Attached below is a post from the E39 board. It seems that the E39 has idiot lights that only LOOK like gauges. I call them "idiot gauges." Glad our E34's have the real thing...
'90 535i(m) 211K+ miles and still going strong
: I noticed that the 540 engine heats up very fast (minutes) to the mid point and stays there. Is this normal?
They all heat up quickly - I noticed same with my 97...
BUT - do not be fooled by the fact that the gauge is at the mid point into thinking that your engine is at full operating temperature.
That guage gets to mid point at about 75-80C. It stays there while the coolant temperature continues to move up to its normal temperature of about 106-108C.
So you will want to wait about another 5-10 minutes after the guage gets to mid point before doing any foot to the floor acceleration.
The operation of that guage is programed to be "dampened" - so that it stays at the mid point unless the temp goes well out of range.
Apparently BMW did this to avoid complaints from so many owners who thought that the guage should always be at the mid point and who complained about possible problems when they saw it moving back and forth.
Now those of us who want to use the guage to keep an eye on minor fluctuations can no longer do this - foiled by the "worry warts".