Another issue that arises with the use of cooler thermostats is how the PCM uses the cooling fans behind the radiator to keep the engine cool. Nearly all contemporary cars now use electric fans to pull air through the radiator to cool the engine coolant. Only when the vehicle is at higher speeds (typically above 35 MPH) does the vehicle's motion naturally force air through the radiators and removes the need for the fans. Problems can even occur at higher speeds when drafting behind another vehicle and your car experiences less frontal air pressure. At lower speeds or in those special situations, the fans are necessary to keep the engine cool. However, these fans are controlled by the PCM and do not run all the time. Furthermore, the PCM typically controls the fans at two different speeds, Low Speed and High Speed. For many GM vehicles, the PCM will turn the fans to Low speed when the PCM sees the ECT reach ~215ºF and switches to High speed when the engine temperature reaches ~225ºF. Consequently, having a 180ºF thermostat will NOT have the engine running at ~180ºF when you are traveling below 35 MPH or in stop-n-go traffic. In this condition, the thermostat will be wide open, coolant racing through the radiator but without any airflow through the radiator, the coolant is not cooled down. Thus, the engine begins to superheat over the thermostat set temperature.
