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.