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Emission Control Systems

There are 3 main groups of compounds that the federal govenment has placed a limit on. These are unburned hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NO). Emission systems are designed to control the output of these fumes by the engine and vehicle. This FAQ will cover a bunch of the emission systems that may be found on your vehicle.

Positive Crankcase Ventilation
As the engine runs, a small amount of the fuel air mixture leaks by the piston rings and enters the crank case. These gases would escape to the atmosphere with unburned hydrocarbons unless otherwise acted upon. If these gases become trapped in the crankcase for any period of time they will also cause sludge to build up. These gases are controlled by drawing filtered air into the crankcase, flowing through the crankcase, picking up the gases, and then being drawn out of the crankcase through an oil separator, PCV valve, and into the induction system. This allows them to once again enter the combustion chamber to be burnt. The PCV valve controls the amount of gases which are recycled into the combustions chamber. At low idle, the valve is partially closed. As idle speed increases the valve opens up to admit more gases into the induction system. If the PCV valve becomes clogged, it will cause these gases to stay in the crankcase and become pressurized. These gases will find a way out via a bad seal. This will also create an oil leak. Also this will tent to cause the engine oil to sludge up.

A quick test to see if the PCV valve is working properly is to clamp the hose while the engine is running. The RPM should decrease by more than 50 rpm. If the rpm does not drop then replace the valve and check the hoses for excessive build up. It may be necessary to replace the hoses if there is excessive build up.

Evaporative Emission Control System

This system is designed to control the evaporated gases from the gasoline in the tank. The tank is connected to a carbon canister which absorbs and stores the vapors while the engine is not running. When the engine is started and the manifold vacuum reaches a certian level, a signal is sent to a purge valve which then opens. This allows air to be drawn into the canister which will pick up the vapors and then be pulled into the combustion chamber and burnt.

Exhaust Gas Recirculation (EGR) System

This system uses a metering valve, a vacuum source, and cast in exhause gas passages in the intake manifold. The system is disabled until the engine is hot and at idle and full throttle. During normal acceleration and crusing the valve opens and allows exhaust gases into the air/fuel mixture. The exhaust gases lower the combustion temperature and reduce the amount of NOx produced.

Thermostatic Air Cleaner

This system uses a temperature sensor to control a door on the air cleaner. When the engine is cold, the door closes and takes air which is pulled through a stove located on the exhaust manifold. The stove helps to heat the air. This aids the engine in warming up. once the engine has warmed up, the temperature sensor cuts off vacuum to the door and the door opens allowing the air to be pulled from the front of the vehicle.

Air Management System

This system utilizes an air pump to inject filtered air into the Catalytic Converter. This also have a diverting valve to redirect the air from the cat to the atmosphere under certian conditions. One condition is when the engine is under deceleration due to the fact that this will create a temporary air fuel mixtures that are too rich to burn completly. If these were combined with injected air they would burn, creating a backfire. The pump injects air into the second bed of the cat. This bed need the addition of oxygen to reduce the levels of HC and CO. The first bed controls the levels of NOx which requires the removal of oxygen, therefore injected air would disable this process.

Catalytic Converter

This is a muffler like component in the exhaust system. They consist of pellets or a honeycomb monolithic substrate coated with a metal such as platinum, palladium, rhodium, or a combination of these. When the exhaust gases flow through these metals, a chemical reaction occurs which will reduce the pollutants into harmless substances like water and carbon dioxide.

source: Chiltons general motors buick/oldsmobile/pontiac full-size 1975-90 repair manual, 1994
 
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