Exhaust Gas Recirculation: Description and Operation
On Federal and Canadian Turbo equipped vehicles Exhaust Gas Recirculation is regulated by 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 pressure while the Turbocharger is in operation.
On California Turbo equipped vehicles, the EGR Valve has three nipples and is computer controlled by 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, which is located downstream of the EGR Valve. California vehicles also require an Exhaust Gas Temperature Sensor within the EGR valve. The sensor is used 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. The same procedures would be followed if the temperature sensor were to malfunction.
CALIFORNIA
While the engine is warming up, the EGR solenoid is open (ungrounded by the MPI Control Unit). Port vacuum from the throttle body (Green 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 MPI Control Unit 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 past the idle position, additional vacuum will aid in opening the valve.
If the engine goes into a boost, while the EGR solenoid is closed, boost pressure from the throttle body will be applied to the lower 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 at all nipples of the EGR Valve, causing spring pressure to close the valve.
NON-CALIFORNIA
The Thermo Valve works on the same principles as the solenoid valve, but closes as the engine coolant temperature increases and is not computer controlled.
While the engine is still cold, the two ports of the Thermo Valve are connected, venting port vacuum back to the throttle body. 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.
Once the engine reaches operating temperature and the thermo valve is closed, port vacuum will be applied to the EGR Valve. Before the valve can be opened to allow exhaust gas to recirculate, additional vacuum will be required. The additional vacuum is supplied when the throttle is opened past the idle position. When the engine goes into a boost, while the Thermo Valve is closed, boost pressure from the throttle body is applied to the lower nipple 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 causing the spring inside to close the valve.