Another way to troubleshoot the heater valve is by unplugging it. Unplugging makes it default to the open position. If heat works then it could be related to the temp sensor in the hvac duct. If still no heat might be faulty heater valve.

I know it might seem like a lot of work but might be worthwhile to inspect the water pump. Maybe the impeller is damaged? While your at it try to reverse flush the heater core.

Good luck, I know it’s frustrating but it is probably something simple.

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2000 E53 4.4
2011 E70 35d