Chapter 15 Fuel and Emission Control Systems 323 Copyright by Goodheart-Willcox Co., Inc. by incomplete burning and a lack of air (oxygen) during combustion. If the analyzer reading is higher than specifica- tions, locate and correct the cause of the problem. The CO reading is related to the air-fuel ratio. A low CO reading indicates a lean air-fuel mixture. A high CO reading indicates a rich mixture. Typical causes of high CO readings are: • Fuel system problems. Bad injector, leaking fuel pres- sure regulator, restricted air cleaner, bad engine sensor, or computer control problems. • Emission control system troubles. Almost any emis- sion control system problem can upset CO readings. • Incorrect ignition timing. Timing too advanced due to computer control system problem. CO 2 Readings An exhaust analyzer measures carbon dioxide (CO 2 ) in percent by volume. Typically, CO 2 readings should be above 8%. Carbon dioxide is a by-product of combustion. CO 2 is not toxic at low levels. Normally, oxygen and carbon monoxide levels are compared when evaluating the content of the engine exhaust. For example, if the percentage of CO 2 exceeds the percentage of oxygen, the air-fuel ratio is on the rich side of the stoichiometric (theoretically perfect) mixture. CO 2 is also a good indicator of possible dilution of the exhaust gas sample due to an exhaust leak. O 2 Readings An exhaust analyzer measures oxygen (O 2 ) in percent- age by volume. Typically, O 2 readings should be between 0.1% and 7%. Oxygen is needed for the catalytic converter to burn HC and CO emissions. Without O 2 in the engine exhaust, exhaust emissions can pass through the converter and out of the vehicle’s tailpipe. The O 2 level in the exhaust sample is an accurate indicator of a vehicle’s air-fuel mixture. When an engine is running lean, oxygen increases proportionately with the decrease in fuel. As the air-fuel mixture becomes lean enough to cause a lean misfire (engine miss), oxygen read- ings rise dramatically. The O 2 level is also a good indicator of a possible exhaust leak, which can dilute the exhaust sample. NOX Readings Oxides of nitrogen (NO X ) are measured on a five-gas analyzer. High NO X emissions can result from: • High combustion chamber temperatures. Excessively high engine compression ratio, carbon deposits in the combustion chambers, low coolant level, blocked cool- ing passages, stuck thermostat, etc. • EGR system problems. Burned gases are not being injected into the intake manifold and combustion flame temperature is too high. Summary Proper engine operation depends on an adequate sup- ply of clean air and fuel. Without the correct mixture of fuel and air, an engine will perform poorly. A fuel system can be divided into three subsystems: fuel supply system, air supply system, and fuel metering system. The fuel supply system provides filtered fuel under pressure to the fuel metering system. The air supply system removes dust and dirt from the air entering engine intake manifold. The fuel metering system controls amount of fuel that is mixed with the filtered air. The two types of electronic fuel injection (fuel meter- ing) are multiport and throttle body injection. In multiport injection, one injector is located just before each intake port. Fuel is injected into the engine in more than one location. This is the most common type. An engine sensor is an electronic device that changes circuit resistance or voltage with a change in a condition. The computer can use the increased current flow through the sensor to calculate any needed change in injector open- ing. There are several typical sensors used in an electronic fuel injection system. The OBD II diagnostic system can produce over 500 trouble codes related to engine performance. It checks the operating parameters of switches, sensors, actuators, in-system components and their related wiring, and the computer itself. A diagnostic trouble code is a digital signal produced and stored by the computer when an operating parameter is exceeded. An OBD II code is a four digit number with a letter designator. The letter designator indicates the system in which the problem is located. The first number indicates whether the code is SAE or manufacturer specific. The second number indicates the function of the system where the fault is located. The last two numbers indicate the specific fault. Emission control systems are used to reduce the amount of harmful chemicals released into the atmosphere from a vehicle. The four basic types of vehicle emissions are hydro- carbons, carbon monoxide, oxides of nitrogen, and particu- lates. Vehicle emissions result from engine crankcase blowby fumes, fuel vapors, and engine exhaust gases. Several systems are used to reduce the pollution produced by the engine and its fuel system. The positive crankcase ventilation (PCV) system keeps engine crankcase fumes out of the atmosphere. The heated air inlet system speeds engine warmup and keeps the temperature of the air entering the engine constant. The evaporative emissions control (EVAP) system prevents vapors in the fuel system from entering the atmosphere. The exhaust gas recirculation (EGR) system injects burned exhaust gases into the engine intake manifold to lower the combustion temperature and reduce NO X pollution. An air injection system uses an air pump to force fresh air at low pressure into the exhaust ports of the engine. A catalytic converter is a thermal reactor for burning and chemically changing exhaust byproducts into harmless substances.