i would suspect a faulty relay.
my water pump pulley seems to be slipping at high rpm (overheats on the highway, and sometimes in town :twisted:)
never let the coolant boil over though... at idle she keeps her temp perfectly, and the coolant fan kicks right on! also the t-stat opens perfectly at 87C
all of my wiring for my fan (and the fan itself) died.. i blame a/c! they are very prone to having issues due to the fact that the rad fan has a super high speed mode (on a nice hot day if it was on for long enough, it could slowly burn out the motor, and corrode the wires, or even blow the relay like on mine!)
best idea, is to just take out the thermoswitch connector (drivers side of the rad, bottom portion) and then there should be 3 connectors. take a wire and jump two at a time. the fan should kick on both high and low speeds depending on which ones you jump (only jump each side with the center wire... i hold no responsibility if you jump both outside wires... i have never tried it, and don't think it would hurt anything, but just a warning)
now if your fan doesn't kick on, then bingo! if your rad fan does kick on, then suspect the thermoswitch as faulty. i replaced mine with an acdelco i think... i don't think brand is as crucial as the t-stat. best part about changing that, is that if you do it fast enough, you won't lose too much coolant :lol: .
i will be changing my belt soon... probably tomorrow. it only has about 45 degrees of belt coverage, so that makes it a good candidate for some bad slippage at high rpm!
Good luck!!!
edit: forgot to mention... there is an a/c resistor located on top of the fan motor... you can't really miss it. mine was blown... had to replace it. it is for the a/c low speed. without this resistor, your a/c will overheat and cook itself real good, and all the gas will blow out one of the expanded o-rings (probably the one going from the compressor to the condensor... ask me how i know this :lol: )
so if you're keeping a/c, get everything fixed asap :wink: