DESCRIPTION
Exhaust Gas Recirculation (EGR) is used to reduce oxides of nitrogen (NOx) emissions. This process is accomplished by lowering combustion temperatures of burning gases. Recirculated and metered amounts of exhaust gases are reintroduced into engine through intake manifold, where they are mixed with air/fuel mixture.
Vacuum modulated system regulates exhaust gas recirculation according to manifold vacuum. Backpressure modulated system regulates timed vacuum according to exhaust backpressure level. Special control valve within the EGR valve housing responds as a pressure regulator.
Some engines use what is called "Pulse Width Modulation". The ECM turns the solenoid on and off many times per second and varies the amount of "on" time to vary the amount of EGR. For more information, see appropriate article in the COMPUTERIZED ENGINE CONTROL 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 the 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 same function as positive backpressure EGR valve, except that the 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 signal to act on diaphragm, opening bleed and causing transducer to modulate providing a specific valve flow. Thus flow of valve is a constant percentage of engine airflow.
Scheme 66
Scheme 67
EGR THERMAL VACUUM SWITCH
EGR-TVS closes to prevent EGR operations when engine coolant temperature is below 85°F (29°C). This improves cold engine driveability. When coolant temperature rises above 85°F (29°C), TVS opens to allow vacuum to be directed to EGR valve.
Vacuum Modulated (Ported) & Negative Backpressure Types
- 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.
- 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 or 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 or 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.
EGR BLEED SOLENOID CHECK (5.0L/5.7L FEDERAL W/ A/T)
- Disconnect EGR bleed solenoid connector.
- Connect a test light from solenoid terminal "A" to ground.
- With the ignition "ON" and engine stopped, the test light should light. If not, there is an open in circuit to ignition switch.
- Connect solenoid connector.
- Disconnect vacuum hose from solenoid to the EGR valve and connect a vacuum gage to solenoid.
- Start engine. There should be no vacuum reading. Replace solenoid if there is a reading.
- Connect a test light from solenoid terminal "B" to ground. Solenoid should energize and there should be a full vacuum reading. Replace solenoid as required.
- Connect a test light from terminal "D" at the TCC solenoid to ground. Solenoid should energize and there should be a full vacuum reading. If not, open is in circuit 386.
