The cold air indicates a problem with one or more of these components:
* Blend Door Actuator: This is the most likely culprit. It's a small motor that controls the blend of hot and cold air. If it's malfunctioning or its linkage is broken, it might be stuck in the "cold" position despite the heater core being hot.
* Climate Control Module (CCM): This is the "brain" of the climate control system. A faulty CCM can prevent the system from correctly interpreting commands and sending signals to the actuators.
* Vacuum Leaks (less likely but possible): Some older systems use vacuum to control some aspects of the climate control. A leak could interfere with the blend door operation. This is less likely on a '97 Mark VIII due to its more electronically controlled system, but it's still possible.
* Low Coolant: While you say the heater core is hot, ensure the coolant level in the radiator is sufficient. A low coolant level will restrict heat flow.
You cannot manually override the system. There's no simple knob or lever to force hot air. You'll need to diagnose and repair the problem. Here's what you should do:
1. Check Coolant Level: Make absolutely sure your coolant level is correct.
2. Check Fuses and Relays: Look for blown fuses or faulty relays associated with the climate control system in your owner's manual or online.
3. Diagnostic Scan: The best approach is to have the vehicle scanned with an OBD-II scanner that can read climate control system codes. This will pinpoint the most likely problem (like a faulty blend door actuator). An auto parts store might offer this service for free.
4. Professional Diagnosis: If a scan doesn't reveal the problem, take it to a qualified mechanic specializing in older vehicles. They'll have the experience to diagnose and repair the malfunctioning component.
Trying to manually force hot air could damage the system further. Focus on properly diagnosing and repairing the underlying issue.
