188 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. Thermostats The thermostat in the cooling system does not open until the coolant temperature reaches at least 190°F to 200°F (88°C to 93°C). The cooling system warms up faster and operates at a higher temperature than the systems found on older vehicles. Thermostats used on older vehicles opened at 160°F to 180°F (71°C to 82°C). The new systems allow the combustion chamber to be kept at higher temperatures for most of the time that the engine is operating. Higher tem- peratures reduce gasoline condensation on the combustion chamber surfaces and cylinder walls. Levels of HC will also be lower, Figure 10-9. OBD II Thermostat Monitor Many OBD II vehicles have a system to monitor ther- mostat operation. The thermostat monitor consists of: • Engine-coolant temperature sensor. • Ambient-air temperature sensor. • Timing circuit in the ECM. When the engine is started and the coolant temperature is lower than the normal operating temperature, the thermostat monitor begins timing how long it takes the engine to reach operating temperature. If the thermostat is closed, coolant temperature should rise to about 80% of the thermostat open- ing temperature within a specified time. This time varies from Figure 10-6. A—Increasing valve overlap results in high power at higher speeds, but can cause mild dilution of the incoming air-fuel mixture at idle. B—By reducing or eliminating valve overlap, high-speed power is reduced, but the engine idles smoothly with fewer emissions. Both valves open Both valves open Exhaust gases pulled back into cylinder Intake Stroke Intake Stroke Intake valve closed Exhaust gases only Piston moving up Exhaust Stroke Exhaust valve closed Air-fuel mixture only Exhaust Stroke A B Piston moving up Piston moving down Piston moving down Incoming air-fuel mixture diluted by exhaust gases