I had weak heat on my 300D. Engine temp fine, plenty of coolant, blower fine, and heat consistent irrespective of engine speed (i.e. didn't drop when idling). But it was weak. So had to be the monovalve.

So I had the monovalve replaced. Now the heat is great....BUT, the heat now drops at idle (whereas it didn't before). From what I understand, that would be symptomatic of a bad aux water pump. But, like I say, what heat I had was perfectly consistent before I replaced the monovalve. Plus the aux water pump isn't all that old.

Any ideas?

Thanks!

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 179k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 77k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife