Car guys on a local radio station suspect a head gasket.
Heater blows cool air at low rpm. on 2004 Mercury Marauder
The first thing I would check is the coolant level. It could just be low. Other causes could be a restricted heater core, low pressure from a worn water pump. The head gasket should be the last thing to check. If you want us to check this out just call and schedule an appointment (313-562-7022). I am sure we can diagnose it and give you the answer you need.
Agreed that the head gasket is a low percentage chance for failure on this engine...