EGR Valve Vacuum System

A leak in the exhaust gas recirculation (EGR) vacuum system can upset the PCM's programmed control of exhaust gas recirculation and adversely affect engine operation. Too much exhaust gas recirculation at idle or cruise can result in the engine stalling after closed throttle deceleration, surging during steady throttle cruising or a rough idle. Too little exhaust gas flow allows combustion temperatures to become too high and bring about detonation ("spark knock") or an overheated engine. In addition, oxides of nitrogen emissions may be high enough to cause the vehicle to exceed emission standards.

Because the source of an EGR system symptom might be traced to a mechanical or electrical condition as well as a faulty vacuum system, an effective systematic diagnosis procedure should examine all three categories of components - not just the vacuum system alone. Therefore, no exclusive vacuum system diagnosis procedures are provided here. Refer to the Light Duty Fuel and Emissions Manual.

In the diesel engine, the operation of the EGR valve is assisted by the exhaust pressure regulator (EPR) valve. At idle, the EPR valve increases exhaust back pressure to force more exhaust through the EGR valve and reduce combustion temperatures. The vacuum which operates the EGR valve and the EPR valve is supplied by an engine-driven vacuum pump.

The Powertrain control module (PCM) controls the EGR valve solenoid to regulate the vacuum to the EGR valve in the same way as with a gasoline engine. The Engine Control Module (ECM) calculates the amount of exhaust gas recirculation based on inputs from the engine speed sensor and the TP sensor, and pulses the EGR valve solenoid to control EGR. The MAP sensor monitors the control of exhaust gas recirculation as indicated by the air pressure in the EPR vacuum line. If actual vacuum differs from the preferred vacuum as calculated by the PCM, the PCM will adjust the ON-OFF intervals of the EGR valve to correct the flow of exhaust gas into the cylinders.

When the PCM recognizes the operating range in which no exhaust gas recirculation is needed, the EGR vent solenoid opens to vent the vacuum at the EGR valve.

At idle, the PCM energizes the EPR valve solenoid to send vacuum to close the EGR valve. This increases exhaust back pressure to force more exhaust gas through the EGR system and lower combustion temperatures to reduce oxides of nitrogen emissions.

The servo unit of the cruise control is a special type of vacuum actuator with an internal control valve that can apply varying amounts of vacuum to the diaphragm. The varying vacuum provides a calibrated range of throttle positions that can accurately control vehicle speed. The function of the servo is explained more fully under "CRUISE CONTROL VACUUM CONTROL" in this section.