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A coolant hose restricted by a kink, a collapse or internal blockage. A hose can collapse under suction as it ages and softens, closing on the pump's inlet side while looking perfectly normal at rest.

The result is reduced flow with a healthy pump and a clear radiator, so temperature climbs under load while the usual checks pass. Feel along the hoses with the engine hot: a hose cold beyond a point is blocked there, and the temperature difference locates it exactly. A collapsed suction hose is visible with the engine running — squeeze it and watch whether it recovers. Internal blockage comes from sediment in a system topped up with water or run on exhausted coolant, which means the rest of the system deserves inspection too.