See Figures 1, 2, 3, 4, 5, 6 and 7
The EGR system consists of a diaphragm actuated flow control valve, coolant temperature override switch, low temperature vacuum signal modulator, high temperature vacuum signal modulator.
All 1977 and later California units have a back pressure sensor which modulates EGR signal vacuum according to the rise or fall of exhaust pressure in the manifold. A restrictor plate is not used in these applications.
The purpose of the EGR system is to limit the formation of nitrogen oxides by diluting the fresh air intake charge with a metered amount of exhaust gas, thereby reducing the peak temperatures of the burning gases in the combustion chambers.EGR Valve
The EGR valve is mounted on a machined surface at the rear of the intake manifold on the V8s and on the side of the intake manifold on the sixes.
The valve is held in a normally closed position by a coil spring located above the diaphragm. A special fitting is provided at the carburetor to route ported (above the throttle plates) vacuum through hose connections to a fitting located above the diaphragm on the valve. A passage in the intake manifold directs exhaust gas from the exhaust crossover passage (V8) or from below the riser area (Sixes) to the EGR valve. When the diaphragm is actuated by vacuum, the valve opens and meters exhaust gas through another passage in the intake manifold to the floor of the intake manifold below the carburetor.Coolant Temperature Override Switch
This switch is located in the intake manifold at the coolant passage adjacent to the oil filler tube on the V8s or at the left side of the engine block (formerly the drain plug) on the Sixes. The outer port of the switch is open and not used. The inner port is connected by a host to the EGR fitting at the carburetor. The center port is connected to the EGR valve. When coolant temperature is below 115°F (46°C) (160°F [71°C] on the 8-304 with manual transmission), the center port of the switch is closed and no vacuum signal is applied to the EGR valve. Therefore, no exhaust gas will flow through the valve. When the coolant temperature reaches 115°F (46°C), both the center port and the inner port of the switch are open and a vacuum signal is applied to the EGR valve. This vacuum signal is, however, subject to regulation by the low and high temperature signal modulators.Low Temperature Vacuum Signal Modulator
This unit is located just to the right of the radiator behind the grill opening. The low temperature vacuum signal modulator vacuum hose is connected by a plastic T-fitting to the EGR vacuum signal hose. The modulator is open when ambient temperatures are below 60°F (16°C). This causes a weakened vacuum signal to the EGR valve and a resultant decrease in the amount of exhaust gas being recirculated.
This unit is located at the right front fender inner panel. The high temperature vacuum signal modulator is connected to the EGR vacuum signal hose by a plastic T-fitting. The modulator opens when the underhood air temperatures reach 115°F (46°C) and it causes a weakened vacuum signal to the EGR valve, thus reducing the amount of exhaust gases being recirculated.
See Figures 8, 9 and 10
With the engine idling and at normal operating temperature, manually depress the EGR valve diaphragm. This should cause engine speed to drop about 200 rpm. This indicates that the EGR valve had been properly cutting off the flow of exhaust gas at idle and is operating properly.
If the engine speed did not change and the idle is smooth, exhaust gases are not reaching the combustion chambers. The probable cause of this is a plugged passage between the EGR valve and the intake manifold.
If the engine idle is rough and rpm is not affected by depressing the EGR valve diaphragm, the EGR valve is not closing off the flow of exhaust at idle like it's supposed to and there is most likely a fault in the hoses, hose routing, or the EGR valve itself.
The EGR valve can be removed and cleaned with a wire brush and a9/16in. drill bit coated with grease (to hold dirt particles) inserted in discharge passage. The drill should be held with a pair of pliers only.
Before checking the operating of the EGR CTO switch, make sure that the engine coolant is below 100°F (38°C).
- Check the vacuum lines for leaks and proper routing.
- Disconnect the vacuum lines at the backpressure sensor, if so equipped, or at the EGR valve, and connect the line to a vacuum gauge.
- Operate the engine at 1,500 rpm. No vacuum should be indicated at the gauge. If vacuum is indicated, replace the EGR CTO switch.
- Allow the engine to idle until the coolant temperature exceeds 115°F (46°C).
- Accelerate the engine to 1,500 rpm. Vacuum should be present at the gauge. If not, replace the EGR CTO switch.
- Make sure that all the EGR vacuum lines are routed correctly and are not leaking.
- Install a tee in the vacuum line between the EGR valve and BPS, and attach a vacuum gauge to the tee.
- Start the engine and allow it to idle. No vacuum should be present. If vacuum is indicated at idle speed, make sure of correct line connections. Also, be sure that manifold vacuum is not the source. If the carburetor is providing the vacuum, look for a partially open throttle plate which could cause premature ported vacuum to the BPS unit.
Accelerate the engine to 2,000 rpm and observe the vacuum gauge for the following:
- If the coolant is below 115°F (46°C), no vacuum should be present.
- With coolant temperature above 115°F (46°C), ported vacuum should be indicated.
- If no vacuum is indicated at any time, make sure that vacuum is being applied to the inlet side of the BPS. If correct, remove the BPS and either clean it with a wire brush (if blocked) or replace it.