Chapter 10 Emission Control and Exhaust System Fundamentals 189 Copyright by Goodheart-Willcox Co., Inc. four to 15 minutes, depending on the outside temperature measured by the ambient-air temperature sensor. If coolant temperature does not rise as specified, the thermostat monitor concludes the thermostat is stuck open. The computer then sets a trouble code and illuminates the MIL. Cylinder Cutout on Engine Overheating Some vehicles have a feature that prevents engine dam- age when the engine cooling system fails. A temperature sen- sor is threaded into the cylinder head, but does not contact the coolant. At least one sensor is installed in each cylinder head. When the engine begins to overheat, the temperature sensor signals the ECM. The ECM then de-energizes fuel injectors to certain cylinders. The affected cylinders receive only air. No combustion takes place in these cylinders and they cool off. After a few seconds, the fuel injectors are re- energized, while fuel injectors on different cylinders are de-energized. Alternately disabling cylinders ensures that no cylinder becomes hot enough to cause valve burning or other damage related to overheating. Figure 10-7. Oil pressure to the variable valve timing mecha- nism is controlled by the ECM, which responds to inputs from the camshaft and crankshaft position sensors. (University of Toyota/Toyota Motor Sales USA) Camshaft timing oil-control valve Crankshaft position sensor VVT-i sensor ECM Figure 10-8. One type of valve lift and duration control. A—The control system is in low-speed mode. Note the position of the rocker arm pin. B—The control system is in high-speed mode. Notice how the rocker pin has engaged the pad and shaft. (Toyota) Rocker arm pad Locked state Hydraulic pressure View Z Z High-speed cam High-speed cam Low- and medium- speed cam Low- and medium- speed cam Rocker arm pad Moves freely Needle roller Rocker arm pin View Z Z A B