So long, and thanks for all the fish
- Mar 6, 2005
- Motor City, Michigan
- 13 Ford Mustang GT, 17 Ford Fiesta ST
If turning off the heat fixes it, your heater core may be plugged.
#1 cause of "temp creep" at idle is a failing fan clutch.
A very common cause of overheating at idle is a bad fan clutch.
Install a new electric fan, the old one is worn out and vibrates. Service the rest of the cooling system and maintain or replace parts as needed.
Creeping temperatures at idle are almost always a sign of viscous fan clutch failure.
Stop and go traffic issue , main suspect would be fan clutch.