Cortina TF Ghia 1981

[Crazy Dazz]

Originally Posted by Reno

Mine functions perfectly when motor cold but after driving 10 klms it then directs 50% of the air flow to the windscreen (something within the valve changes).

Is it related only to engine temperature?
Does it change with throttle-position?
My XF used to shift to the floor vents when I put my foot down.

If it is only temp related, then the most likely culprit is the notorious Emissions control valve.
Mounted near the thermostat, is a 3-pronged (temperature-controlled) vacuum-valve. IIRC the originals were a reddy-brown colour. When the car gets up to operating temp, the valve opens and redirects the vacuum. (I think it may go to the EGR valve?)
Any way, they are notorious for getting brittle and snapping, opening up the vacuum to the atmosphere.
If that control-valve is intact and working, it could be the EGR valve itself that's stuffed and leaking.