DESCRIPTION
To reduce NOx emissions, all vehicles are equipped with an exhaust gas recirculation (EGR) system. The EGR system recirculates exhaust gases from an exhaust port in the cylinder head to a port in the intake manifold located below the carburetor. EGR flow is controlled by the EGR valve and thermo valve.
EGR CONTROL VALVES
Increased EGR flow (which adversely affects driveability) is required to meet stringent emission standards. The EGR control valves compensate for the EGR system's effect on driveability. The amount of vacuum is controlled by throttle opening. The EGR system increases EGR flow during high load operation and decreases during low load operation. There is no EGR operation at idle or wide open throttle because vacuum level is low.
Sub EGR Control Valve
This mechanically controlled valve is linked to the throttle valve. It controls EGR flow by responding directly to throttle valve position.
THERMO VALVE
The Thermo valve senses engine coolant temperature which prevents EGR operation below preset values. When engine warms, thermo valve opens, supplying vacuum to the EGR valve. At temperatures below 131°F (55°C) thermo valve is open and vacuum is vented so no vacuum is supplied to the EGR valve. At temperatures above 131°F (55°C) the thermo valve closes and normal EGR operation takes place.
SUB-EGR VALVE
Operate sub EGR valve manually. Valve should move smoothly through full range of motion. If operation is difficult, remove and inspect for deposits. Before removing a sub EGR valve that is difficult to operate, spray with solvent from the outside with rubber boot removed, then remove valve. If necessary, clean valve with solvent. Apply a light coat of oil. If operation remains difficult after cleaning, replace sub EGR valve.
Typical EGR Control Valve Configurations. Scheme 6
SYSTEM TESTING
- Cold start engine and run at idle until normal operating temperature is attained. Remove Green-striped hose from injection mixer and connect vacuum pump to hose end. Disconnect solenoid valve connector and apply 3.9 in. Hg of vacuum. If idle becomes unstable or engine stalls, system is okay.
- Reconnect solenoid valve connector and apply 3.9 in. Hg of vacuum. If engine performance remains stable (no stall or rough idle) the negative chamber of EGR valve is open to atmosphere and EGR is okay. If unstable idle occurs, solenoid valve is staying closed and must be replaced.