1. "My car overheats (at least the guage indicates this - the car doesn't feel hot) when idling, climbing hills (sometimes)".

This is a tip-off that something is wrong with your cooling system intermittently.
I suspect your thermostat failing. Thermostat average lifespan is 50-80K.

2. The aux fan is a different issue. The Aux Fan is needed when:
a. AC is ON
b. Ambient temp is HOT like 100F day
c. Car stationary, when car runs more than 10 mph, no need for the Aux Fan.

Aux Fan problem could be:
a. Circuit
b. Sensor
c. fan itself. Info on how to change the Aux Fan is here (Make sure you make the correct diagnosis first!!!):

http://bimmerboard.com/forums/posts/356191/

There are alot of info on bimmerfest.com as well.
Search "Aux Fan".

Good Luck and let us know how it turns out after you have done cooling system overhaul.

CNN-090
1998 28i 5sp 98K

• 1998 528i - random overheating - dsionne 2007-12-01 15:41:45 (269 views)
  • Two separate issues (url) - CNN-090 2007-12-02 23:21:56 (200 views)
  • Re: 1998 528i - random overheating - poolman 2007-12-02 20:49:25 (176 views)
  • Yes I'd do a COOLING OVERHAUL (url) - CNN-090 2007-12-01 21:18:15 (203 views)
    • Re: Yes I'd do a COOLING OVERHAUL - dsionne 2007-12-02 18:44:42 (164 views)
      • reporting back - dsionne 2008-01-16 21:31:08 (96 views)