october 1990 runs rich after fuel pump install

I had the fuel pump replaced from the gas tank since the fuel was not being pumped to the engine. Before this the ranger was on average about 20 mpg. After the fuel pump got replaced back in the tank after i took it out of a ranger about one year older the gas milage went bad. they look to be the same fuel pump. the first 10 gallons i was around 13.32 mpg on average. the next 10 gallons i was around 7.35 mpg. then i put in one gallon and went on the highway a short distance then exited and only got 2.5 mpg. I replaced the mass air flow sensor a while back and i was told by one guy that if you replace the mass air flow sensor once, you do not have to replace it again,and another guy told me that if you change a fuel pump you have a 50/50 chance of the mass air flow sensor needing to be replaced. Which of this is true, if any? I think the codes need to be reset. I was told, if you take the battery cable off and put the ignition switch in the accesory position for a length of time, this will reset the codes. Is this true? If it is true, which cable do you remove and which do you do first , remove the cable or put the ignition switch in the accesory position and for how long do i leave it like that? where can i get a free engine scan loaner or free engine scan besides aamco, since mine is to old for advanced auto parts, napa, auto zone and pep boys? since it was good before the fuel pump stoped puming fuel it looks loke all the parts should be good and the codes need to be reset , but i am not sure.It is a four cylinder ,the one that has only one fuel pump in the gas tank, far as i know, 8 spark plug wires, fuel injected,with close to 100,000 or 200,000 or 300,000 miles on it. do you think the guy put the fuel pump in right? what can i do?

I'm not sure what putting the ignition to accessory would do if the battery is disconnected, but you might was well start by removing the negative lead from the battery terminal for a few minutes and go from there.

as far as pulling codes, you can use a 12v test light instead of a volt meter like this following these instructions for the analog volt meter method and here are the codes this method should put out, they will be 2 digit

how are you testing this gas mileage, putting in a certain amount of gas and running it dry? getting an average out of one gallon of fuel used isn't very accurate, it's best to go by a full tank of fuel, and best to use the same pump at the same gas station to get the closest you can

the best place to start is puling the codes, throwing parts at the problem only confuses things further and you end up replacing parts that are perfectly good...

for resetting the computer, pulling the negative terminal off and letting it sit for 15 minutes or so should be sufficient

The fuel pump has nothing to do with the fuel mileage, all it can do is supply the engine with fuel. Your problem lies elsewhere. Start by pulling the codes and checking the fuel pressure. Does the truck run normally otherwise?

I'm guessing that you mean that the truck is a late '91 production, yes?