Thermostat Replacement (08 S550)

[Benzjag]

Occassionally, I was getting code P0128 on my 08 S550 so I changed the thermostat today. Runs fine, but the gauge is showing around 85 highway and up to 90 at a stop light. It does come back down to 85 once moving. Previously, it was around 80 highway and 85 in traffic. I put the anti-freeze in the recovery tank leaving the cap off with full heat while the car idled for about 15 minutes. I am just concerned why it's running hotter than before.

Any ideas? The car is new to me;therefore, I am not familiar with the W221 chassis!

Thanks!

Richard