Why is the upper radiator hose hot while the lower one is not?

[EloiseFitz]

The reason why the upper radiator hose is hot while the lower one is not is: the engine coolant temperature is not high enough, and the thermostat has not yet opened. The coolant in the engine cooling system is still in a small circulation state. When the engine temperature rises above 80 degrees Celsius, the thermostat opens, and the coolant enters the engine water passages for large circulation, causing both the upper and lower hoses to become hot. The upper hose is the engine's outlet hose, while the lower hose is the inlet hose. If the engine coolant temperature has not reached the thermostat's opening temperature, the coolant will not circulate in a large loop but only in a small loop. When the engine temperature reaches the thermostat's set temperature, the thermostat opens, allowing the coolant to circulate in a large loop, at which point both the upper and lower hoses will become hot.

[DelGavin]

I've encountered this issue once before when driving my old Jetta up a mountain and noticed the temperature gauge skyrocketing. The upper radiator hose was too hot to touch, while the lower one was just warm. Essentially, the engine's heat couldn't dissipate, usually due to a faulty thermostat. If it's stuck closed, the coolant just circulates within the engine, and the radiator doesn't get involved at all. Another possibility is a failed water pump—if the impeller isn't turning, the coolant won't circulate. The scariest part was hearing the fan running loudly while the coolant was practically boiling. It's best to pull over immediately; pushing the engine further could blow the head gasket, potentially leading to major repairs. For a quick check, feel the temperature at the lower radiator hose connection—if there's a noticeable difference, call a tow truck right away.

[AliceLynn]

Last time my car had the same issue, with a huge temperature difference between the upper and lower radiator hoses. I initially thought it was normal, but the mechanic said it's a classic symptom of failure. The key culprit might be a stuck thermostat—like a gate jammed shut, preventing hot coolant from reaching the radiator. A failed water pump can also cause this, leaving coolant stagnant and overheating locally. The worst-case scenario is clogged radiator channels, requiring removal of the entire front bumper for repairs. Nowadays, some new cars are prone to thermostat failures—my neighbor’s SUV even seized its engine due to this. If the temp gauge spikes, don’t wait for a warning light; shut off the engine immediately and inspect it.

[VonFinley]

I've specifically researched this issue. A car's cooling system is like the human circulatory system, where the upper hose outputs high-temperature coolant that should cool down after passing through the radiator. If the lower hose isn't hot, it indicates no heat exchange is occurring. There are three main reasons: a faulty thermostat stuck closed, preventing coolant from entering the main circulation; a detached water pump impeller or broken shaft; or internal blockage in the radiator. When handling this myself, I first test-drive to check the water temperature, then feel the radiator's bottom temperature. A temperature difference exceeding 30 degrees is definitely abnormal, and the engine should be shut off immediately. For modified cars, special attention should be paid to incorrect coolant ratios, which can also cause such failures.