Thank you for your replies. My mechanic is not a Mercedes specialist but he does work on a number of 126s. That is why I am posting here. I will relay your suggestions to him. How do you "burp" the system? What is the correct way to orient the thermostat? I will assume that the mono valve is working because there was no temperature issue until the new pump and thermostat were installed.

Thanks again.

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1988 420 SEL
1991 420 SEL with 560 motor
1993 Miata