EGR Valve and Passages

The EGR system includes the EGR valve, the passages between the exhaust and the intake, and a control source such as an electronic signal or a vacuum supply. These are controlled by the PCM according to input from various sensors. It's function is to reduce the amount NOx or nitrous of oxides created in the combustion chamber. When nitrogen and oxygen are combined with high cylinder temperatures harmful NOx gases are formed. The EGR valve reduces the amount of NOx by recirculating a regulated amount of inert gas (exhaust) into the intake. Reducing cylinder temperatures reduces the amount of nitrous of oxides produced in the chamber.

There are different types of EGR systems used on different engines. The vacuum operated EGR valve used in the illustration below is a typical design. The valve is opened by vacuum supplied through a duty cycle solenoid and closed by a spring. The duty cycle solenoid controls the valve to open in proportion to throttle opening. It is a controlled opening between the intake and the exhaust. By mixing this inert gas into the air fuel charge the cylinder temperature is reduced. This prevents the engines cylinders from reaching damaging temperatures and protects the environment from NOx.

Vehicle manufacturers have their engines programmed for a certain amount of flow. If the flow is changed either allowing too much or too little flow, driveability and performance will be effected. Before replacing the valve make sure the engines injection and ignition timing is correct. Also check the vacuum supply and solenoid on vacuum operated valves and check the power supply and ground on electronically controlled valves. The solenoid's coil is checked for resistance with an ohmmeter.

This question reminds us that the passages in these systems can become clogged and a restriction forms. This restriction blocks the flow of exhaust gases into the intake. This results in insufficient EGR flow, detonation, and high NOx. If the EGR valve is good, cleaning out the EGR passages will return the system to proper EGR flow. Cleaning an EGR valve and passages is typically done with a wire brush and a throttle body or carburetor cleaner. Always check manufacturers specifications for any details. The symptoms of a stuck open EGR valve are stalling, surging, and a rough idle. A stuck closed or restricted EGR valve results in detonation and ping.