See Figures 1 and 2
Oxides of nitrogen (NOx) are formed in the engine under conditions of high temperature and pressure. Elimination of one of these two conditions reduces the formation of NOx. Exhaust gas recirculation is used to reduce combustion temperatures in the engine.
Except Turbo-Equipped Engines
All 1973-74 models have an EGR. Only the 1975-76 models sold in California have an EGR. All 1977-79 models and all 1980 models (except California) use an EGR system. All 1981 and later models have an EGR system.
An EGR valve is mounted on the intake manifold. The exhaust gas is drawn from the exhaust manifold, through the EGR valve and into the intake manifold. The EGR valve is closed when the engine is idling; exhaust gas recirculation would cause a rough idle. As the throttle is opened, vacuum is applied to the EGR valve vacuum diaphragm. When the vacuum reaches about 1.96 psi (2 in. Hg) the diaphragm moves against spring pressure and is fully open at 3.92 psi (8 in. Hg) of vacuum. As the diaphragm moves up, it pulls the EGR valve pintle from its seat, allowing exhaust gas to be pulled into the intake manifold by vacuum. The valve closes at full throttle as a means of improving fuel economy.
The 1973-74 models have an electrically operated solenoid valve mounted on the EGR valve. The vacuum signal to the EGR valve must travel through the solenoid valve. The solenoid valve prevents the EGR valve from opening when the engine is cold. Temperature signals to the solenoid valve are sent by a passenger compartment mounted temperature sensor in 1973 models. As long as the temperature inside the vehicle is below freezing, the solenoid valve blocks vacuum to the EGR valve, preventing exhaust recirculation. On 1974-76 models, a temperature switch installed in the engine coolant outlet housing; as long as engine coolant temperatures remain below approximately 106°F (41°C) for 1974 or below approximately 122°F (50°C), 1975-76, the solenoid valve blocks the vacuum to the EGR valve. When the temperature of the engine coolant or in the passenger compartment, reaches normal operating temperature, the solenoid is deactivated, allowing the intake manifold vacuum is allowed to act upon the EGR valve diaphragm and exhaust gas recirculation takes place.
On 1977 and later models, a Thermal Vacuum Valve (TVV) controls the application of vacuum to the EGR valve. When the engine coolant reaches a predetermined temperature, the TVV opens and allows vacuum to be routed to the EGR valve. Below the predetermined temperature, the TVV closes and blocks vacuum to the EGR valve.
The 1977-79 models and all 1980, 49 State U.S. models have a Back Pressure Transducer (BPT) valve installed between the EGR valve and the thermal vacuum valve. The BPT valve has a diaphragm raised or lowered by exhaust back pressure. The diaphragm opens or closes an air bleed, which is connected into the EGR vacuum line. High pressure results in higher levels of EGR, because the BPT diaphragm is raised, closing off the air bleed, which allows more vacuum to reach and open the EGR valve. Thus, the amount of recirculated exhaust gas varies with exhaust pressure.
The 1977-78 (California) models, the 1979 (catalytic converter) models and the 1980 (49 State) models have a Vacuum Delay Valve (VDV) installed in the line between the thermal vacuum valve and the EGR valve. The valve delays rapid drops in vacuum in the EGR signal line, thus effecting a longer EGR time.Turbo-Equipped Engines
The EGR system is controlled by the central electronic control unit adjusting to the engine operating conditions.
The cylinder head temperature, the engine rpm, the engine load, the air temperature and the barometric pressure are used for determining the amount of the EGR vacuum.
These signals are transmitted to the control unit where optimum EGR quantities are recorded. To obtain the optimum EGR quantity that corresponds to the engine operating conditions at the time, an electric signal is sent to the Vacuum Control Modulator (VCM). The vacuum control modulator transforms the electric signal to a vacuum signal, which in turn controls the EGR valve.
A one-way valve is utilized for the purpose of preventing the VCM from applying positive pressure in high speed conditions.
This valve is installed in the vacuum line leading to the VCM.
See Figure 3
- Visually inspect the entire EGR control system. Clean the mechanism of any oil or dirt. Replace any rubber hoses found to be cracked or broken. On the 1973 models, check the vacuum tube which runs from the solenoid to the carburetor. If deformed, replace it. Tighten the tube to 3.0 ft. lbs. (4.0 Nm).
- Check the solenoid electrical connections for corrosion or breaks in the insulation. and correct as necessary.
- Start the engine and allow it to reach normal operating temperature. On 1973 models, the temperature in the passenger compartment must be over 60°F (15°C). Increase the engine speed to 3,000-3,5000 rpm. The plate of the EGR control diaphragm and the valve shaft should move upward. This can be more easily seen with a mirror placed under the EGR valve.
- Disconnect the EGR solenoid electrical leads and connect them directly to the vehicle battery with a pair of jumper cables. Rev the engine again with the solenoid connected to the battery. The EGR valve diaphragm should remain stationary.
- With the engine running at idle, reach up under the EGR valve and raise the diaphragm by pushing it upwards with your fingers. Wear a heavy glove to protect your hand from engine heat. When the diaphragm is raised, the idle should become rough, indicating that exhaust gases are recirculating. If the roughness does not occur, the EGR passages are blocked.
Inspect the individual components as follows:
- Remove the EGR valve from the intake manifold.
- Apply 2.94 psi (6 in. Hg) of vacuum to the EGR valve vacuum connection. The valve should open. Pinch off the connection with the vacuum still applied. The valve should remain in the raised position for at least 30 seconds.
- Inspect the EGR valve for any signs of warpage or damage and replace as necessary.
- Clean the EGR valve seat with a brush and compressed air.
- Connect the solenoid to a 12 volt DC power source. The solenoid should click when power is applied. If the valve clicks, it is considered to be working properly.
- Check the 1974-76 temperature switch by removing it from the engine (drain the engine coolant first) and placing it in a container of water together with a thermometer. Connect a self-powered test light to the temperature switch electrical leads. Heat the water. The switch should conduct current when the water temperature is below 77°F (25°C) on 1974 models or below 122°F (50°C) on 1975-76 models. The switch should stop conducting between 88-106°F (31-41°C) on 1974 models or between 134-145°F (57-63°C) on 1975-76 models. Replace the switch if it behaves otherwise.
- Remove the EGR valve. Apply enough vacuum to the EGR valve vacuum connection to raise the diaphragm and open the valve. Pinch off the vacuum connection. The valve should remain open for at least 30 seconds. If not, the diaphragm is leaking and the valve must be replaced.
- Check the valve for damage, warpage, cracks or etc. and replace as necessary.
- Clean the valve seat with a wire brush and compressed air.
- Install the EGR valve on the engine. Start the engine and allow it to idle. With the engine idling, reach up under the EGR valve and raise the diaphragm by pushing it upwards with your fingers. Wear a glove to protect your hand if the engine is hot. When the diaphragm is raised, the engine idle should become rough, indicating that exhaust gases are recirculating. If the roughness does not occur, the EGR passages are blocked.
- To check the operation of the thermal vacuum valve, drain the engine coolant and remove the valve. Connect two lengths of vacuum hose to the two TVV vacuum connections.