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Q: Overheating 2004 Z4 60,000 miles. fans all work, coolant is full. on 2004 BMW Z4

Took it to the dealer (rip off). They tested everything and replaced the sensor sending unit. Flushed the cooling system, bled out the air...problem still exists. Any ideas on what could be causing this? I don't want to just start replacing parts randomly. Mechanic said the Z4 runs hot but I don't buy that. Temp. gauge gets up to 3/4 mark. I don't want to blow a head gasket. I think the temp gauge needle should remain near the vertical position not vacillate up and down. Also A/C shuts off when the temp. goes up. Mechanic also said the gauge is not actually a gauge but instead a buffered switch. Any insight on this would be greatly appreciated. Car is in perfect shape and has been well maintained.
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If it has a problem and that needs to be determined first. These cars assuming its a 3.0 liter engine have had issues with thermostat and water pump that becomes weak. Cooling fans are computer controlled and can cause problems also. I think it needs to be determined if its running too hot or not first. Its a very involved cooling system that needs to be checked by a good qulified technician.
Thanks for the answer. I had it in to the dealer and they said the temperature was ok but the gauge or sending unit was bad so they replaced the sending unit. the fans work but after the change out of the sending unit it still reads it is overheating. How do you check for a bad water pump? the thermostat should be somewhat easy to change out. I just don't want to start spending money changing out everything as trial and error. I have taken it back to the dealer.
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Basic... its inspecting the coolant flow back into the coolant tank. I would assume dealer had done that but who knows. The dealer should have some other opinion of what to check next if its showing a over heat condition even after changing sender. I would be interested in what tempuature does it show in the computer data stream readings. That would have to be read on a scanner that commicates with the cars computer. That temp is read by a differant sensor on the engine and is considered to be more accurate of a reading.
Interestingly, the BMW dealer says the fix they do for an overheating problem is to just replace everything. starting with the water pump and thermostats, it has two thermostats, then if that doesn't fix the problem replace the radiator. apparently this is a common problem in BMW's. I'm a little ticked that they don't have a way to measure impedance in the water flow.
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I have done some research and true its a matter of elimination sometimes. You might want to seek an independant shop in your area that specializes in BMW for another opinion.
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