Way too much hot air at the wrong time!
I have a 1985 Nissan 300ZX non-turbo. Lots of miles but runs like a top. With all switches off, I'm getting a stream of hot air blowing through my ducts as if the heater and fan were both on but only as I accelerate - at 65 on the freeway it's like the fan is cranked, but at idle, nothing - I can't find any breach where hot air is getting in from the firewall or from below but my ducts are getting very hot, and again, it's as if there was a hole somewhere letting air in but - when I turn my AC on it goes cold then hot then cold, like it wants to work fine - the hottest part seems to be where the duct meets the floor right behind my stereo and switches and my manuel is of no help. Man, it's getting hot in Vegas and this is driving me nuts so if this rings any bells, please help.
some heater systems have a damper door that closes off the air passage to the heater core this could be controlled by a cable or vacuum actuator's sound like the damper doors not closeing the other system is a valve in the heater hose that would open and close might be open allowing the coolant to flow throught the heater core
Thanks for such a fast reply to my question. I've already manually closed all the dampers and the duct just heats up, but I didn't know about the valve in the heater hose - I'll check it out in the morning and post my findings - it's the only possibility left so maybe that's the answer. Thanks again...
I appreciate your help very much. You were right on the mark. Along with a few vacuum leaks from some very old tubing, it was the valve next to the firewall where the hoses feed the heater core that was stuck. Just in time for a forecast of 80 degrees tomorrow. I'm grateful....Thanks again.