Try scanning it with a jumper wire. The Check Engine Light will flash out the codes. Connect the wire between Signal Return and Self Test Input (see the illustration, then turn the key to "run", and it should flash out the Key On Engine Off codes, and the Continuous Memory codes. They should be 2 digit codes with a single flash between the KOEOs and CMs. Once it throws the last CM code, start the engine. It'll flash four times, then run through the testing procedure, and after a minute or so, it'll flash the Key On Engine Running codes.
I'd say fix all the faults, if any, and do a complete tuneup. Mileage should improve.
Also, if you have an exhaust leak between the engine and the O2 sensors, it'll run rich.
__________________ 1992 Deep Emerald Green Mustang GT Hatch, 5 speed, 2.73s, Mac fenderwell cold air intake, '93 Cobra MAF, Mac 70mm throttle body & spacer, Explorer intakes, Mac unequal headers & offroad pipe, Magnaflow catback, Walbro 110lph fuel pump, Jet adjustable FPR, MSD Blaster coil, Accel 8mm wires, FRPP aluminum quadrant, UPR firewall adjuster, stock cable, Granatelli upper control arms.
Waiting to go on: Granatelli lower control arms, and GT40 Iron heads! | 
Bad gas mileage 
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