The radiator is not clean.

The radiator must be removed from the vehicle and some time must be spent to thorougly get the crap out from between the fins. I like to use a chemical cleaner (condenser cleaner). Others have been successful with a pressure washer, but, I'd be very careful with one of them to prevent rounding the outside face of the fins.

Hold the radiator up to the sun...........peer through the fins...........see anything??