* Heater Core Blockage: This is the most probable cause. Sediment, rust, or debris can clog the heater core, preventing coolant flow through it. The hot hose is receiving coolant from the engine, but it can't flow through the blocked core to the return hose.
* Heater Core Leak: A leak in the heater core itself would prevent proper circulation. Coolant might be escaping into the car's interior (you'd likely smell sweet coolant or see a leak on the floor).
* Heater Valve Problem: The heater control valve regulates coolant flow to the heater core. If it's stuck closed, no coolant will reach the heater core, resulting in a cold hose on the output side. If it's stuck open (less likely to cause this specific symptom), you'll have continuous heat regardless of the controls. The valve itself might be mechanically stuck or electrically malfunctioning (depending on your Roadmaster's system).
* Water Pump Issues (Less Likely): While a weak water pump wouldn't typically create this *specific* symptom (one hot, one cold), if it's extremely weak, it might not have enough pressure to push coolant through a partially blocked heater core. Usually a weak water pump leads to general overheating issues.
* Hose Clogs or Kinks: While less likely, a severe kink or clog in one of the hoses *could* restrict flow enough to create this imbalance.
Troubleshooting Steps:
1. Check for Leaks: Carefully inspect the heater core hoses, connections, and the area under the carpet near the heater core (you might need to remove some trim). Look for any signs of coolant leakage.
2. Test the Heater Control Valve: Locate the heater control valve (often near the firewall). See if you can manually operate it (if it's a cable-operated valve). If it's electrically controlled, you might need to test it with a multimeter or have it checked by a mechanic.
3. Flush the Heater Core (If Possible): This requires disconnecting the heater hoses and flushing the core with a solution designed for cleaning cooling systems. This is a more involved task and may not be feasible for everyone. A shop will have the equipment for pressure-flushing.
4. Check Coolant Level: Ensure your coolant level is correct. Low coolant can exacerbate any existing blockage issues.
5. Consider a Pressure Test: A pressure test on the cooling system can reveal leaks that aren't immediately obvious.
If you are not comfortable working on your car's cooling system, it's best to take it to a qualified mechanic. Working with coolant can be dangerous. A mechanic will have the tools and expertise to diagnose the problem accurately and repair it safely.
