The Exhaust Gas Recirculation (EGR) system's purpose is to reduce NOx emissions that contribute to air pollution. The first EGR systems were added to engines in 1973, and today most engines have an EGR system. As long as the EGR system is functioning properly, it should have no noticeable effect on engine performance. But if the EGR system is leaking or inoperative, it can cause driveability problems, including detonation (knocking or pinging when accelerating or under load), a rough idle, stalling, hard starting, elevated NOx emissions and even elevated hydrocarbon (HC) emissions in the exhaust. WHY EGR? Exhaust gas recirculation reduces the formation of NOX by allowing a small amount of exhaust gas to "leak" into the intake manifold. The amount of gas leaked into the intake manifold is only about 6 to 10% of the total, but it's enough to dilute the air/fuel mixture just enough to have a "cooling effect" on combustion temperatures. This keeps combustion temperatures below 1500 degrees C (2800 degrees F) to reduce the reaction between nitrogen and oxygen that forms NOx. HOW EGR WORKS To recirculate exhaust back into the intake manifold, a small calibrated "leak" or passageway is created between the intake and exhaust manifolds. Intake vacuum in the intake manifold sucks exhaust back into the engine. But the amount of recirculation has to be closely controlled otherwise it can have the same effect on idle quality, engine performance and driveability as a huge vacuum leak. Most older EGR systems use a vacuum regulated EGR valve while newer vehicles tend to have an electronic EGR valve to control exhaust gas recirculation. When the engine is idling, the EGR valve is closed and there is no EGR flow into the manifold. The EGR valve remains closed until the engine is warm and is operating under load. As the load increases and combustion temperatures start to rise, the EGR valve opens and starts to leak exhaust back into the intake manifold. This has a quenching effect that lowers combustion temperatures and reduces the formation of NOx. In addition to EGR, other methods may also be used to minimize NOx. These include increasing camshaft valve overlap, redesigning the combustion chamber and modifying ignition advance curves. Three-way catalytic converters also reduce NOx in the exhaust. Some engines run so clean that they do not need an EGR system to meet NOx emission standards. If the EGR system is rendered inoperative because it was disconnected or tampered with, the cooling effect that was formerly provided by the EGR system will be lost. Without EGR, the engine will often knock and ping (detonate) when accelerating or lugging the engine. This can cause engine damage over time. TYPES OF EGR VALVES There are six different types of EGR valves: Ported EGR valves (1973 to 1980s). The typical ported vacuum EGR valve consists of a vacuum diaphragm connected to a poppet or tapered stem flow control valve. The EGR valve itself is usually mounted either on a spacer under the carburetor or on the intake manifold. A small pipe from the exhaust manifold or an internal crossover passage in the cylinder head and intake manifold routes exhaust to the valve. When vacuum is applied to the EGR valve, it opens. This allows intake vacuum to suck exhaust into the engine. To prevent the EGR valve from opening when the engine is cold, the vacuum line to the EGR valve may be connected to a parted vacuum switch or a computer-controlled solenoid. Vacuum is not allowed to pass to the valve until the engine is warm. EGR isn't needed when the engine is cold, only when it is warm and under load. Positive backpressure EGR valves (1973 & up). Backpressure EGR valves use exhaust backpressure to vary the point at which they open and their flow rates. On GM cars, they are identified by the last letter on the part number on top of the valve. A letter "P" indicates a positive backpressure valve, and a letter "N" indicates a negative backpressure valve. Inside a backpressure EGR valve is a second diaphragm that reacts to backpressure in the exhaust system. The backpressure diaphragm opens and closes a small bleed hole in the main EGR vacuum circuit or diaphragm chamber. Opening the bleed hole reduces vacuum to the main diaphragm and prevents the valve from opening fully. Closing the bleed hole allows full vacuum to reach the main diaphragm so the valve can open wide and allow maximum EGR flow. With positive backpressure EGR valves, any increase in exhaust backpressure causes the EGR valve to open. This reduces backpressure somewhat, allowing the backpressure diaphragm to bleed off some control vacuum. The EGR valve begins to close and exhaust pressure rises again. The EGR valve oscillates open and closed with changing exhaust pressure to maintain a sort of balanced flow. Negative backpressure EGR valves (1973 & up). The negative backpressure type of EGR valve reacts in the same way, except that it reacts to negative or decreasing pressure changes in the