it's a$$-cold around here but i've noticed that my 97 E420 only gets up to about 70C no matter how long I drive. Car warms up fine, and blows hot air just fine.

I've considered a failing thermostat, but I realized that when I replaced the coolant 3 months ago, I used OEM MB coolant but ended up using a lot more coolant than distilled water. So the final mix ratio was more like 7:3 coolant:h2o rather than 5:5.

could the excess coolant keep the engine from reaching 80C?

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Current cars:
2001 MB E55 AMG
2003 Honda S2000
2000 Honda CRV

Gone but not forgotten:
2004 BMW 325i 5sp sport
1999 BMW 540i sport
1997 MB E420
1995 BMW M3 coupe