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Passing Automotive Emission Testing

By Jerry Lemke

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Do you live in an area where you car has to meet inspection requirements? Normally your vehicle will also have to pass emissions requirements. This article will help explain what emissions are, and how to fix a possible problem your vehicle may have.

Emissions are simply the exhaust or leftovers of combustion coming out of your vehicle's engine. An emissions test is normally done with a probe placed into the exhaust tailpipe. The emission sampler will "sniff" or sample the contents of the exhaust coming out of the tailpipe. Every road going vehicle produced, has certain clean requirements that it is required to meet. This requirement is specified by EPA to limit the amount of pollutants exiting the vehicle.

Most emission samplers are five gas analyzers. That means they "see" or measure five types of gases. The five gases that will be measured are HC, NOX, O2, CO, and CO2. We will start with HC or hydrocarbons. HC's are simply another term for unburned fuel that makes it through the engine and out the exhaust. HC's cause smog and and are not good to breath. NOX means oxides of Nitrogen. NOX is a by product of highly heated and compressed air that has nitrogen in it. NOX is another bad emission to breath at high levels. O2 is leftover unburned oxygen in the exhaust. Although O2 is obviously not bad, it is tested for to look for people trying to cheat the test.

The percentage of oxygen in the exhaust will also tell the fuel ratio of the engine as it runs. CO and CO2 is carbon monoxide and carbon dioxide. CO is odorless and will give you headaches and eventually kill you by robbing O2 from your body, if in high quantities. CO2 is present in the air but large amounts contribute to global warming. Before doing any emission test the check engine soon light, should never light up or it will cause the car to fail. HC's are usually the worst problem for cars that will have to pass the test.

Many things can produce high HC's such as too much or even too little fuel, not all cylinders firing, advanced timing, bad catalytic converter, or an air pump that isn't working. These are the most common causes. Tune up's can prevent most emission problems. NOX is generally worse on higher compression engines. All engines produce NOX but the use of EGR valves will cool and slow down the burn rate of the engine's combustion. This considerably lowers NOX values.

O2 levels are controlled by the fuel ratio being correct from the fuel injection. If there is an exhaust leak all the test numbers will be low and incorrect except the high O2 numbers which will void the test. CO has to do with the efficiency of the burn in the engine and also is highly effected by the fuel to air ratio of the engine. CO2 is also an indicator of the engines set up. The HC's and NOX are by far the largest problem areas. Catalytic converters scrub or clean the majority of the emissions and need to be replaced when they break internally causing a loss in power and no longer clean the air.

Jerry Lemke is the webmaster at A site focused on engine information for repair, troubleshooting, performance, upgrades, and new technologies for all types of vehicles.
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