Replaced my exhaust manifold yesterday and now the heater is blowing hot air of one temp. It can't be changed, hotter or cooler. A/C has no effect. All the wiring seems to be intact. Coincidence or not? On a second note, for the past 8 months or so, why is it when I briskly accelerate, with the heater on (floor only), does the defroster come on, then return ti floor heat when I decelerate? Sounds like a vaccum glitch.. This system is really confusing!