Okay.. I have a 2003 G35 sedan with 150K miles. The car started overheating so I took it to the mechanic.. per his recommendations I replaced the thermostat, radiator and belts.. so after $1K I assumed everything would be ok. Took car home and did not heat up. Next morning drove to work and on my way back home the car overheated again!! WTF.. took it back with the mechanic and after he did a test "under pressure" he's now saying it has a cracked head.. estimated $2500!!!... I took the car to another mechanic.. he said there was air in the valves and a bad water pump.. replaced it and after $500 the car is still overheating.. now he's also saying it has to be a blown headgasket.. Same mechanic is suggesting replacing the engine for one under 50K miles.. there's no white smoke or white stuff on the oil cap.. has anyone here had this problem?? what do you suggest?
thanks..

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Change your oil. Is it foamy and milky looking?
Drain and fill your radiator. Does it have oil in it?

If either of those is true, your head gasket is blown. You only get white smoke if the head cracks in an odd way and you get coolant into the exhaust side. Everything the first mechanic charged you for most likely means it was the radiator (all the little stuff like hoses is standard). The second mechanic might have been "shooting in the dark" about the water pump. I say that because the first guy seems to have done a nice radiator job, I doubt he would have missed a leaking water pump. (pretty much the second thing you check)

The head gasket sounds likely at this point. Its one of the reasons I dread overheating so much... once is enough sometimes. You have a new water pump, radiator, hoses, and thermostat... they have replaced your entire cooling system

I'll try that this weekend.. thanks