mileage drops from 17 mpg to 11 mpg on 1991 Ford Explorer

check engine light on but dont know where to have it checked, parts stores dont check beyond 1996.

light comes on after city driving for twenty or so minutes, goes off when engine off, comes back on in another twenty minutes.
does not come on during highway driving.

Asked by for the 1991 Ford Explorer
Some parts store still carry the OBD 1 code reader you require. Try Advance autoparts and Autozone. Expect it to cost from $40 to $60. Then post the code here. Or, you can call shops in your area and see if they can do the read for you. Make sure to get a copy of their read if you do. Could be a misfire condition or an o2 sensor code.
thank you, i am going to advance auto today,and will also check autozone, i will see if i can find a shop with an obd 1 code reader and have it read.