Exhaust Gas Recirculation: Description and Operation

The Exhaust Gas Recirculation System is designed to reduce Oxides of Nitrogen (NOx) by lowering the peak combustion temperatures. During specified engine operating conditions, the ECU will recirculate some of the exhaust gases to back into the intake manifold. Since exhaust gases are inert (inactive) and do not contain the needed oxygen for the burning process, the air/fuel ratio is less by volume which lowers peak combustion temperatures and reduces NOx formation. If too much exhaust gas is recirculated or recirculation occurs at the wrong time, such as during idle or Wide Open Throttle, power and engine performance will be affected.

There are two methods used for controlling EGR operation. California vehicles incorporate a computer controlled EGR Solenoid Valve to control exhaust gas recirculation, while Non-California vehicles use a coolant temperature sensitive Thermo Valve. Both valves bleed vacuum when EGR operation is not desired.

California EGR System Vacuum Hose Routing:

CALIFORNIA
The EGR Solenoid is spring loaded open and vents ported vacuum to the air intake chamber, until the ECU internally grounds the circuit. Exhaust gas recirculation is not beneficial during engine warm-up, at idle or at Wide Open Throttle. After the engine has reached operating temperatures, the ECU will ground the solenoid circuit allowing vacuum to the EGR Valve. Until the throttle is past the idle position, this vacuum is not strong enough to open the valve against the EGR's opposing spring. When the throttle is fully opened, the lack of vacuum is overcome by the EGR's spring and exhaust gas recirculation is shut off.

Non-California EGR System Vacuum Hose Routing:

NON-CALIFORNIA
The Thermo Valve on Non-California vehicles, works on the same principles as the solenoid Valve. The Thermo Valve detects engine coolant temperature. Until operating temperatures are reached, the Thermo Valve is open and vents the ported vacuum signal back to the air intake. Once the valve closes, vacuum is then routed to the EGR Valve. Again, this ported vacuum signal is not strong enough to open the valve against spring pressure until the throttle is past the idle position.