That's kind of weird, especially when the temp stays below 80C. The thermostat doesn't even start to open until it reaches that temp. A fully warmed up engine should be at slightly above 80C. Assuming you installed the thermostat correctly, you might want to look at replacing the coolant temp sensor. Also it never hurts to pressure test the cooling system and check for residual pressure in the system after the engine completely cools down. Residual pressure would indicate a head gasket leak which is known to contribute to overheating. Guess how I know.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL