Sometimes even if they look clean , they aren't .
I''ve had folks bring vehicles to me with overheating and por AC performance problems and they tel me that they've hosed them out, but i pop the grill off and spend an hour or so with a pick tool and a flat screwdriver picking all the insect carcases and tiny rocks out and straighten the fins, and no more problem. I've seen the airflow as much as 80% blocked with just little crud, but a whole lot of it.
-------Robert