Over time, deposits accumulate on a radiator's interior surfaces. Radiator deposits have a dual effect: they reduce heat conduction at the radiator surfaces, and they create flow constrictions which reduce heat delivery to the radiator. These deposits can include corrosion (rust) and hard water precipitates, grease and oil, and precipitates formed from incompatible coolant mixtures.
Dirt and bugs, stones and other road debris accumulate on a radiator's exterior. These materials generally get stuck between cooling fins where they block air flow through the core, further reducing radiator efficiency.
Internal clogs are sometimes identified by an infrared thermometer, an instrument that scans a radiator's surface for cold spots. Sometimes chemical cleaning and back-flushing will remedy an internal blockage. A badly clogged radiator usually needs to be removed for thorough cleaning, or replaced.