Exhaust Gas Recirculation: Description and Operation

Exhaust Gas Recirculation is regulated by the EGR valve which is computer controlled via a solenoid valve. On non-turbo equipped vehicles the EGR valve houses only one vacuum chamber. On Turbo equipped vehicles the EGR Valve has an additional vacuum nipple and chamber. The additional nipple and chamber on the EGR Valve is used to allow the valve to be opened by boost pressure while the turbocharger is in operation.
Both valves house an Exhaust Gas Temperature Sensor for detecting exhaust gas recirculation flow as an input to the MPI Control Unit. If the EGR valve should become stuck in the closed position, the sensor would not be able to reach its specified temperature, causing the MPI Control Unit to set a trouble code and illuminate the Malfunction Indicator Light on the instrument panel. The same procedures would be followed if the temperature sensor were to malfunction.

TURBO

While the engine coolant is below operating temperature, the computer will not apply voltage to the solenoid. Port vacuum from the green striped hose is routed through the solenoid and bled back to the air-intake chamber. Once the MPI Control Unit detects that the engine has reached operating temperature, voltage will be supplied to the solenoid. The solenoid will close and vacuum to the EGR valve will increase, causing the valve to open.
If the solenoid is open and the Turbocharger goes into operation, boost pressure will be applied equally to both vacuum chambers of the EGR valve and the valve will not open. If the solenoid is closed, boost pressure will be applied to the lower chamber of the EGR valve, helping the vacuum being applied to the upper chamber in opening the EGR valve.

NON-TURBO

The EGR Solenoid is spring loaded open to vent port vacuum back to the air intake chamber, until the MPI Control Unit grounds the circuit. After the engine has reached operating temperatures, the MPI Control Unit internally grounds the solenoid causing vacuum to the EGR Valve to increase. When the throttle is past the idle position additional vacuum will be supplied to open the EGR Valve, allowing exhaust gases to recirculate. When the throttle is fully opened (WOT.), the drop of supply vacuum within the intake manifold causes the EGR Valve to close. The MPI Control Unit will cycle the EGR Solenoid ON time based on engine rpm and engine load.