With over 240k miles, my 99 6sp 540i has a overheating problem. Here is a brief story. While driving home, noticed engine temp gauge creeping from midpoint up to about three-quarters and back and again. This happened ever 3 to 5 minutes (1 hour commute). The gauge never went all the way, the car never ran hot. No signs of leaking or any other drama under the hood. Purchased and replaced thermostat, drove around block to get up to normal temp…. no fix, problem persist but now I notice coolant leaking by the droplet from the heater valve (sits on drivers side fender with 3 hoses connected). So I purchase a new heather valve…. Same thing happens with new unit! Still leaking at heather valve, still got fluctuating temp gauge and hoses are fine. Replaced final stage unit (fsu), had purchased one in error about a year ago so I had it on hand… problem persist! It as if whatever device controls coolant flow through the heater valve is not telling the valve to open and let the coolant pass. Car has no heat at this point but worked flawless prior to the problem. Can someone suggest a fix? What controls coolant flow through the heater valve? This really appears to be my problem. Please respond directly to email@example.com
or here on the board. Surely there is a sensor or something basic that is controlling the heater valve.