Q: bad gas mileage 18 to 10 miles to gallon. and idle will be normal then high at times on 1991 Ford Explorer

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gas mileage went down 5 to 8 miles to gallon. the idle will be normal at times and high at others and sometimes up and down. check engine lite will come on sometimes and stayes on till i restart car then will stay off for 30 miles then other times comes on in 10 miles nothing steady except really bad gas mileages.
(2) Answers
Agree with the air intake leak - unmetered air is coming in likely. Vacuum leak, air tube leak, etc. Your check engine codes will likely be "lean fuel mixture" codes.

Frankly, i'm so surprised you were getting 18 MPG with your Explorer ever! Nice.
see if your throttle body is loose,check your air filter & housing,also check your (tps) throttle positioning sensor, unplug it and plug it back in see if there is a change in idle, if there is, it's working. if not replace it. also check for cracked or broken vacuum lines, if this does not help take the car to your closest advance auto, or auto zone stores. they will check it with a scanner free of charge. ps. check engine light must be on, most don't know how to read a freeze frame.