Let's start here:

Have you had it out of the water and running on a hose at idle? What were the results? Does the temperature continue to be elevated, or is it where it is supposed to be?

If not, do so. Let us know the results. Hook the hose up to the pickup, at the point where the water is usually drawn in from.

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Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport