.

I checked & rechecked everything. The hoses are the same size as old ones, the new ‘T’ matches the old perfectly (the old ‘T’ is just a ‘T’ no springs, valves, etc inside.), there are no kinks, or blockages, and the clamps are secure, but not over tightened.

I am stumped. What makes it worse is I am starting to wonder how the thing worked in the first place. I mean there is NO valve directing the return to one tank or the other! How did the fuel ever go back to the right place (but I assure you, IT DID!)

Anyone have any Ideas?

Thanks…

mike reeh
03-02-2001, 09:29 PM
hmm, interesting..

are you sure that the Tee isnt a Vent? it is on my trucks (77s)

if you trace it, it should go to the charcoal canister in the engine compartment..

what does the selector valve look like? does it have 6 ports? if so then its just like my setup. on a side note, the dealers computers say that only big blocks used the 6 port, ive had 3 77s and they're all small block, 6 port selector valves.. the other kind is a 3 port, i think..

at any rate, let me know what kind of selector valve.. I had some problems with mine a while back and i learned a lot trying to figure out exactly what was the problem :)

do your tanks vent ok? my drivers side tank would build a LOT of pressure.. so much that it would blow past the locking gas cap every few seconds as it built up.. so much that if i took the cap off fast, I would get a face full of hydrocarbons :) and almost pass out.. i pondered this one for a while.. finally I found (after replacing or testing all the lines and stuff) that the sending unit was bad.. apparently there is some sort of check valve for the vent system, and mine was frozen shut.. i ended up drilling it out very carefully (didnt have the $$ for a new sending unit-- they're expensive!) and now its good as new.. the "cap" part of the sending unit, that is..the part that seals off the hole in the tank.. not actually affiliated with the sending unit, just a part of whats called the "sending unit" :) ok hope you're not confused..

one thing that helps is to disconnect the lines from the selector valve and hook up a clean tube to each port and suck and blow (get your mind out of the gutter) while someone moves the switch, to make sure that they at least seal a little bit.. if you can overcome the selector valve with your mouth, you best replace it..

keep me posted,

mike

Chopper Lover
03-04-2001, 12:54 PM
Not sure I can help but I want to toss out something simple.

Is it possible that the lines have something to do with gravity? Could they be routed different than originally causing fuel to flow to or from places it did not before? Understand what I am trying to ask?

Just a thought.
Mark

RTallday
03-04-2001, 03:42 PM
I shared your problem. I have a 79 K-20, with dual tanks. Its weird to explain. I changed the little solenoid that is worked by the selector switch. Thats what did it for me. Also, just check to make sure that your selector swithc is working properly. Good luck and let me know how things work.


~Rich

KDB
03-05-2001, 10:50 AM
Thanks for the replies.

Mike, the selector valve has 3 ports, one fitting to pump, and one for each tank. My tanks don’t build up pressure, so I guess however they are venting is working; the charcoal canister was long gone when I bought the truck. I assume a previous owner removed it. Strangely though I can’t find any cut hoses coming from the tanks. I am going to drop the perpetually empty tank and see if the sending unit has any unused connection. Have any idea when canisters were required on pickups? I know a couple of early 70’s cars I owned had them. What is the electrical setup on your 6-port selector? My 3-port has just one wire coming from a dash switch. Maybe I should buy a 6-port selector and re-do the whole thing.

Rich, is your valve 3 or 6 port? Is your charcoal canister still in place? How are the vent hoses routed to it?

Mark, this was my first thought. But strangely, the tank that seems to be getting all the return fuel was the one where the hose is coming out of the “high” side of the “T”?? I tried leveling things out but this was fruitless. Next time I’ll take a picture before I take things apart!!


Thanks everyone,

Kevin.

RTallday
03-05-2001, 06:13 PM
Kevin, First things first, are you referring to the soleinoid as the valve? Because i am not understanding that part, but anyway, The soleinoid has 3 hoses. One to each tank, and one to the fuel pump. The charcoal canister, has two hoses deffinately, maybe 3, i have to look at it. But i'm pretty sure that two of them are from the sending units on each fuel tank. I guess they are vents or something, ive never really done anything to them. Ill look when this weather clears up and ill let you know.

