What is the effect of installing a colder than specified thermostat in a vehicle?

Installing a colder than specified thermostat in a vehicle affects engine performance and emissions by allowing the coolant to circulate at lower temperatures than intended. This can lead to incomplete combustion of fuel, which often results in an increase in unburned hydrocarbons (HC) in the exhaust. A cooler engine may not reach optimal operating conditions, causing fuel to not combust fully, which contributes to higher HC emissions.

In optimal conditions, the engine's operating temperature helps in achieving better combustion efficiency. When the thermostat opens too early due to being a colder version, the engine may not reach its designed temperature and hence not operate efficiently. This inefficiency can lead to a rise in hydrocarbon emissions as unburned fuel exits with the exhaust.

While colder temperature scenarios may influence other types of emissions, such as decreasing CO emissions or altering NOx emissions, the primary and most direct impact is observed in the increase of HC emissions when the engine operates outside its intended temperature range.