cold air on freeway...

[RazorRacer]

Guys,

My 1985 535is heater works great in stop & go driving, but get on the freeway and the heat goes away. Any ideas on where to look? My guess is a leak in the heater hoses under the car.

[Bob540iA]

If you are not loosing coolant I would suspect the thermostat is stuck open. In low speed driving the radiator doesn't get much air flow so the temperature of the coolant is enought to keep you warm. At highway speeds the radiator is getting lots of cold air and coolant temperture is reduced. The temperature gauge should tell you this.

[86E28sandman]

Hey, I have an 86 535i and my car is doing the same thing, I replaced the thermostat, but it has changed nothing, still no heat at hwy speeds. I would really like to know if you find out anything.

[Jared at Pelican Parts]

I remember hearing something about these cars having an additional water pump for the heater. Maybe this is gone bad?

[CarreraS2]

It may be your heater control valve. You will see it on the firewall, in the engine compartment, directly in front of the driver. It has the small water lines coming out of it.

Call any supplier and ask for the "heater control valve repair kit." Should be $10 - 15. Is an easy fix, very common on these cars.

It could be your thermostat, but even if your thermostat were stuck fully open, the coolant gets hot enough to give a decent amount of heat in the cabin.

P.S.
There is no "additional water pump" for the heater.