exhaust system to regulate EGR action. A drop in backpressure occurs when there is less load on the engine. This causes the backpressure diaphragm to open a bleed hole and reduce EGR flow. It's the same principle as with the positive type except that the control function occurs when backpressure goes down instead of up. NOTE: Most precomputer EGR systems have a temperature vacuum switch(TVS) or ported vacuum switch between the EGR valve and vacuum source to prevent EGR operation until the engine has had a chance to warm up. The engine must be relatively warm before it can handle EGR. If an engine runs rough or stumbles when cold, it may indicate a defective TVS that is allowing EGR too soon after starting. A TVS stuck in the closed position would block vacuum to the EGR and prevent any EGR operation. The symptom here would be excessive NOx emissions and possible pinging or detonation. Pulse-width modulated electronic EGR valves (early 1980s & up). First used in 1984 by General Motors, this type of EGR system uses a pulse width-modulated EGR control solenoid. With this technique, the powertrain control module (PCM) cycles the EGR vacuum control solenoid rapidly on and off. This creates a variable vacuum signal that can regulate EGR operation very closely. The amount of "on" time versus "off" time for the EGR solenoid ranges from 0 to 100 percent, and the average amount of "on" time versus "off" time at any given instant determines how much EGR flow occurs. Digital electronic EGR valves (late 1980s to 1990s). On some applications, a "digital" EGR valve is used. This type of valve also uses vacuum to open the valve but regulates EGR flow according to computer control. The digital EGR valve has three metering orifices that are opened and closed by solenoids. By opening various combinations of these three solenoids, different flow rates can be achieved to match EGR to the engine's requirements. The solenoids are normally closed, and open only when the computer completes the ground to each. Linear electronic EGR valves (early 1990s & up). Another type of electronic EGR valve is the "linear" EGR valve. This type uses a small computer-controlled stepper motor to open and close the EGR valve instead of vacuum. The advantage of this approach is that the EGR valve operates totally independent of engine vacuum. It is electrically operated and can be opened in various increments depending on what the engine control module determines the engine needs at any given moment in time. GM started using this type of valve on many of its engines in 1992. Linear EGR valves may also be equipped with an EGR valve position sensor (EVP) to keep the computer informed about what the EGR valve is doing. The EVP sensor also helps with self-diagnostics because the computer looks for an indication of movement from the sensor when the it commands the EGR valve to open or close. The sensor works like a throttle position sensor and changes resistance. The voltage signal typically varies from 0.3 (closed) to 5 volts (open). APPLICATIONS WITH NO EGR VALVE On many late model engines with Variable Valve Timing(VVT), there is no EGR valve because the VVT system varies the timing of the exhaust valves to provide the same effect as EGR. By changing the point at which the exhaust valves close when the engine is working hard under load, a small amount of exhaust gas can be retained in the cylinders for the next combustion cycle. This has the same effect on reducing combustion temperatures and NOx as recirculating exhaust gas from an exhaust port back into the intake manifold through an EGR valve. The big difference is that the VVT system can react to changing engine loads much more quickly and precisely than a traditional EGR valve. Using VVT for EGR also eliminates many of the problems associated with EGR valves such as carbon buildup and valve sticking or failure. COMMON EGR PROBLEMS Pinging (spark knock or detonation) because the EGR system is not working, the exhaust port is plugged up with carbon, or the EGR valve has been disabled. Rough idle or misfiring because the EGR valve is not closing and is leaking exhaust into the intake manifold. You may also find a P0300 random misfire code on OBD II vehicles. Hard starting because the EGR valve is not closing and is creating a vacuum leak into the intake manifold. EGR DIAGNOSTICS Find out what kind of EGR valve is on the vehicle so you can use the appropriate test procedure. Examine the valve or refer to a service manual. On some vehicles, you may find this information on the underhood emissions decal. Also, find out what kind of vacuum controls are used in the vacuum plumbing. Does it have a ported vacuum switch or a solenoid? Follow the vacuum connections from the valve, refer to a service manual or the underhood emissions decal for vacuum hose routing information. There are several ways to troubleshoot an EGR system. You can follow the EGR troubleshooting procedure that's listed in a service …

Fonte: AA1Car.com