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This is an ancient thread, but I wanted to add one thing that I've done since I "completed" the rebuild. The monovalve has always been a problem spot on my car, I've replaced the housing and the core several times but never gotten it to work properly. I even ran a light bulb parallel to it to make sure it was cycling properly (fyi, it looks like a bang-bang circuit to me, either full on or full off, cycling every few seconds to minutes based on temperature requested).
I sourced a monovalve replacement kit online, essentially it's a vacuum switchover valve (electronic input controls a vacuum valve), a vacuum controlled monovalve (I think it's a part from a w124), an elbow to replace the aux water pump, and vacuum and water plumbing parts.
This completely replaces the monovalve (which has extremely weak action) and the aux water pump (widely deemed unnecessary in moderate climates) with new elements. Installation took a couple of hours, with help from a friend, and since it's been installed, the heater has been working reliably, and not producing hot air except on demand. The new system is more complex, but seems to be more reliable, and can be swapped out for original parts without any permanent damage to the car.
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