ok heres a kicker for all u truck guys out there i brought my truck to a shop to have it looked at (87 F-150 5.0L efi dual tanks)becuse the front tank will not work the gauge will read way past full even when it's empty and it will pump from the back tank. They told me it was the fuel pump and sending unit so i spend yesterday doing it. Droped the tank changed the fuel pump and sending unit and put on a new tank just to be safe and filled it up well same problem as before drove it for 3 hours and was still taking from the back ne ideas would be greatly taken into consideration. The gauge will read proper on the rear tank.


if u need ne more info just tell me


thanks kyle

 

Hi and welcome to FTE.
I would suspect either the dash switch is bad, or the switching valve on the frame is bad. Search thru this forum, there have been several threads on this issue.
Jimmy

 

Kyle, Are you saying that even when the tank selector is in the "front" position, it also pumps from the rear tank? Very unusual. The only place I can even dream up a way it could happen is if the selector switch was shorted to the rear tank AND either pin 4 or Dark Blue wire (673) from the front tank sender was also shorted hot. The two pins are at opposite ends of the connector so I don't think its the connector, but where ever you have multiple items involved, first place to look is where they might come in contact. (the connector).
If rear tank works normally, but front doesn't, and front guage is maxed out, check the grounding splice (S154) where Orange (89)(front tank pump) and Black (57)(front tank guage sender) join the same colored wires from the rear tank. Should be somewhere on the frame between the tanks and the In-line fuel pump. If it is good, I would bet its in one of the connectors (C177) (C100) (C244) where all tank pump and guage wires go thru. Check for Red, Dark Blue/Yellow (front tank) Brown/White, Yellow/Light Blue (rear) in the same connector somewhere on the frame.

 

yes! even when in the front position it pumps from the back tank i tryed a new switch on the dash no help so i can out rule that. Thanks for the imput tho it gives me some where to start. I just wondered if any one has ever seen this problem cuz it's annoying *** heck

thanks kyle

 

Did you just get the truck, or was it "normal" before? Do you have (& know how to use) a multimeter? I recommend getting a GOOD digital one. Doesn't have to be expensive, just not one of those $5 ones. It will save you so much money in your lifetime.
My books' good wiring diagrams stop at 1986 but here are my thoughts, but remember, Anything is Possible!
I would agree with the selector valve except for the front guage maxing out and the rear not. This indicates front tank specific problem (before the valve). IF ?? the fuel guage sender works as in an aircraft, it measures resistance. Then your problem could be procuced. give me a while to ponder on it.

 

well my dad had the truck for about 7 years and it's mine now along with this problem but i really sucks cuz i need that tank to go fishing and hunting i hate not making it to where i wanna go and have to carry extra gas but the problem started like a year ago and now i got some $$ so i wanna fix it cuz it's nice to have and take all the time u need i got along time lol

thanks kyle

 

Did he say it had the problem for some time?
The reason I ask is that is could easily be caused during maintenance and connecting the rear tank ground to the front tank hot wires. If it just happened one day for no known reason, then I would suspect something else.

 

ok ok i asked him what went on with it he said at first the gauge went and if u drove for a while and the tank got under half the gauge would jump around tell u the reading then go back to over full then one day the pump just stoped working like a year ago and the gauge went like a year and half ago if u get that and my e-mail is kyle_fodchuk@hotmail.com if u dont feel like posting any more

 

well if it wont take my e-mail i cant give it to u one other way kyle_fodcuk at hotmail .com try this

 

They block out e-mail addresses. Thanks anyways.
OK, here you go... remember though, I am not an automotive genius, but I have been working on helicopters 15 years.
I would have suspected the fuel pump and sender but you said you replaced them. So lets talk electrical.
The rear tank pump MUST be energized hot. It may or may not be the from fuel wires. Also, the front tank guage sender must be shorted hot or grounded depending upon how the guage reads the resistance or voltage. They usually read resistance because of the chance of fire with electricity and fuel.
First, quit throwing money at the problem! If you stand in front of your truck, hood open, looking into your engine compartment, there are 2 large connectors low and right of your power brake booster. They carry most of the fuel type wires. Look for Red (front pump), Brown/White (rear pump), Dark Blue/Yellow (front guage sender), Yellow/Light Blue (rear guage sender). Then you will have to follow the wires along the frame back to the tanks. I show 3 times they go thru a connector. With the key on, and tank selector to "front tank", carefully insert the red multimeter probe in the wire socket till it touches metal, ground the black probe to the metal body of your truck (best if not painted), and read the voltage on the brown/white wire. There should be none. If there is then the short is under your dash somewhere. If none, go to the next connection in line and try again. You are trying to find the last place its good. Then you can find the problem and splice in some new wires. You will do the same with the Dark Blue/Yellow, starting at the front tank. I bet both problems are in the same place. Worst case, if the wires are good out of the firewall, you can run 4 new ones all the way back to where they go and be done with it. But first make sure you know they are good, and you KNOW where they go. Good luck.