It is indeed a marginally functioning auxiliary water pump that is the problem. I clearly needed a new monovalve, but I'm guessing the bad monovalve was masking the fact that I also had a bad AWP. Surprised that it only lasted 4-5 years. Oh well...at least these are cheap fixes.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 160k miles
06 E320 CDI "Rutherford", Black on Tan, 180k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 79k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife