I recently bought a '91 dual tank, manual tran, 4.9 6 cyl. The front tank didn't have a fuel cap I soon found out and put one on. As I got to know the truck better I realized that the front tank (fuel guage always on empty) ran fine until the fuel level got low and the engine began stuttering. I could switch tanks for a minute (truck ran fine off back tank and this guage works) then back and it would usealy run fine for a little while longer then start acting up, I would switch back and each time it would last a shorter time until I would just have to run off back tank only. I used this fully working tank as my reserve and would fill the front tank completely after which the process would start over. This worked OK for a while but recently I switched over to the rear tank like usual and IT was stuttering (1/2 full at the time)! Now I am worried, niether tank is running well enough to get up the street safely.
On further inspection I find that the fuel pump noise heard under the hood (fuel return?) when the key is first turned on is higher pitch for the rear tank.
Figured I would start with the fuel filter, then check fuses and relays for each.
Just trying to get to work Monday!

 

the 91 has hi pressure pumps in each tank only and no boost pump under the cab like mid 89 and older.

the fuel return is benign-it just happens - and IMO should not make noise.

all fuel selection for mid 89 up is done electrically and the fuel pumps via a dash switch with the re4ar harness wired a wee bit differently.

however....even if the fuel pumps put out - they can still be bad.

in 1991 Ford issued a recall - the outlet side of both pumps are joined at a Y. a one way valve in the pump prevented fuel from going back in the inactive pump. these would fail and the active pump would slowly fill the other tank - which was a problem if it was full to begin with.

so ford grafted on simple external one way valves and problem solved.

when you look under neath - see if you have these, if not, the one ways in both pumps may have failed so spectacularly that you simply are not building enuf fuel pressure. any way you look at it, I think you are looking at 2 new pumps (but you dont have to replace the assemblies - just the pumps. about 1/4th the price.

And of course the FPR might be bad