Recently took a road trip from Phoenix to San Jose and she started running warmer on the way there, about 100cel instead of her typical 84-86. During the trip home, she spiked to near the red zone and we drove most of the way home with the heat. Ugh.

I took the car to an MB dealer who could not find anything wrong with it.

It is hot and humid today, and into the red it went for the first time. The interesting twist, I run the a/c at 86, which should be hot, and instead it is cold air and only one tap away from heat. I hate the thought of randomly replacing items that aren't broken to fix the issue. Has anyone heard of this? What could be wrong?

 

Most likely it is the thermostat. It is easy to test. It is also inexpensive and easy to replace.

 

That dealer is pittiful for not diagnosing it. I agree with k_alavi that the problem is most likely a bad thermostat. Check the amount of coolant you have left in the car and definately don't drive it around while it's hot otherwise you may end up with a huuuge repair bill for new cylinder heads, engine rebuild,etc.

 

I agree T-Stat. But make sure that the auxilarry fans are working at both speeds. High speed should come on at 100-105 deg c. The waterpumps are notorious for going bad on the 104 engine too.