1985 Ford Thunderbird running rich

Options: The throttle linkage is stuck, the butterfly plate in the throttle is gummed and sticking, the IAC valve is stuck/the vent passages are plugged or the intake gasket is cracked or broken allowing extra air into the intake. The only thing stopping a gas motor from revving to the moon once started is the throttle plate. I would be suspect of the idle air control parts, or the intake gasket. In that order.

Ed, a vaccum or intake leak would be a constant flaw, the idle would not fluctuate unless the iac system was trying to balance it out, the iac valves on that generation of Ford products were known to fail. He has a 1985 Thunderbird, with Ford only obd 1. Auto part stores do not have readers that can scan those codes unless he PURCHASES one (actually a good idea), and they have no data capability, only the ability to read codes, which there are probably none of. The part store monkeys will only be able to sell him several hundred $$ worth of parts for no reason. It's an older car, check the basics first.