DESCRIPTION
The exhaust gas recirculation (EGR) system, used on vehicles with gasoline engines, is designed to reduce emissions of oxides of nitrogen (NOx).
This process is accomplished by lowering combustion temperatures of burning gases. Recirculated and metered amounts of exhaust gases are reintroduced into the engine through the intake manifold where they are mixed with the air/fuel mixture.
On some models, the vacuum signal to the EGR valve is controlled by an ECM controlled solenoid valve. The amount of exhaust gas admitted into the engine is then controlled by a vacuum operated EGR valve under all operating conditions.
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 back-pressure modulated, and the pulse width modulated systems. The major difference between the EGR systems is the method used to control how far each valve opens.
VACUUM MODULATED (PORTED VACUUM) EGR SYSTEM
With this system, the amount of exhaust gas admitted into 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.
View of Vacuum Modulated EGR Valve. Scheme 52
EXHAUST BACK-PRESSURE MODULATED EGR SYSTEM
Two types of back-pressure EGR valves are used, either a positive or negative back-pressure valve. Operation of these 2 systems is as follows
Positive Back Pressure EGR Valve Operation
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 back-pressure 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.
Cutaway View of Positive Back-Pressure EGR Valve. Scheme 53
Negative Back-pressure EGR Valve Operation
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, back-pressure in the hollow shaft decreases. As back-pressure decreases, vacuum opens the control valve and bleeds EGR control vacuum to the atmosphere, thus closing the EGR valve. This cycle occurs about 40 times every second under normal operating conditions.
Cutaway View of Negative Back-Pressure EGR Valve. Scheme 54
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 up to 32 times per second. The ECM uses a ported vacuum signal to determine the flow rate signal to the solenoid. You'll note that in see scheme 4: Cutaway View of Pulse Width Modulated EGR Valve, it is the same as the Vacuum Modulated EGR Valve.
EGR VALVE CLEANING
| CAUTION | Do not clean valve in solvents or degreaser. Do not sand blasting 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.
POSITIVE BACK-PRESSURE EGR VALVE
- 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.
- Make sure 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 (.7 kg/cm 2 ). 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 BACK-PRESSURE 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 flow chart (if applicable) in appropriate article in the COMPUTER ENGINE CONTROL section.