Also before you do the monovalve job. a good coolant flush product (from MB dealer-$6.95) followed by the OE Mercedes coolant AND distilled ph neutral water made my W126 heat system work alot better.
1993 Mercedes Benz 300D 2.5 205K (ex wife's)
1984 Mercedes Benz 300SD 320K (SOLD)
2004 Mercedes Benz C240 75K
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1987 BMW 325i 220K (SOLD FOR SALVAGE)