All emissions if you're serious about clean air -- or at least worried you won't pass an emissions test. Most emission testing programs to date only check for only two pollutants: carbon monoxide (CO) and hydrocarbons (HC). In areas that have the new "enhanced" I/M 240 emissions testing program, they also check for oxides of nitrogen (NOX) and the operation of your "evaporative emissions" control system (the system that captures and holds vapors from your fuel tank).
Of the three main pollutants, carbon monoxide is the deadliest because you can't see it or smell it. A concentration of only half a percent (0.5%) CO in the air can render a person unconscious -- and kill within 10 to 15 minutes! Even concentrations as small as four hundredths of a percent (0.04%) can cause headaches and be life threatening after several hours exposure.
WARNING: Never run an engine inside an enclosed garage, not even for a few minutes. The fumes can build up quickly and overcome you before you realize what's happening. Carbon monoxide is invisible and odorless so you can't really tell when it's around.
Carbon monoxide is formed when the fuel mixture is rich and there is insufficient oxygen to completely burn all the fuel. The richer the fuel mixture, the greater the quantity of CO produced. So high CO emissions indicate incomplete combustion typically caused by carburetor maladjustment, a clogged air filter, sticking choke, defective heated air intake system, plugged PCV valve, faulty oxygen sensor, excessive fuel pressure or a fuel injection metering problem.
Carbon monoxide production is highest when the engine is first started because the fuel mixture is richer than normal during this time and the catalytic converter has not yet reached operating temperature.
Carbon monoxide emissions are minimized by maintaining a balanced to slightly lean fuel mixture. This requires careful adjustment of the carburetor idle mixture screws (which may have "limiter caps" to limit the amount of adjustment or are covered with plugs to prevent tampering). On some fuel injected engines, there is also an adjustment for the idle mixture (but it is usually factory sealed to prevent tampering). The fuel mixture is further balanced by the oxygen sensor and computer system. Most of the carbon monoxide that is produced by the engine is converted into carbon dioxide (CO2) by the catalytic converter.
Hydrocarbon emissions are unburned gasoline and oil vapors. Though not directly harmful, they are a major contributor to smog and ozone pollution (which are toxic). Hydrocarbons in the atmosphere react with sunlight and break down to form other chemical compounds that irritate the eyes, nasal passages, throat and lungs.
HC emissions, which are usually measured in parts per million (PPM), can go up as a result of ignition misfiring (a fouled plug or bad plug wire), "lean" misfiring (incorrect carburetor idle adjustment or vacuum leaks that creates a lean mixture that misfires), loss of compression (such as a burned or leaky exhaust valve), or engine wear that causes the engine to burn oil (worn valve guides, rings and/or cylinders).
Hydrocarbon emissions are controlled by maintaining the fuel mixture so it is neither too lean nor too rich to ignite, by keeping the combustion chamber tightly sealed (good rings and valves), and by maintaining the ignition system (changing the plugs periodically). HC that is produced in the engine is reburned in the catalytic converter and changed into water vapor and carbon dioxide.
Nitrogen makes up about 78% of the air we breathe. Though normally inert and not directly involved in the combustion process, combustion temperatures above 2500 degrees F cause nitrogen and oxygen to combine and form various compounds called "oxides of nitrogen," which is abbreviated NOX. This mostly occurs when the engine is under load and the throttle is open wide.
NOX is a nasty pollutant both directly and indirectly. In concentrations as small as a few parts per million, it can cause eye, nose and lung irritations, headaches and irritability. Higher concentrations can cause bronchitis and aggravate other lung disorders. Once in the atmosphere, it reacts with oxygen to form ozone (which is also toxic to breathe) and smog.
To reduce the formation of NOX, Exhaust Gas Recirculation (EGR) is used. By recirculating a small amount of exhaust gas back into the intake manifold to dilute the air/fuel mixture, EGR has a "cooling" effect on combustion, thus keeping temperatures below the NOX formation threshold.
On 1981 and later engines with computerized engine controls, a special "three-way" catalytic converter is used to reduce NOX in the exhaust. The first chamber of the converter contains a special "reduction" catalyst that breaks NOX down into oxygen and nitrogen. The second chamber contains the "oxidation" catalyst that reburns CO and HC.
High NOX emissions are almost always due to a defective EGR valve (or some component that controls the operation of the EGR valve). A related symptom that usually occurs when EGR is lost is spark knock (detonation) during acceleration.
The fuel vapors that evaporate from your fuel tank can be another source of smog and ozone pollution. So fuel systems for the past twenty years have been sealed to prevent the loss of vapors.
Some venting of the tank must be provided so it can "breathe" during temperature changes and when the engine is running, so this is provided by hoses connected to a charcoal filled canister usually located in the engine compartment. The charcoal particles in the canister soak up and store fuel vapors when the engine is not running. Then, when the engine is started, a "purge valve" opens to siphon the vapors into the engine where they are burned.
If the canister or any of its hose connections leak (or the gas cap does not seal tightly), fuel vapors can escape into the atmosphere around the clock. The amount of pollution can really add up, especially during hot weather, so it's important to make sure the system is functioning properly. The new OBD II test program includes a pressure check of the fuel tank system as well as a flow test of the purge valve.