* One hot, one cold hose at the firewall (heater core): This strongly suggests a blockage or failure within the heater core itself. The hot hose is getting hot coolant from the engine, but that coolant isn't flowing through the core to heat the air, resulting in a cold outlet hose.
* Engine hotter than normal: This is a significant clue. If the heater core is blocked, coolant flow through the entire system is restricted. This reduced flow can cause the engine to overheat because the coolant isn't circulating and dissipating heat effectively. This is more likely the *cause* of the hot hose, rather than an effect of the heater core being clogged.
* Bad Heater OR bad thermostat: While a bad heater core is the most likely culprit given the symptoms, a faulty thermostat can also cause overheating and affect heater performance. A stuck-closed thermostat prevents proper coolant circulation, leading to overheating and no heat in the cabin. However, with one hose hot, the thermostat is at least *partially* opening to allow coolant to reach one side of the heater core.
Here's how to proceed:
1. Check the Thermostat: This is the easiest and cheapest check first. A simple way is to feel the upper radiator hose when the engine is at operating temperature. It should be hot. If it’s cold or lukewarm even after the engine is up to temperature, the thermostat is likely stuck closed and needs replacing. If it's hot, then focus your attention on the heater core.
2. Check for Blockages (Before Replacing Anything): Before assuming a bad heater core, try flushing the system. This involves draining the coolant, and using a garden hose to flush the system from the heater core hoses, etc, clearing out any possible debris that could be causing a blockage. This is a relatively inexpensive option to attempt first.
3. Investigate the Heater Core: If the thermostat is working and flushing doesn't solve the problem, the heater core is the prime suspect. A blocked or leaking heater core is relatively common in older vehicles. Replacing it is a more involved job, requiring accessing it under the dash. Sometimes this can be a messy job since coolant will likely leak upon removal and access may be difficult
In summary: While a faulty thermostat is a possibility, the combination of one hot hose, one cold hose, and engine overheating strongly points towards a problem with the heater core, likely a blockage. Check the thermostat first; if it's fine, flush the system to try clearing any blockages before considering replacing the heater core. A faulty heater core would be the most likely culprit, given the other symptoms.
