The heat on the drivers side and cold on the passenger was the tell tale sign to lube up my duovalve unit. One time I had to do it twice even though it appeared fine.

You can clamp off one of the hot water lines to the valve which will prevent hot water flow and see if this corrects the issue. The valve might be fine but not getting a signal to close properly.

Its simple to try and definitely would isolate that issue.

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1991 300D 172,000 (totaled)
1998 E300 Turbodiesel 82,500