Hi there!
I’m sure you guys are not surprised that I have a few “issues” with my 90 XJ-S as I bring it out of mothballs after a two year hibernation. What follows is the least of my problems. However, it is a nuisance. I have managed to get my A/C up and running. However, when not using the A/C, hot air comes out of the side vents. This air is significantly hotter than the ambient temperature outside the car.
Any thoughts?
Michaek
You have the Delanair Mk III system. The control flaps are driven by servomotors, and those servomotors are unreliable. One has probably failed and left a flap in the heat position.
My bet would be heater valve as it is much more unreliable after all…
Switch to manual mode or from manual to auto - maybe this will do the trick…
Any thoughts? Yes.
When you say “Not using the A/C” I hope that means “When the climate control is in the OFF position.”
If that is the case, continue. If not, ignore!
Each blower has an upper and lower flap. They are connected so that when one is open, the other is closed. They are operated by vacuum, through a solenoid valve.
When the engine is not running, and the system is OFF, the lower flap (Recirc) on both blowers will be closed, and consequently the upper flaps (outside air) will be open.
Reach under each blower case and make sure that is true- or use a light and a mirror.
When the engine is started, even with the system OFF, vacuum is applied through the solenoid valve and the lower recirc flaps will open, and the upper fresh air flaps will close.
If that doesn’t happen, and the upper flaps remain open, as you drive the car then some outside air will flow into the car, even without the blower fans running. This air will be heated by hot water flowing through the heater core. If you switch to A/C at that point, (Full cold) then the hot water valve will close, and cool air will be the result.
I would be looking at the vacuum lines behind the removable panels at the front of the trans. tunnel, and the vacuum solenoid valve which controls the upper/lower flaps on the two blowers, if it were my car. (Blue vacuum tubing.)
Hi there!
Thank you all for the advice. It will be a while before I get around to trying to resolve this problem as other more pressing issues regarding this car have reared their ugly heads. You will hear about them in due time. 
Thanks again for your help!
Michael