I have a 08 suburban with a 5.3 engine. When I leave work I have about a 5-10 minute drive 35-40 mph before I get to the highway. Right before I get to the highway my temp is pushing 240. Once I get on the hwy and going 55-60 mph, it cools down to 210. After I get off the hwy, and I’m pulling into the neighborhood, it starts creeping back up on me. Anyone have any suggestions?

 

Sounds like you fans are not working. Highway speed pushs enough air through to cool it.

 

You can hear the fans running until it cools down

 

Make sure both fans are actually moving air. Blades not slipping on the shafts, condenser or radiator fins not plugged. If thermostat was stuck closed it would never cool down. I don't know if there are water pump impeller issues on the 5.3 myself. Cooling system full and holding pressure.