Overheating

Burn59

I have a 2002 300M 3.5L HO the other day it over heated to the red...I didn't notice till it set a code and dinged...
Luckily I was less than a half block from home...when I opened the hood ...I discovered a crack in the tank reservoir
spraying coolant...I got a replacement online changed it out and added some universal coolant less than half a gallon!
I did open the bleed valve while adding coolant it was a steady flow no bubbles, but now the car overheats under driving conditions very fast 10-15 minutes it's fine idling but driving it goes up past half quickly but if I crank the heat it comes back down to half, is there another issue or just air in the system???

PF_Flyer

Perhap the thermostat is broken and the coolant is not flowing to the radiator to bleed the heat off the engine. Since you bled the system and have no bubbles, it has to be something else. When you crank the heat, you're just moving coolant through a smaller radiator and bleeding off heat.

Definitely check/replace the thermostat