So to give you all some background, when I first got the 300d, the coolant was kinda brownish and gross. The water pump housing eventually started leaking, so I took it in and had the housing, pump and thermostat replaced and the coolant flushed.

Ever since then, the coolant temp has seemed wonky. Before all that, the temp gauge would stick right around 85 to 90 deg C (the marks on the gauge are 60-80-100-120, with 120 being the overheat temp). After the work, the gauge will behave semi erratically. Some days it’ll stick around 90 deg C for my 40-60 minute commute, some days it’ll hover closer to the 100C mark (usually like, 97?). It’ll never overheat, and it never crosses that 100C mark. Then most days it’ll do a combination of the two, kind of fluctuating back and forth between 85C and 95C. Now, most of this isn’t too concerning, but it is a little annoying.

Onto the troubleshooting; my gut tells me its just a faulty thermostat, but I’m not positive. If I turn on the heater, the coolant temp will drop which implies coolant isn’t being circulated properly? No coolant in oil, but the coolant does have some brown liquid in it. Doesn’t seem like oil though because it’s always separated, I’m more thinking that at some point the Mercedes stock coolant isn’t the green stuff that it was filled with, and the difference in color is just the remnants of the old coolant. I’ve burped the cooling system a few times and I’ve gotten some bubbles out, but it seems pretty well bled. I asked my mechanic about headgasket and he didn’t think that was it.

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Any thoughts?