1999 grand marquis
back in the middle of the winter the ac compresser baring locked up so installed a bypass pully. so only heat worked no cold air. couple days ago i heard a knocking coming from vents turned off the heater and the knocking stopped. turned back on the knocking continued after a while the knocking stopped but the heat stopped working i would have the heat on 90 and on high but cold air blows only..put it on defrost but only blow out of vents... i checked the blower motor and dont think thats the issue... need help i was told might be heater core but looking for second opinion..
Probably the HVAC heater blend door actuator that controls the flow of air, and a HVAC Heater Control Valve that controls the heat.
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