Could be alot of things. If you have any performance goodies in there, do you have a mass airflow sensor? "85 didn't come with MAF. Ford didn't add that until "89.
The MAF will read any change in airflow to the engine due to performance parts and adjust the tune accordingly. You can check real quick by looking at the air inlet tube between the air filter and the throttle body.
Could be alot of things depending on what mods you have done. Just guessing on the MAF.
Good luck!
