Alright, this weekend I hard wired the monovalve to a switch I installed in my cabin....basically detoured the temp wheel. With a flip of the switch I have full heat or full cold.

This is NOT a sloution, but part of my diagnostics

Same issue.....heat idle and in traffic, loss of heat when up to speed on interstate....this is with me flipping switch to the monovalve to give me full heat.

So, how is it that hot water is bypassing the heater core with monovlave open? (Operating temp never really moves past 80*)

Could it be a sign of not enough water pressure? like bad water pump, or a restriction? I am trying to rule evrything out before I drop $200+ on a monovalve!

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'89 300SE
'02 E320 Wagon