Hey guys, new to the page, just picked up a 94 F150 XLT 5.0 a couple days ago. The front tank had a leak so I just replaced it last night with the tank/pump assembly from RockAuto. After replacing it I believe that it is moving gas from the brand new front tank to the rear tank. I cant think of any thing I did wrong as it was pretty much just plug and play. Anyone have any ideas or quick checks?

 

Very common issue on this vintage truck. Each tank/pump assembly has a check valve built into it to prevent the powered pump from pumping fuel into the non-powered pump. In your case the rear pump assembly (Fuel Delivery Module aka FDM) has a bad check valve therefore the front tank is filling the rear.

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Welcome to FTE!

 

Thank you for your help, the thing that doesnt make sense to me is that it didnt do this until I replaced the front tank.

 

The front fuel pump assembly may have been putting out lower pressure perhaps. Now that everything is in spec, the dreaded fuel transfer problem has reared it's ugly head.

 

Ohh I see what youre saying... is it hurting anything other than having to flip back and forth between tanks?

 

New healthy pump at peak pressure. Old worn pump likely had reduced pressure from wear. May explain issue arising after new pump install.

F series "gasser" fuel systems from 1990-96 are finicky. Did you check fuel pressure before removing old pump/tank? I would bet you had low pressure compared to new pump which would have masked the week check valve in rear FDM.

Ford had a recall related to one tank filling other tank, do not remember affected years. Maybe someone who knows can chime in. The fix was an in line check valve. Check junkyards and/or ask around, you may find a pair and avoid replacing other pump.

 

Unfortunatly I did not check the fuel pressure of the old pump before replacing it, the fuel leak was the only reason I replaced the whole deal as the truck ran fine and wasnt doing any of this cross flow stuff. I may of just lived with the small leak than gotten into this mess haha

 

The check valve in the rear FDM may get bad enough to cause low fuel pressure to the engine resulting in poor performance. Some have found hard starting conditions too. Let's not forget spewing expensive fuel out the fill tube and flooding the EVAP system.

 

The Ford recall check valve was a band aid fix at best and a few have found it did not mask the problem.

 

This issue will not be fixed with the valves. They were to prevent the problem-not repair it. Plugged fuel filters will cause the pressures to go real high. This blows out the pump. The over-fill is happening thru the return line. The valves go on the pressure side. Will need the pump in the tank that is over-filling, fuel filter & the valves to repair correctly.

1L3Z-9J274-BA
Check Valve for 1990/91 4.9L/5.0L/5.8L/7.5L Engines
(Also used for Chassis Cab Vehicles)
1 package per vehicle
(2 per pkg.)

1L3Z-9J274-AA
Check Valve for 1992/93 4.9L/5.0L/5.8L/7.5L Engines
1 package per vehicle
(2 per pkg.)

 

I replaced the fuel filter when I had the tank out, theres not a valve listed for a 94?

 

One more question for you guys, driving to work this morning I (about 3 miles) I lost about 1/8 to 1/4 of gas to the rear tank. My question is I had never tested the rear tank until this morning but once I got to work I switched it to rear and it fired right up for me, if I run it off of the rear the fuel should stay where its at correct? I would hope it wouldnt flow into the front as I just replaced the entire assembly and from what I understand the tank that has fuel going into it is the one with the bad valve. Thank you guys for your help

 

Assuming the replacement front Fuel Delivery Module is good then you should have no problems running off the rear tank.

 

Well hopefully when I drive it home that proves to be correct, that will buy me a little bit of time to buy a pump assembly for the rear. Thanks for your help

 

The High Pressure Check valve is part of the Fuel Delivery Module.

courtesy of Subford

Some have been successful replacing just the fuel pump assembly. Supposedly some of them come with the Check Valve.