By Steve Schaeber, MACS Technical Editor
The main purpose of a thermostat is to help the cooling system regulate engine operating temperatures to within the manufacturer’s specification. This is normally done by first remaining closed to allow engine coolant to warm up, and then gradually open all the way as temperatures near spec. But sometimes this doesn’t happen, as in the case of a fully or partially stuck open thermostat.
When a thermostat sticks open, engine coolant is allowed to circulate without much restriction throughout the cooling system. This sounds great, and it’s important for this to happen after the proper operating temperature is reached. However, if it happens too soon the engine coolant may not be able to reach a high enough temperature. This problem occurs because modern cooling systems are actually quite efficient, and are able to transfer a lot of heat from a lot of liquid in a relatively short period of time. When the coolant is not allowed to warm up fully, it basically circulates from the engine to the radiator and back to the engine, losing so much heat with each pass that it stays relatively cool.
Problems attributed to a cold running engine include lower HVAC heater output, engine performance issues and increased tailpipe emissions. But it also prevents the evaporation of condensation that forms on the inside of the engine block. If this water doesn’t burn off it can oxidize the metal (producing rust) and cause an oil sludge buildup in the bottom of the pan.
Engine oil is also affected by the thermostat, and it’s important for it to reach an operating temperature of approximately 190°F. Oil that is cold is not quite as viscous as the substance when it’s warm. Transmission oil coolers are also affected because they are generally in direct contact with engine coolant.
An easy way to determine if a thermostat may be stuck open is to simply monitor coolant temperature. Some vehicles have a dashboard temperature gauge, but many technicians prefer to use a scan tool with graphing capabilities. Begin when the engine is cold and connect your scan tool before starting the engine. Graphing the coolant temperature sensor PID will allow you to watch the thermostat to see what happens over time. If the thermostat is only partially stuck open, you may notice a rise in value to a relatively warm number, perhaps around 150°F. But if it’s stuck open all the way, temperatures may not increase much past 120°F.
Many PCMs will also monitor thermostat operation, illuminate the check engine light and produce a trouble code if there’s a problem. Ford uses P0125, Insufficient Coolant Temperature for Closed Loop Fuel Control, along with P0128 Coolant Thermostat (Temperature Below Thermostat Regulating Temperature). Issues could be related to slow warm up times, low coolant level, or sensor output issues, as well. Sensor readings may be double checked by using a mechanical thermometer or a thermocouple connected to a digital meter.