-Rich

mike reeh
03-05-2001, 08:36 PM
i dont know exactly when canisters were required.. definately required on your truck though.. I dont know why people remove that stuff, there is really no reason to..

the 6 port valve uses just one wire from the switch too.. let me think.. theres

1) output to pump
2) input from tank1
3) input from tank2
4) input from pump return
5) return to tank1
6) return to tank2

so thats the 6 ports.. im interested in just how the 3 port returns the fuel to the correct tank. surely it cant just send it to either/or can it??

heres a thought.. maybe one of the return lines is clogged and/or has more resistance to fuel flow than the other..(after the Tee?). when the motor is running there IS fuel being returned at all times..there has to be.. I guess mechanical low pressure pumps are regulated internally.. ? well, during light throttle/cruise speeds, not a lot of fuel is entering the carb, and so its all being returned to the tank which lets fuel in the easiest.. fuel return lines and regulators are not something I usually think of when talking about mechanical/low pressure systems, I guess I just take it for granted that it does it all itself.. anywho, makes sense to me... i'd take off the tee and blow into each tanks return line and see if one is significantly easier (or harder) than the other..... OOH. lets say the vent system is fudged up, not a whole lot of fuel could enter, the working tank would tend to take all the return..

makes sense to me .. good luck keep us posted!!

mike

RTallday
03-05-2001, 10:06 PM
KEVIN!!!!

I forgot about something here. Look at your fuel pump. There is a line going from the fuel pump to the carb, and there is one going from the tank to the bottom of the fuel pump. Is there another line in yours? Tell me how many lines there are into your fuel pump. If there are only 2, then you dont have the fuel return system. You would only have the one from the tank into the pump to the carb. I dont even know what mine is, i never really looked. LET ME KNOW!!
-Rich

KDB
03-06-2001, 10:50 AM
Thanks again guys.

Here is my fuel line setup:

Fuel Pump: Three Connections
1 – Pump to carb
2 – Pump to solenoid valve
3 – Pump to the infamous “T” fitting (Return?)

Solenoid valve: Three Connections
1 – To Pump
2 – To Tank 1
3 – To Tank 2

Tanks: Two hoses coming from sending unit on each tank
1 – To “T” fitting
2 – To solenoid valve
3? - ? I can’t see the sending unit without dropping a tank so there may be an unused (disconnected) vent fitting? I will check this soon.

There are two metal lines running along the frame rail from the area near the tanks/solenoid to the engine compartment. These serve the pump to solenoid connection, and the pump to “T” fitting connection. On this basis I assume it came from the factory with some type of fuel return as I don’t guess a metal line would have been used for a vent hose…or would it???

I talked to a retired GM mechanic and he said that often dual tanks would be dealer installed, so the setup could vary!?!?

Rich, if you get a chance when you guys dig out up there I’d really appreciate knowing how your fuel return is setup (or if you have one) since you have the three-port solenoid (valve). Maybe then I’ll get to the bottom of this. Anyway, thanks for your time!

Kevin.

RTallday
03-06-2001, 05:41 PM
Ok kev, im hoping to get it up on the rack at work to do a few things anyway, so ill check out my fuel setup and ill keep you posted.
-Rich

KDB
03-07-2001, 01:46 PM
After talking to you guys, a professional mechanic, the local NAPA store, and reading the chapter on “Fuel and Emissions systems” in my Chilton's book I have come to the following conclusion:

-My truck was originally equipped with dual tanks, and almost certainly a three port solenoid, and fuel pump with NO RETURN LINE (Internally Regulated so says the NAPA Store).

-The metal lines on the frame rail are indeed a fuel line and a VENT line to the now absent charcoal canister.

-While I haven’t dropped a gas tank yet, I believe I will most likely NOT find an unused connection on the fuel tank because there are probably only two (evap. vent and fuel outlet).

Here is my theory:
-Sometime in the past someone has replaced the fuel pump with an improper one that has a return fitting.

-Being clever (or stupid as the case may be) they ran the return into the vent lines.

-The returned fuel would go in through the vent, and if one tank got to full, the internal check valve (which prevents fuel from leaking out of the vent) would close causing the return fuel to go to the other tank!


This still leaves the question of how in the heck the thing seemed to work before. Not being a hydrodynamics/mechanical pressure expert I have two admittedly “wild” guesses as to how and why it worked then/doesn’t work now:

-Maybe if the “T”, lines, gas caps, and filler-neck vents were “just right”, the fuel pump could cause slight vacuum in the “source” tank and “draw” the majority of the return fuel that way??!!?? When I changed the Position/angle of the “T”, and/or the length/route of the lines this delicate engineering (or sheer blind luck) was ruined. I admit this sounds like bad science but who knows?!?!

-If the tank which is no longer getting any return fuel suddenly developed a blocked/sticky check valve on the vent hose, this MAY have led not only to a leak (due to slight, constant pressure buildup in the soft hose and “T” with no clamp (more bad science?)), but also may explain the fuel now going exclusively to the other tank.

Well, I’m off to buy a two-port fuel pump NAPA ($11.99)!!

Thanks for your help!
Sorry if you already racked your truck Rich!

Kevin

RTallday
03-07-2001, 06:51 PM
ITs cool Kevin, i am a mechenic, so i was gonna do a few things anyway, so it would have been no sweat to check that out. Im gonna do it anyway, cause now i want to see all this stuff. Im glad you found the problem. I know it can be VERY annoying.

-Rich