See Figure 1
On carbureted engines, the system is regulated by a pair of vacuum operated control valves using a compound ported vacuum strategy. Ported vacuum upstream of the throttle opens valve A, which allows manifold vacuum to open valve B. As B opens, some of the ported vacuum is bled off to the carburetor venturi, causing A to begin closing and the EGR valve to open. Eventually a balance is reached that is dependent upon a manifold vacuum and ported vacuum. This ties EGR valve opening to throttle valve opening and, therefore, to engine load. When the engine is cold or the vehicle is not moving, the purge cut-off solenoid valve for the evaporative emission control system turns off the vacuum to valve B, preventing the venturi vacuum from reaching the EGR valve. The system is far easier to test than to understand. Except for the cut-off solenoid valve, the system is entirely mechanical. Any malfunctions are due to vacuum hose leakage/misrouting or EGR control valve failure.
See Figure 2
On fuel injected engines, the system is modulated by controlling the amount of engine vacuum to the valve diaphragm. The ECU modulates the position of the control solenoid valve in order to regulate the EGR valve lift according to an internal program. Upstream of the control solenoid valve, the Constant Volume Control (CVC) valve provides a constant supply of vacuum. This allows precise EGR control under all manifold vacuum conditions. An air chamber inline between the CVC and control solenoid acts as an expansion chamber to dampen any vacuum pulses. The control solenoid valve, CVC and air chamber are in the control box on the firewall.
- With the engine cold, connect a vacuum gauge to the EGR valve vacuum hose and run the engine at about 3000 rpm. There should be no vacuum. If there is, test the evaporative emission control system.
- Allow the engine to reach normal operating temperature (radiator fan will run).
- Remove the control box lid on the firewall.
Remove the top hose from the purge cut-off solenoid valve and cap the valve. Check the vacuum to the EGR valve hose under the following conditions:
At idle-no vacuum.
- To test the EGR valve, plug the vacuum hose and connect a hand vacuum pump to the valve. Draw a vacuum of about 6 in. Hg (150 mm Hg) with the engine at idle. The engine should stall (or run very rough) and the vacuum should remain steady, indicating the diaphragm is good. If the engine did not stall or run rough, either the valve is not opening or the passageway is blocked.
- First check that all vacuum lines and electrical connections are in good condition.
- Disconnect the vacuum supply hose to the EGR valve and connect a hand vacuum pump to the valve.
- Start the engine and allow it to reach normal operating temperature.
- With the engine at idle, draw a vacuum on the EGR valve. The engine should stall (or run very rough) and the valve should hold vacuum. If not, replace the EGR valve.
- Connect a vacuum gauge to the vacuum hose from the control solenoid valve; there should be no vacuum at idle. If there is vacuum to the EGR valve at idle, check the wiring for the control solenoid. One wire should have 12 volts anytime the engine is running. The ground wire goes to the ECU, which modulates the control solenoid opening by controlling the ground circuit. Turn the ignition switch OFF and use a digital ohmmeter to see if the wire between the ECU and control solenoid is shorted to ground. If the wiring is OK, the ECU is getting an incorrect input signal or the ECU is faulty.
- The vacuum going to the control solenoid valve should be about 8 in. Hg (200 mm Hg) at idle. Connect a vacuum gauge to the hose coming from the air chamber. If the vacuum is not correct, read the vacuum at the CVC valve outlet. Full manifold vacuum should be available at the CVC inlet. If the air chamber or CVC valve is are not functioning properly, the units must be replaced.
REMOVAL & INSTALLATION
See Figures 3, 4, 5 and 6
EGR components are generally simple to work on and easy to access. The air cleaner assembly may need to be removed on some models. Always label each vacuum hose before removing it-they must be reinstalled in the correct position.
EGR valves are held in place by two nuts. The nuts can be difficult to remove due to corrosion. Once the EGR valve is off the engine, clean the studs and the nuts of any rust or debris. Always replace the gasket any time the valve is removed. Tighten the nuts to 16 ft. lbs. (22 Nm).
Most of the other valves and solenoids are made of plastic. Be very careful during removal not to break or crack the ports; you have NO chance of gluing a broken fitting. Remember that the plastic has been in a hostile environment (consisting of heat and vibration); which makes fittings brittle and less resistant to abuse or accidental impact.