The temp gage is buffered so at 12 o clock or 1/2 or vertial, the engine temp is between 75-115 degrees, I've only seen my temp gage go past 12 o clock when running at the track and even then only up to 3/5 no higher.
According to Jim Cash on Bimmerboard (he works for a dealer), the E39 540 cooling system is designed to regulate at 109-110 degrees. The tstat has a heater on it (prolly why it cost so much) that it uses to pre heat the tstat to open earlier during spitited driving or when the computer sees something odd with the rate of cooling.
Some guys say that there was a bad batch of after market expansion tanks that cracked or cause overheating, I haven't seen any tho as I buy my expansion tanks through Mike at Maranello BMW, they are a site sponsor and give discounts on parts when you mention the Max forum.
Last edited by MLue1; 12-27-2011 at 10:58 AM.
Reason: Comment on Maranello BMW