Nissan Repair: 1997 Quest Heater Problem, automatic climate control, heater hoses

Question
I have a 1997 Quest GXE with the automatic climate control and rear heat/air.  I am getting intermittent heat from the system.  I have replaced the thermostat and flushed the radiator and front heater core.  What happens is this.

If the car is sitting still at idle the air from the vents will get HOT.  As soon as I start driving the air begins to cool and is soon COLD.  It almost seems like once air starts moving through the radiator and the thermostat closes, the hot air stops.  There is a very brief reference to a Heat Control Valve in my repair manual, but no indication of where it is.  At Autozone I learned that the control valve was actually on the heater core on 1993 to 1995 models, but I have a 1997 (my manual covers 1993 to 1998).  If there is a Heat Control Valve where should I look for it?

I just found a procedure in my repair manual on bleeding air from the cooling system and I plan to try that tomorrow.

The engine on this vehicle was replaced (not by me, by a local mechanic). Is there a chance that the heater hoses could be attached in reverse?  Would it make a difference?

Thanks,
Jim

Answer
Jim,

If the temperature gague does not indicate cold when you are driving and warm at idle it would most probably be the blend door not working properly.  That would lead me to think the vacuum reservoir is not working properly.  The blend door is opening up to the cold side and then you have no warm air entering the cabin.  Remove the glovebox and see what is going on at idle and when you drive.  A passenger would help.  But, I have done it with a mirror on the floor to see what the blend door control is doing.