I also suspect it is the monovalve. I was getting no heat. Changed the monovalve and that fixed it. It is an easy job on the 123. Just do a search and you will find instructions with pictures. Part cost about $25.00. Also check you auxillary water pump. Just follow the heater hose until you see a small electric pump. You can jumper wire it to the battery and make sure it spins. Good luck.

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Bruce

73 220D (never left Germany)
81 300D (totaled)
84 300D (Purchased '03, sold '17)
85 300SD (purchased 10/01/03)