BMW E38 heater not warming up: circulation pump and water valve
Why does the E38 heater only warm up at high revs?
The most common cause is the electric circulation pump, whose brushes are worn or whose fuse has blown. Then cold air comes out at idle, and you only get heat by raising the revs. First check the coolant level and bleeding.
Why does heat come out on one side only?
The water valve unit contains two valves, one for each side. A stuck valve or a broken coil leaves one side cold. You can hear the valve click when you turn the temperature control from one extreme to the other.
The heat did not come back after the pump was repaired, what is wrong?
There may be an air lock in the heating circuit, because the heater core is its own loop where air easily gets trapped. Bleed with the heat on full and drive at higher revs, several times if needed. The thermostat may also be stuck open.
Where are the E38 circulation pump and water valve located?
The pump is integrated with the water valves, and the unit is behind the engine on the driver's side in the bulkhead. The valve and pump are often sold as one package, and a used package is cheaper than a new one.