The exhaust gas recirculation (EGR) valve is an automobile part used to reduce exhaust emissions. A bad EGR valve causes an increase in emissions as well as a number of problems for an engine.
The EGR valve allows a small amount of exhaust gas to enter the air intake when needed to reduce the combustion temperature in the engine. Without this cooling effect, the engine runs hot which increases exhaust emissions. These emissions increase pollution and result in the vehicle failing emissions testing.
With a bad EGR valve, the combustion temperatures rises which can cause detonation. Detonation, often referred to as spark knock, is caused by fuel burning in the cylinders due to heat and not ignition spark. Detonation causes the fuel to be burnt prematurely which makes the engine inefficient, resulting in decreased gas mileage.
A bad EGR valve creates vacuum problems for the engine. The engine uses vacuum to adjust air intake and fuel injection. Vacuum problems may make an engine difficult to start and causes the engine to not idle properly. The engine may also stall, hesitate or show other performance problems while driving.