I have a 1997 Isuzu Rodeo, 3.2l, auto, 4x4, a/c non-functional. It has been boiling over at idle for the past several days, but runs fine at driving speeds. Pump fan is good, electric fan cycles on start-up, but, as the a/c doesn't work, that's the only time it runs. Could my pump impeller have quit working? It doesn't look like coolant is circulating.