Chapter 34 Advanced Diagnostics 681 Copyright by Goodheart-Willcox Co., Inc. temperature. Never test a vehicle with a cold engine, as inac- curate readings will result. Although most states provide emission specifi cations in parts per million (PPM) and per- centage, some states give emission specifi cations in grams per mile (GPM). When this is the case, the vehicle must be operated on a chassis dynamometer so that the amount of emissions produced per mile driven can be calculated. Hydrocarbon Readings An exhaust gas analyzer measures hydrocarbons (HC) in parts per million (PPM) by volume. For example, an analyzer reading of 10 PPM means there are 10 parts of hydrocarbons for every million parts of exhaust gas. A vehicle that is 15 to 20 years old, for example, will have a relatively high hydrocarbon specifi cation, such as 900 PPM. A newer vehicle, having stricter emission requirements, could have a 220 PPM hydrocarbon specifi cation. If a vehicle’s hydrocarbon reading is higher than the standard permits, the vehicle’s hydro- carbon emissions (unburned fuel) are excessive. Always refer to the emission control sticker in the engine compartment or a service manual for emission level specifi cations. Values vary year by year. A rotten egg smell from the exhaust is oft en an indica- tion of the presence of unburned fuel and, as a result, exces- sive hydrocarbons. Higher-than-normal HC readings can be caused by one or more of the following conditions: • Fuel system malfunction (leaking fuel injector, faulty pressure regulator, improper fuel pressure, or carbure- tor problems). • Improper ignition timing (distributor, computer, or adjustment problem). • Engine problems (blowby, worn rings, blown head gasket). • Faulty emission control system (bad PCV, catalytic con- verter, EGR valve, evaporative control system). • Ignition system troubles (fouled spark plug, cracked distributor cap, open spark plug wire). • Computer control system problems (defective input sensor, output actuator, or ECM). Carbon Monoxide Readings An exhaust gas analyzer measures carbon monoxide (CO) in percentage by volume. For instance, a 1.2% analyzer reading would mean that 1.2% of the engine exhaust is made up of carbon monoxide. Th e other 98.8% consists of other substances. High carbon monoxide is caused by incomplete fuel burning or a lack of air (oxygen) during the combustion process. If the exhaust analyzer reading is higher than specifi ca- tions, the engine is producing too much carbon monoxide. You would need to locate and correct the cause of the problem. Th e exhaust analyzer’s carbon monoxide reading is related to the air-fuel ratio. A low carbon monoxide reading indicates a lean air-fuel mixture (too much air compared to fuel). A high carbon monoxide reading indicates an Figure 34-39. Since exact procedures vary, always follow the operating instructions for the particular exhaust analyzer. Th is will ensure accurate measurements. Warning: When using an exhaust gas analyzer, do not let engine exhaust fumes escape into an enclosed shop area. Engine exhaust can kill. Use the shop exhaust ventilation system to trap and remove the toxic fumes. Most analyzers measure hydrocarbons and carbon monoxide at idle and approximately 2500 rpm. If you have a fi ve-gas analyzer, it will also measure oxygen, oxides of nitrogen , and carbon dioxide. Compare the analyzer read- ings with specifi cations. When testing some electronic fuel injection systems without a load, only idle readings on the exhaust analyzer will be accurate. A dynamometer must be used to load the engine to simulate actual driving conditions. Exhaust Gas Analyzer Readings Before testing the vehicle, take it on a thorough road test. Th is allows the vehicle to reach the proper operating Figure 34-39. Note how the exhaust analyzer probe is installed in a vehicle’s tailpipe. The adapter on the vent hose prevents toxic vapors from entering the shop. Warm-up the engine and calibrate the analyzer before placing the test probe in the tailpipe. Compare the readings to specifications for the vehicle’s model and year. Exhaust Gas Analyzer Probe Shop Vent Hose