An exhaust gas recirculation (EGR) vacuum solenoid is an electrically controlled vacuum switch. It is a component of the exhaust gas recirculation system found on motor vehicles that are equipped with exhaust emission controls. This system sends a portion of the engine exhaust gases from the vehicle's exhaust manifold back to its intake manifold, which helps reduce the combustion cylinder temperature and allows the ignition timing to be advanced. Advancing the point of ignition increases the length of the combustion cycle, which helps reduce the amount of nitrous oxides and unburned hydrocarbons that the engine produces without producing pre-ignition or engine knock.
The exhaust gas recirculation system is designed to operate at normal engine operating temperatures and at speeds of 2,000 revolutions per minute or more. To ensure that the EGR valve opens only under certain conditions, an EGR vacuum solenoid is used to open and close a vacuum line to the EGR valve. Some EGR systems use a modulating vacuum solenoid to adjust the position of the EGR valve during different operating conditions, and others use a simple on-or-off vacuum switch.
EGR vacuum solenoids are employed in different ways by different vehicle manufacturers. Some systems use a series of sensors to directly control the solenoid. Newer vehicles employ an engine control module (ECM) to monitor engine sensors and control the various performance and emission control systems.
A modulating EGR vacuum solenoid operates in a manner similar to that of a household lamp dimmer. The ECM monitors the changing engine conditions and adjusts the size of the EGR valve opening by sending voltage pulses of varying duration to the solenoid. The longer the pulse duration, the farther the valve opens. Some manufacturers have eliminated the component completely, choosing to let the ECM control the EGR valve directly with a built-in electric solenoid.