682 Auto Electricity and Electronics Copyright by Goodheart-Willcox Co., Inc. over-rich mixture (too much fuel compared to air). Typical causes of high carbon monoxide readings are: • Fuel system problems (sticking or leaking injector, leak- ing fuel pressure regulator, high fl oat setting, clogged carburetor air bleed, restricted air cleaner, defective input sensor, computer control problem). • Emission control system troubles (almost any emission control system problem can upset the carbon monoxide readings). • Incorrect ignition timing (timing too far advanced or improper vacuum going to the vacuum advance unit). If a high carbon monoxide reading is accompanied by a high hydrocarbon reading, the problem is related to some- thing that will make the engine run rich, such as a computer or sensor problem, injector problems, and restricted air intake. Oxides of Nitrogen Readings A fi ve-gas analyzer can measure oxides of nitrogen (NO x ), while a two-gas and a four-gas analyzer cannot. Since oxides of nitrogen are toxic, some state air quality agencies have made exhaust emission measurements with a fi ve-gas analyzer man- datory. Typical causes of high NO x emissions include: • High combustion chamber temperatures (caused by excessively high engine compression ratio, carbon deposits in the combustion chambers, low cooling sys- tem, blocked water jackets, etc.). • EGR system problems (caused by burned gases not being injected into the intake manifold and an exces- sively high combustion fl ame temperature). Carbon Dioxide Readings Four- and fi ve-gas exhaust analyzers measure carbon dioxide (CO 2 ) in percent by volume. Typically, CO 2 readings should be above 8%. CO 2 readings provide more data for checking and adjusting the air-fuel ratio. Carbon dioxide is a by-product of combustion. It is produced when one carbon molecule combines with two oxygen molecules in the combustion chamber. Carbon dioxide is not toxic at low levels. When you breathe, for example, you exhale carbon dioxide. Normally, oxygen and carbon dioxide levels are com- pared when evaluating the content of the engine exhaust. For example, if the percent of carbon dioxide exceeds the percent of oxygen, the air-fuel ratio is on the rich side of a stoichiometric (chemically correct) mixture. It is also a good indication that the exhaust gas sample has become diluted due to an exhaust leak. Oxygen Readings Four- and fi ve-gas exhaust analyzers measure oxygen (O 2 ) in percentage by volume. Typically, oxygen readings should be between 1% and 7%. Oxygen is needed for the catalytic converter to burn HC and CO emissions. Without oxygen in the engine exhaust, exhaust emissions can pass through the catalytic con- verter and out the vehicle’s tailpipe. Th ere are two systems that add oxygen to the engine exhaust: the air injection system and the pulse air system. As oxygen is added to the exhaust, CO and HC emissions decrease. As a result, oxygen readings can be used to check the operation of the fuel injection system, air injection sys- tem, catalytic converter, and computer. Th e oxygen level in the engine exhaust sample is an accu- rate indicator of a vehicle’s air-fuel mixture. It is also a good indicator of a possible exhaust leak, which can dilute the exhaust gas sample. When an engine is running lean, oxygen increases proportionately with the air-fuel ratio. As the air-fuel mixture becomes lean enough to cause a lean misfi re (engine miss), oxygen readings rise dramatically. Th is provides a very accurate method of measuring lean and effi cient air-fuel ratios. If you fi nd any exhaust gas analyzer reading to be abnormally high or low, use your knowledge of system oper- ation to pinpoint the trouble. By knowing which emissions are aff ected by which engine problem or emission system trouble, you can narrow down the source of the problem to specifi c components. You would then need to test each component or circuit to verify your conclusions. Note: An engine with a defective thermostat can fail an emission test. If the engine operating temperature is too high, it can affect engine combustion efficiency and the operation of the computer control systems. The computer control system will try to compensate for the overheating engine. If the engine cannot reach the proper operating temperature, the computer will not be able to go into closed loop mode. Keep these basic system malfunctions in mind when diagnosing problems. Summary • Strategy-based diagnosis involves using a consistent, logical procedure to narrow down possible problem sources. Basically, you verify the complaint, make pre- liminary checks, read service bulletins, perform service- manual recommended checks, repair the problem, and recheck system operation. • Electromagnetic interference (EMI), or radiation inter- ference, occurs when an induced voltage enters another system’s wiring. Sources of EMI can include loose, misrouted, or unshielded spark plug wires police and CB radios and aft ermarket accessories. • A digital thermometer is an electronic device that measures temperature. Measuring actual temperature can help you verify scan tool readouts and fi nd hard-to- locate problems.