Seconded on the monovalve. Just did mine last winter, and now my heat works perfectly. Just make sure when you remove the monovalve, you re-insert everything back the way it was before.

I mis-aligned mine after I installed it, and the heat was stuck on full blast until I did it properly.

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(Oo{=|=}oO) 1983 MB 300D USA, 212,000mi. 80's yellow/white. "Gunther"
(Oo{=|=}oO) 1984 MB 300D Euro, Turbo Added in Germany, 186,000mi [SOLD] Missing her dearly.
(Oo{=|=}oO) 1984 MB 300D USA Turbodiesel, 269,000mi. [SOLD]