Exhaust Gas Recirculation: Description and Operation

Exhaust Gas Recirculation:

SYSTEM DESCRIPTION

The EGR system is controlled by ported vacuum, and an ECM controlled solenoid. The EGR system routes exhaust gases from the exhaust manifold to the intake manifold at specified engine operating conditions. The EGR valve is opened to allow exhaust gases to flow into the intake manifold. The exhaust gases combine with the air/fuel mixture in the combustion chamber. Since there is less air/fuel mixture to burn in the combustion chamber, during EGR operation, the temperature is reduced to limit NOx formation. If too much exhaust gas enters, combustion will not occur. Because of this, very little exhaust gas is allowed to pass through the valve, none at idle. The valve should only open when the engine is at operating temperature and above base idle speed.

Negative Back Pressure EGR Valve:

Positive Back Pressure EGR Valve:

Ported Vacuum EGR:

EGR CONTROL

The EGR is controlled by a vacuum solenoid that uses "Pulse Width Modulation". The valve allows the ECM to turn ON and OFF the solenoid many times a second. The ECM varies the amount of ON time (Pulse Width) to change the amount of exhaust gas flow. The Coolant Temperature Sensor (CTS), Throttle Position Sensor (TPS), and Park/Neutral Switch transmit signals to the ECM in order to regulate the EGR solenoid. During cold and idle operations, the solenoid circuit is not grounded by the ECM, which does not allow vacuum to flow to the EGR valve. The amount of exhaust gas flow depends on manifold vacuum and variations in exhaust back pressure.

The Positive Back Pressure EGR valve has a bleed valve that controls the amount of vacuum in the vacuum chamber. The bleed valve allows the vacuum to vent to atmosphere during open phase. When the bleed valve receives sufficient exhaust back pressure through the hollow shaft, the bleed valve closes. The maximum amount of vacuum is available once the bleed valve closes, which allows the EGR valve to open. If there is no exhaust back pressure present, or if the throttle is wide open or at base idle, then the EGR valve is closed. The amount of exhaust gas flow is determined by manifold vacuum and throttle valve opening.

The Negative Back Pressure EGR valve has an internal vacuum bleed hole which is forced closed by a small spring when no exhaust back pressure is present. Vacuum opens the EGR valve against the pressure of a large spring. When manifold vacuum combines with exhaust back pressure, the vacuum bleed hole opens and the EGR valve closes.
NOTE: The negative back pressure EGR valve will open if vacuum is applied to the valve, with the engine not running. The positive back pressure EGR valve will not open when vacuum is applied to the valve, with the engine not running.

EGR Valve Identification:

EGR VALVE IDENTIFICATION

1. Negative backpressure EGR valves will be stamped with a "N" on the top side of the valve after the part number.
2. Positive backpressure EGR valves will be stamped with a "P" on the top side of the valve after the part number.