The EGR system is used to lower NOx (oxides of of nitrogen) emissions levels caused by high combustion temperatures. It does this by decreasing combustion temperature.
The main element of the system is an EGR valve operated by vacuum, and mounted on the intake manifold.
The EGR valve feeds small amounts of exhaust gas back into the combustion chamber.
The EGR valve is opened by ported manifold vacuum to let exhaust gas flow into the intake manifold. The exhaust gas then moves with the air/fuel mixture into the combustion chamber. If too much exhaust gas enters, combustion will not occur. For this reason, very little exhaust gas is allowed to pass throw the valve, especially at idle. The EGR valve is usually open under the following conditions:
The amount of exhaust gas recirculated is controlled by variations in vacuum and in some cases, exhaust back pressure.
Two types of EGR valves are used on G body engines:
This valve is controlled by a flexible diaphram which is spring loaded to hold the valve closed. Ported vacuum applied to the top side of the diaphragm overcomes spring pressure and opens the valve in the exhaust gas port. This allows exhaust gas to be pulled into the intake manifold and enter the engine cylinders.
An air bleed, located inside the EGR valve assembly, acts as a vacuum regulator. This bleed valve controls the amount of vacuum in the vacuum chamber by bleeding vacuum to atmosphere during the open phase of the cycle. When the EGR valve receives sufficient exhaust backpressure through the hollow shaft, it closes the bleed. At this point, maximum available vacuum is applied to the diaphragm and the EGR valve opens.
This type of valve will not open if vacuum is applied to it with the engine stopped or idling.
To regulate EGR flow, an ECM controlled solenoid is used in the vacuum line. The ECM uses information from various combinations of the following sensors to regulate the solenoid:
During cold engine operation, a "pulse width modulated" signal from the ECM energizes the EGR solenoid blocking vacuum to the EGR valve. "Pulse Width Modulation" means the ECM turns the solenoid on and off many times a second and varies the amount of "on" time ("pulse width") to vary the amount of EGR. The solenoid is also energized during cranking and at wide open throttle. When the engine warms up, the EGR solenoid is turned off and the EGR works on normal port vacuum and exhaust backpressure signals.
A diagnostic switch is part of the control and monitors vacuum to the EGR valve. This switch will trigger a "Service Engine Soon" light, and set a Code 53 in the event of a vacuum circuit failure.
The EGR vacuum control is through a normally open solenoid vacuum valve which is closed when the ECM completes the solenoid ground. The ECM will energize the EGR solenoid (EGR off) when the engine is cold and also when other specified parameters are met.
Too much EGR tends to weaken combustion, causing the engine to run roughly or stop. With too much EGR at idle, cruise, or cold operation, any of the following conditions may happen:
If the EGR valve should stay open all of the time, the engine may not idle.
Too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This could case:
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