Exhaust Gas Recirculation: Description and Operation

The Exhaust Gas Recirculation System is designed to reduce Oxides of Nitrogen (NOx) by lowering the 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 combustion temperatures and reduces NOx formation. If too much exhaust gas is recirculated or recirculation occurs at the wrong time, such as during engine warm-up, at idle or at Wide Open Throttle, power and engine performance will be affected.

The Non-California Turbo equipped models make use of a Dual Nipple EGR Valve which is controlled by a coolant temperature sensitive Thermo Valve. The second nipple on the EGR Valve is used to allow the valve to be opened by boost during the operation of the Turbocharger.

California Turbo equipped models use a Triple Nipple EGR Valve which is computer controlled through a solenoid Valve. Similar in operation to the Dual Nipple EGR Valve, the third nipple on California systems is used for bleeding vacuum from the EGR Valve via the solenoid. California Models also incorporate a Exhaust Gas Temperature sensor within the EGR Valve. The temperature sensor is used for detecting exhaust gas recirculation flow as an input to the ECU. If the temperature sensor should malfunction, the ECU will set a trouble code but the EGR system may not be affected.

California EGR System Vacuum Hose Routing:

CALIFORNIA
While the engine is warming up, the EGR solenoid is open (ungrounded by the ECU). Port vacuum from the throttle body (Blue Striped Hose) passes through both upper nipples of the EGR Valve, through the solenoid and is bled back to the throttle body via the yellow striped hose.
If the engine goes into a boost condition while the EGR Solenoid is still open, boost pressure will be applied equally to all three nipples at the EGR Valve and the Valve will remain closed.
After the coolant reaches operating temperature, the ECU will internally ground the EGR solenoid. Port vacuum is then trapped within the EGR Valve's vacuum chamber, but is not strong enough to overcome the EGR spring. Once the throttle is opened past the idle position, the additional vacuum will aid in opening the valve.
When the engine goes into a boost, while the EGR solenoid is closed, boost pressure from the throttle body travels through the yellow striped hose and a vacuum tee to the lower nipple (boost pressure chamber) of the EGR Valve. The valve is then opened by boost pressure as well as vacuum.
If the engine is operated at Wide Open Throttle, boost pressures will be near equal at all throttle body nipples. This will put equal boost pressures above and below the EGR Valve and will cause the spring inside to close the valve.
The ECU will cycle the EGR Solenoid ON time based on engine load and engine RPM.

EGR Solenoid Parameters
Coolant Temperature 20°F to 82°F = 0% Duty Cycle
Coolant Temperature at 82°F = .2% Duty Cycle
Coolant Temperature at 170°F = 10% Duty Cycle
Coolant Temperature 194° = 55 to 65% Duty Cycle

Non-California EGR System Vacuum Hose Routing:

NON-CALIFORNIA
The Thermo Valve works on the same principles as the solenoid Valve, but closes as the engine coolant temperature increases.
Until the coolant reaches operating temperatures, the two ports of the Thermo Valve are connected. Port vacuum from the throttle body (Blue Striped Hose) passes through the Thermo Valve ports and is bled back to the throttle body (yellow striped hose). If the engine goes into a boost condition while the Thermo Valve is still open, boost pressure will be equally applied to both nipples of the EGR Valve and the Valve will remain closed.
When the engine has reached operating temperature, the thermo Valve will close. Port vacuum will then be routed through a vacuum tee to the upper nipple of the EGR Valve. This ported vacuum is not strong enough to open the valve against the EGR's internal spring, until the throttle is opened past the idle position and additional vacuum is applied.
When the engine goes into a boost, while the Thermo Valve is closed, boost pressure from the throttle body travels through the yellow striped hose and a vacuum tee to the lower nipple (boost pressure chamber) of the EGR Valve. The valve is then opened by boost pressure as well as vacuum.
If the engine is operated at Wide Open Throttle, boost pressures will be near equal at all throttle body nipples. This will put equal boost pressures above and below the EGR Valve and will cause the spring inside to close the valve.