The plastic crossover coolant tube molded into the plastic intake manifolds on 4.6L engines are cracking on vehicles here in AZ. This is happening on engines 4-6 years old, resulting in a sudden loss of coolant. The new replacement manifold has an aluminum crossover tube, so Ford has done a fix on the problem. But there a lot of plastic manifolds out there - shine a flashlight beside the alternator under the intake manifold to check for coolant between the "V" of the block. Some engines have been ruined when the driver didn't stop in time; fortunately I caught mine, but the replacement awasn't cheap.