Ok, my 86 5.0 EFI, it has a problem, when its cold it fires up and runs great, for about ummmmm 10 miles, if the front tank is under a half full it bogs over 30mph, it idles fine and slightly accelerates ok, but if i punch it at idle, or climb a hill or try to go over 35 it bogs down, surges, etc, if i let of gas it runs fine. I replaced the High pressure pump and still, if the tank is absolutly filled it does no do this. Funny thing when its cold, i have 4 gallons in rear tank, i can switch over and it runs fine on back tank cold, but when the front tank does the surging thing i switch to back tank and its 10x worse. I just need some suggestions. I replaced High Pump (Junk Yard)?????? maybe a bad one? and it only surges if under a half tank? and when it surges the fuel pressure is low. So just to recap, if under a half tank, and its warmed up. It idles and slightly accelerates ok, but if i try to punch it from a complete stop, or try climbing a hill or go faster than 35 it losses all power, sometimes if i floor it, it will surge, or sometimes just nothing, let of gas and she runs ok, any options? upper intake gasket? fuel pressure Reg? Pump bad? In tank pump bad? Selector switch for tanks? I also put high pump on a toggle switch, to by pass the relay, someone before me , made there own relay system, not factory, but that doesnt help. Thanks and dorry for the long thread

 

Assuming your HP pump is working ok, and we don't know for sure it is, then it seems you may have an issue with your LP pumps in the tanks.

May not be the pumps themselves but they may be clogged. Sounds like you are starving the HP pump suction. When the tank is full, you are able to get enough pressure to the HP pump but when low, the LP pumps have a problem that then cascades over to the HP Pump.

If this only happens depending on tank level....then it has to do with something with the tank level. The transfer valve not functioning would not be dependent on tank level.

Have you changed the frame mounted fuel filter?

 

I would drop the tank and take a look inside at the sending unit/pump assembly. Test the pump to see if it's burned out, see if the pickup tube is obstructed somehow, etc.

 

Sounds to me like its your low pressure pumps, either the low pressure fuel resevoir on the driver side frame rail, may be clogged, or the in take pumps ma be faltering some how. Some of these trucks, (i.e mine) had a crappy rubber hose connecting the passage of fuel inside the tank, wich wrotted out and gave me a hell of a time with my fuel system. You should drop the tanks to be sure. If both tanks are affected then check the resevoir, may have pinched the o-ring at the bottom where it screws on, if you dont get a proper seal it will give you similar symptoms. good luck and let us know what happens.

- Mike