my 2009 328xi coupe only blows cold air

Hi guys, have a question and a problem. During the winter months - my hvac unit in my 2009 328xi coupe only blows cold air at highway speeds. It blows warm/hot air at idle or slow driving speeds. Dealer can not figure this out and have dealt with a cold car for 2 years now in the winter. Thermostat replaced last year. Could it be the heater valve monovalve? The AC works fine in the summer. Help. Suggestions?

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[Nick at Pelican Parts]

E90 models do not use a monovalve. This could be either low coolant, a faulty thermostat, poor coolant flow or a restricted heater core. Start by checking the coolant level and flow. If the heater core is getting good flow of hot coolant, it may be restricted.

- Nick