My mechanic just replaced my water pump and thermostat on my 91 Mercedes 420 SEL with a 560 motor. Now the engine barely warms up. It used to run at just over 80C. Now it is barely getting to 50C and the heat is no longer very hot in 25 degree Fahrenheit weather. The thermostat was bought at Mercedes. Any ideas would be appreciated.

Thanks

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata