Chapter 10 Emission Control and Exhaust System Fundamentals 191 Copyright by Goodheart-Willcox Co., Inc. second seat in the valve to close it. This prevents the flame from traveling back into the crankcase and causing an explosion. Outside air is constantly being drawn into the crank- case through a vent because of the vacuum pulling blowby gases through the PCV system. This incoming air must be filtered to prevent dirt from entering the crankcase. Older PCV systems may have a small filter, usually installed in the air cleaner housing. On newer vehicles, PCV vents are piped into the air-induction system after the main air filter. In this case, a separate filter is not needed. This arrangement is most often found on fuel injection systems with sealed intake ducts. On a few engines with multiport fuel injec- tion, the PCV system is cast as part of the intake plenum. A few engines use an oil/air separator or other system that eliminates the PCV valve. The OBD II system typically monitors the operation of the PCV system. If the PCV system is improperly connected or not properly operating, the PCV monitor sets a trouble code. Additionally, the PCV valve is attached to the engine with a bolted flange or a cam lock similar to those used for the gas tank sender. This keeps the PCV valve from being knocked loose. Also, the hoses and fittings from the PCV valve have a large diameter. Disconnecting the PCV system causes a massive vacuum leak, stalling the engine or keep- ing it from starting. Reconnecting the hose restores normal operation. Heated Oxygen Sensors (HO 2 S) The oxygen sensor must reach an operating tem- perature of approximately 600°F (315°C) before it operates properly. This usually does not occur until the engine is near operating temperature. On an extremely cold day, this temperature may not be reached for an extended period of time, if at all. In the early 1990s, manufacturers began to install oxygen sensors with built-in heating units. A heated oxygen sensor (HO 2 S) warms to operating temperature in a short period of time. This allows the computer system to enter closed loop operation sooner, optimizing vehicle operation. Many vehicles equipped with OBD I systems and all vehicles equipped with OBD II systems have more than one oxygen sensor. One oxygen sensor is used as a catalyst monitor, which is discussed later in this chapter. Thermostatic Air Cleaner Older vehicles with carburetors and throttle body fuel injection used thermostatic air cleaners to heat the incom- ing air. The heated air enters the air-induction system to help atomize the fuel and reduce condensation of the fuel. This is important on throttle body fuel injection systems. A thermostatic air cleaner also reduces icing in the throttle body. Icing occurs when the lower air pressure in the throttle body causes water vapor to freeze in the throat. This ice formation can cause the engine to run roughly or stall in cold, humid weather. Icing can also occur at the throttle plate under the right conditions. The problem is most likely to happen on a throttle body (TBI) fuel injection system, but can occur on a multiport system having a restricted area at the throttle valve. A typical thermostatic air cleaner is shown in Figure 10-11. A metal pipe or heat-resistant flexible hose connects the intake snorkel to a metal shroud that closely fits over the exhaust manifold, Figure 10-12. Air is heated as it passes between the shroud and exhaust manifold. A valve in the air cleaner housing closes off the cold air intake and opens a passage for the manifold-heated air, Figure 10-13. The valve is operated by a vacuum motor controlled by a thermostatic sensor in the air cleaner housing. The valve on some engines is controlled by a thermostatic element in the air intake passage. The heated air prevents ice formation at the throttle plate and improves gasoline atomization. Figure 10-11. A thermostatic air cleaner helps the engine quickly warm up and efficiently operate on a lean mixture. The heated air also reduces the chance of fuel condensation and throttle body icing. (Chrysler) Vacuum diaphragm Heated air Air-control valve Thermostat Cold air enters Exhaust manifold Heated air enters the throttle body Sheet metal cover Figure 10-12. Note the flexible hose from the metal shroud to the air cleaner housing. The hose carries air heated by the exhaust manifold to the air cleaner. Exhaust manifold Shroud Flexible hose