If it is overheating at highway speeds, the fan (bad or not) won't make a difference. At 60mph, there is enough air flow through the radiator to cool the engine. Since everything is new (rad and pump) it has to be one of three things. First, are you certain that there are no air pockets that could be obstructing the flow of coolant (did you bleed the system)? Next, when the engine is overheating, I need you to check all radiator hoses to make sure they are not collapsed. I've seen where they have pinched themself shut and stopped coolant flow. Finally, and this is assuming that the coolant is full, you need to consider a possible headgasket issue. A compression may indicate lower compression in one of the cylenders which is allowing this problem to happen.
Check those things and let me know what you find. As far as the fan clutch, it is normal for the first minute or two for it to run full speed and then slow down. However, if the under hood temp is high enough (like you said when it overheats) it should be locked in full.
I do have one last question. When the temp gauge goes to hot, does the coolant boil over or is there any other indication that the engine is overheating?
Wednesday, January 5th, 2011 AT 1:10 PM