Sounds like you have a leaking vacuum pipe on the heater controls
Heater Control Valve
The heater control valve is actuated by the heater control valve vacuum switch, if equipped (manual climate control), the recirculation door vacuum circuit (manual climate control) or the heater control valve vacuum control circuit (DATC). When the heater control valve is closed (full vacuum), coolant flow through the heater core is shut off and A/C cooling efficiency is increased.
Friday, November 28th, 2008 AT 2:04 PM