DESCRIPTION
Exhaust Gas Recirculation (EGR) is used to reduce oxides of nitrogen (NOx) emissions. This is accomplished by lowering peak combustion chamber temperatures. To do this, recirculated and metered amounts of exhaust gases are reintroduced into engine through intake manifold, where they are mixed with the incoming air/fuel mixture.
Two types of EGR systems are used, depending upon vehicle application. These are vacuum modulated and backpressure. Some engines may also use what is called "pulse width modulation". The ECM turns an EGR vacuum control solenoid on and off many times per second, varying amount of "on" time to vary amount of EGR. For more information on ECM controlled EGR systems, see the appropriate article in the COMPUTERIZED ENGINE CONTROLS section.
Ported EGR Valve
With this system, amount of exhaust gas admitted into intake manifold depends on vacuum signal (ported vacuum), controlled by throttle position.
When throttle is closed (at idle or deceleration), there is no vacuum signal to EGR valve because EGR vacuum port is above the closed throttle valve. As the throttle valve is opened, ported vacuum is applied to top of the diaphragm which overcomes spring pressure and opens the valve in the exhaust gas port. Exhaust gas then is admitted into the intake manifold.
Negative Backpressure EGR Valve
Negative backpressure EGR valve assembly has an internal vacuum bleed feature to regulate EGR based upon engine load. Bleed valve spring is moved from above the diaphragm to below, and valve is normally closed. Flow of valve is controlled by manifold vacuum, negative exhaust backpressure and carburetor ported vacuum signal.
When carburetor ported vacuum signal is applied to main vacuum chamber, partially opening valve, vacuum signal from manifold side (reduced by exhaust backpressure) is transmitted up the hollow stem of valve. This enables the signal to act on diaphragm, opening bleed and causing transducer to modulate, providing a specific flow. Thus, flow past the valve is a constant percentage of engine airflow.
EGR THERMAL VACUUM SWITCH
EGR-TVS closes to prevent EGR operation when engine coolant temperature is less than 85°F (29°C). This improved cold engine driveability. When coolant temperature increases to greater than 85°F (29°C), TVS opens to allow vacuum to be directed to EGR valve.
Scheme 184
Scheme 185
- 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 greater than 20 seconds to return to its seated position. If diaphragm takes less than 20 seconds to return to its seat, replace EGR valve.
- Depress diaphragm and plug vacuum port. Immediately start engine and observe diaphragm movement. Diaphragm is operating properly if diaphragm moves to seated position during cranking and initial starting.
- If diaphragm does not move during cranking or initial starting, EGR valve should be cleaned.
EGR-TVS TEST (HOT)
- Remove EGR valve vacuum hose at EGR valve and connect hose to vacuum gauge. Start engine. With transmission in Park of Neutral, open throttle partially. As throttle is opened, vacuum gauge should respond with increase in vacuum reading. If operation is satisfactory, remove gauge and reconnect hose to EGR valve. If gauge does not respond to throttle opening, proceed to step 2).
- Remove carburetor-to-TVS hose from switch and connect hose to vacuum gauge. Start engine. With transmission in Park of Neutral, open throttle partially. If vacuum gauge responds to throttle opening, switch is defective. Remove switch and replace with new part. If gauge does not respond to throttle opening, check for cracked or plugged hose.