WARNING, if your temp gauge seems faulty jumping to hot and back to normal while you know for sure that your car isnt hot, it is an easy fix, you need a new thermostat. It comes with a sensor attached. I figured that it wasnt that big of a problem and I would fix it next time I took it in. WRONG ANSWER! The sensor on the thermostat shorted out the ECM and usually burns the wiring harness. Yes thats right, 2 wires coming off the thermostat killed the brain of my car. If you see this problem in your car fix it immediately or your pockets will suffer the wrath of the BMW dealer cause thats only where you will find these parts. 1998 was the experimental year for separating the Engine and Trans wiring harness. I guess they figured that it didnt work well cause the 1999 540 wiring harness is identical to 1997. 1998 is the only year they separated them. My mechanic says its not a common problem but he has seen it before. the Engine ECM was the price of a used Honda, the wiring Harness was the same but I was lucky just the ECM went out. New thermostat was $260.