And a coolant "flush" implies just that!

If all that's done is drain the old coolant and refill with MB coolant, there is still residual residing in the engine...so the green is still there.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car