Why would the heater on a 1998 Mercury Mountaineer blow only cold air?

Answer 1

Worst case:SOunds like your heater core could be blocked. If it never blows warm air then I suspect it is blocked. Antifreeze flows through the heater core like a miniature radiator. There is usually an inlet tube and outlet tube. If your heater core has corroded due to lack of maintenance on the cooling system, then no water will flow through the heater core.

Best case:Or you may have a switch that will not allow the hot water to enter the heater core. I would check the switch first. Some switches work off of vacuum, some work from mechanical means with levers and some are electric. A door or flap sould open to allow the hot air to come into the cabin. If that flap or door is not opening, then that may be your problem.

Ford added a heater valve in 02. This is a vacuum actuated valve that cuts off flow of coolant through the heater core when max AC is requested. Your model should just have coolant flowing directly into the core at all times. A simple check of the flow is to feel both heater hoses going into the firewall from a cold start. The hoses should warm up at about the same rate and get uncomfortably hot. This is a good indication that heat is getting into the passenger compartment.

If it appears that the core is getting hot, the next suspect is the blend door. This door diverts some or all of the air flow through the heater core and commonly fails on the Mountaineer/Explorer/Ranger/Mazda pickup...all of which use the same plenum box. The door is on a vertical axis and has a pervasive problem with the the plastic connection to the motor breaking. The door can swing back and forth randomly or wedge in a strange position, killing heat and AC efficiency. This is an expensive dealer repair, but there is an aftermarket repair kit from HeaterTreater that will address and solve the problem. Complete diagnostic information and diagrams are available.

Check the blend door motor. Remove glove box, there's a plastic case/box motor 3"X4" in there. If the plug-in is lose and wiggles around re-soder. I had this problem and now works (replaced the blend door also) Check out Heater Theater for sure...