Probably bad heater valve... (archive)

Heat would only come on when car idled in traffic. At highway speed heat would almost cut off, then return at stop light and low speed. Coolant level OK.

Sounds like the heater valve at the firewall is toast. At higher RPM's (therefore faster coolant flow) the coolant will tend to bypass the bad valve, leaving you with no heat above 2000 RPM. At idle, some of the hot coolant will enter the heater core, because it is not flowing as fast. Test it by dropping to idle at interstate speeds (neutral or clutch). If the heat comes back, then it's definitely the valve.

The replacement can be found at most import auto parts stores and any BMW or Mercedes dealer. Or try a mail order place such as bimmerworld.com. Give them a call--they're usually cheaper than everyone else. About $20-25, and less than 5 minutes to replace. You don't have to drain any coolant, but you will spill a tiny amount. Just make sure you take off the expansion tank cap to relieve the system pressure. It should be mostly cooled off, but cold is not necessary.

Cold weather sucks. I'm moving to Florida.

Good Luck
James
james_535@yahoo.com
88 M5
88 M3
83 535i

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