my guess is thermostat and it's also cheapest fix. My water pump just went yesterday (100* ambient here too), so I'm doing new G05 and a thermostat since it is original and also, according to the dealer svc dept, "a $10 part." I saw them on fastlane a little higher than $10, but either way, it's a cheap fix. Is yours original? If you've gotten a decade out of it, it's cheap insurance to keep you from being stranded with an overheat situation. Any leaks anywhere at all?

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former: 83 300D, 97 C230, 93 400E

current: 08 C300 Luxury , 92 500SL