DESCRIPTION
The Exhaust Gas Recirculation (EGR) system, used on General Motors vehicles with gasoline engines, is designed to reduce oxides of nitrogen (NOx) emissions.
This process is accomplished by lowering combustion temperatures of burning gases. A metered amount of exhaust gas is recirculated into the intake manifold and mixed with the air/fuel mixture.
On some models, the Electronic Control Module (ECM) controls EGR valve operation by controlling vacuum to EGR valve. An ECM controlled vacuum solenoid valve is located in series between vacuum source and EGR valve. The ECM uses information from input sensors to determine the correct amount of EGR.
A Thermal Vacuum Valve (TVV), Thermal Vacuum Switch (TVS) or an electrically operated solenoid controls operating vacuum, depending on engine operating temperature, to maintain good cold driveability.
There are 3 types of EGR systems used, the vacuum modulated (ported vacuum), the exhaust backpressure modulated, and the pulse width modulated systems.
Cutaway View of Positive Backpressure EGR Valve. Scheme 33
VACUUM MODULATED (PORTED VACUUM) EGR SYSTEM
In this system the amount of exhaust gas entering the intake manifold depends on a vacuum signal (ported vacuum), controlled by throttle position.
When the throttle is closed (at idle or deceleration), there is no vacuum signal to the EGR valve because the EGR vacuum port is above the closed throttle valve. As the throttle valve is opened, a ported vacuum signal is supplied to the EGR valve, admitting exhaust gas into the intake manifold.
Cutaway View of Vac Modulated & Pulse Width Mod EGR Valve. Scheme 34
EXHAUST BACKPRESSURE MODULATED EGR SYSTEM
Two types of backpressure EGR valves are used, either a positive or negative backpressure valve. These valves may be identified by the letter in the last position of the part number. A "P" designates a positive backpressure valve and "N" a negative backpressure valve.
Some 4.3L V6, 5.0L and 5.7L V8 models have backpressure EGR valves with a temperature sensor built into valve base.
On these models, ECM monitors EGR valve base temperature. If EGR valve does not open properly, base temperature will be cool. Temperature sensor will then signal ECM to turn on check engine light.
Positive Backpressure EGR Valve
A control valve, located in the EGR valve, acts as a vacuum regulator valve. The control valve controls the amount of vacuum to the EGR diaphragm chamber by bleeding vacuum to the atmosphere during certain operating conditions.
When the control valve receives a backpressure signal, through the hollow shaft of the EGR valve, pressure on the bottom of the control valve closes the control valve. When the control valve closes, the maximum vacuum signal is applied directly to the EGR valve allowing exhaust gas recirculation.
Negative Backpressure EGR Valve
If there is little or no vacuum in the vacuum chamber of the EGR valve, the EGR valve will not open. When there is enough vacuum in the chamber, from the manifold vacuum port, the pintle will rise off of its seat and allow the EGR valve to open.
When the EGR valve opens, backpressure in the hollow shaft decreases. As backpressure decreases, vacuum opens the control valve and bleeds EGR control vacuum to the atmosphere, thus closing the EGR valve.
Cutaway View of Negative Backpressure EGR Valve. Scheme 35
PULSE WIDTH MODULATED EGR SYSTEM
This type of EGR system is controlled entirely by the ECM. The ECM controls the flow rate through a solenoid. The solenoid is pulsed at a rate of up to 32 times per second. The ECM uses a ported vacuum signal to determine the flow rate signal to the solenoid.
EGR VALVE CLEANING
| CAUTION | Do not clean valve in solvents or degreaser, or by sand blasting. |
One-Piece Valve
- Remove EGR valve and discard gasket. Lightly tap sides and end of valve. Shake valve to remove any loose deposits. Buff exhaust deposits from mounting surface with wire wheel. Visually inspect valve seating area to ensure that it is clean.
- Inspect for exhaust deposits in valve outlet. Carefully remove any deposits with a screwdriver. Using a new gasket, reinstall EGR valve.
Replacing EGR Solenoid Filter (3.0L & 3.8L V6) Replace filter every 30,000 miles. Scheme 36
3.0L & 3.8L V6 Only
Replace EGR solenoid filter every 30,000 miles. When installing filter, ensure that solenoid wires are aligned in cut-out section of filter. (Scheme 36)
Vacuum Solenoid Controlled EGR System (Typical). Scheme 37
- Disconnect electrical connector from EGR solenoid. Place transmission in Neutral (man. trans.) or "P" (auto. trans.). Set parking brake and block drive wheels. Connect tachometer to engine.
- Ensure that fast idle speed is set to specified RPM. With engine at normal operating temperature, place fast idle screw on highest step of fast idle cam.
- Disconnect and plug vacuum hose at EGR valve. As vacuum hose is removed, watch for downward movement of diaphragm. This should be accompanied by an increase in engine speed.
- Reconnect vacuum hose. Diaphragm should move upward and engine speed should decrease.
- If engine speed change and diaphragm movement are noticed with vacuum hose removed or installed, EGR valve is operating properly. Reconnect electrical connector to EGR solenoid.
- If engine speed and diaphragm movement did not occur, remove EGR valve from engine. Connect a vacuum pump to EGR valve and apply a constant vacuum of 10 in. Hg. EGR valve should not open. If EGR valve does open, replace it.
- With vacuum still applied to EGR valve, direct a 15 psi air pressure stream directly into valve seat. EGR valve should open fully. If not, EGR valve should be cleaned.
VACUUM MODULATED & NEGATIVE BACKPRESSURE EGR VALVE
- Turn engine off. Disconnect vacuum hose from EGR valve. Place finger underneath valve and push up to depress valve diaphragm. With diaphragm depressed, plug vacuum port on EGR valve.
- Diaphragm should take over 20 seconds to return to its seated position. If diaphragm takes less than 20 seconds to return to its seat, replace EGR valve.
- Again depress diaphragm and plug vacuum port. Immediately start engine and watch for diaphragm movement. Diaphragm is operating properly if diaphragm moved to seated position during cranking and initial starting.
- If diaphragm did not move during cranking or initial starting, EGR valve should be cleaned.
PULSE WIDTH MODULATED EGR VALVE
For testing of Pulse Width Modulated EGR system and valve, see appropriate diagnostic chart in COMPUTER COMMAND CONTROL article.
EGR CONTROL CIRCUIT
For testing of EGR control circuit, see appropriate diagnostic chart in COMPUTER COMMAND CONTROL article in the COMPUTERIZED ENGINE CONTROL section.
