poor fuel mileage help

i have a 98 windstar with a 3.8 and little over 100k. in the past couple of months my fuel mileage has dropped significantly like for 20 mpg to 10 mpg. the van runs great with an exception of a little hesitation when going into passing gear. every now and then i'm getting the codes 171 and 174 which are the lean codes. ive had my all sensors checked and everything shows that it is running good. ive taking the battery cable off to reset the computer. any suggestions would be helpful. thanks

Check the operating temperature as it needs to be at operating temperature.
Check for cracks around the oxygen sensors.

Codes P0171 and P0174 are lean codes.
The most common cause is a vaccum leak.
There are a lot of places that this can happen.....lots of vaccum lines.
Check the vaccum elbow that is on top of the upper intake manifold....next to the throttle body.
This is the PCV line...and on your year, it connects to a line that goes toward the passenger compartment......going to the PCV valve in the rear valve cover.
These can develop a crack on the inside of that elbow.

Check the crankcase breather line.....from the front valve cover.....to the big flex hose that goes from the air filter/MAF to the throttle body.
This stiff line connects to the flex hose on the back side.....if disconnected will let extra air in.

If everything is connected (there are a LOT of vaccum line connections on the back side of the upper intake manifold), then check on the operation of the IMRC.
This will be on the passenger side of the lower intake manifold.
These should be OPEN with the engine OFF.....and CLOSE when you start the engine.
If this is happening, they are working.

You could also try a can (metal can) of Berryman's B-12 Chemtool fuel system cleaner into a tank of gasoline.
The fuel injectors tend to build up a bit of crud inside the end cap.....over the spray end of the injector.
This product will clean this deposit.

This is the time of year to get the vehicle in tip top shape with tuneup items.....fuel filter, spark plugs, spark plug wires, etc....etc.
This is also one of the first things to look into when a problem comes up.
A dirty fuel filter restricts fuel flow and places a additional load on the fuel pump...which can lead to fuel pump failure.
The windstar does NOT monitor fuel flow or pressure....so you will not get any code that directly indicates any problem there.

The isolator bolt TSB does NOT apply to your 1998, that starts with the 1999 year...when they started using a totally different upper intake manifold assembly than what you have.