Sounds like the heater core may be clogged. Check the warmth/lack of warmth of the heater inlet and outlet hoses close to the firewall connections. If the core is clogged, the outlet will be much cooler than the inlet. If it is clogged, have it backflushed to knock the debris out of the core tubes. If it is not clogged, try installing a 195 degree thermostat.
Saturday, January 2nd, 2010 AT 9:58 PM