Now with the temps below freezing the fuel mileage has gone from 14-16 to 8MPG. Pulled the codes and got code 51, I replaced the engine coolant temp sensor and nothing changed, also replaced the connector to it as well with no difference. Also replaced the thermostat, and put a winter front on it as well. Air and fuel filters are almost new. Full tune up with plugs, wires, cap and rotor was done only a few months ago.

Dont know if this has anything to do with it or not but occasionally when I close the door or hood I can hear a buzzing sound that sounds like the fuel pump priming

 

Are you warming the engine up idling a lot?

That uses fuel that most people forget about when computing fuel economy.

I know that when it is below 10 degrees that my truck gets warmed up for 1/2 hour or so. 3 time a day. Once before work, lunch time, and one to go home. That is almost the equivalent of driving an additional 30 miles. It really throws off those fuel economy numbers.

Over the course of a week almost 1/3 of the tank ends up going into just getting it warm! My fuel numbers when comparing miles to gallons is low single digits! My fuel economy on the other hand is around 18mpg.

 

The way that my truck is getting used this winter I wish that I was getting 8 mpg. So far this winter I have been getting 6.5mpg with my 94 F150 (300-6, 5spd, 33X12.50X15 M/T's, 4.10 gears). But my 94 has yet to come out of 4wd this winter, plus pulling the Bobcat around. Its a good thing gas is cheap again.

 

Not too much idling, probably 10 minutes a day. The last two tanks have been 90% highway miles. Took a good look and there is no fuel leaking. Last winter it was averaging around 13

 

Performance thermostats (MR.Gasket) cut warm up time in half and mediates running temp better. And components related to the PCM sometimes have replacement/repairs ignored for some time afterward unless KAM ( Kept Adaptive Memory) is cleared. OBD2 is 5 mins. w/ battery disconnected. On OBD1 I've read 5 and also 20 mins. I've used 20 mins. w/ my 87' to be sure. Also #51 code can be false unless engine is at normal op. temp at time of scan. To maximize MPG the sooner the engine reaches op. temp (closed loop function requirement) the engine operates at an over rich air to fuel ratio and is not trimmed until closed loop. If there is a noticeable difference where I live (occasional low 40's), You should really see a mpg improvement w/ a performance thermostat