don't use acid on the grills it would cause them to rust severely down the road and you'll have to replace them, steam washing is best, or you can use a pressure washer but hold it at some distance as you dont want to bend the grills either, pressure washer is vert effective, you can use a radiator cleaner like liqui moly radiator cleaner to clean the inside then fill up with coolant, also check for air bubbles, open the radiator cap when engine is cold and let the engine run until water starts to overflow, a trapped air bubble can cause over heating as well