The EGR is supposed to be closed at idle, but if it is stuck partly open then idle quality will be poor: slow and rough (uneven) and you may have poor gas mileage.
The EGR controls the exhaust gas flow into the intake.
To test if the EGR is stuck:
1. On a cold engine start the car and let it idle.
2. If the EGR valve gets hot right away it is pulling in hot exhaust gas when it should be shut.
Too much EGR flow tends to weaken combustion, causing the engine to run roughly or stop. With too much EGR flow at idle, cruise, or cold operation, any of the following conditions may happen:
* Engine stops after cold start.
* Engine stops at idle after deceleration.
* Car surges during cruise.
* Rough idle.
If the EGR valve should stay open all of the time, the engine may not idle.
Too little or no EGR flow allows combustion temperatures to get too high during acceleration and load conditions. This could cause:
* Spark knock (detonation).
* Engine overheating.
* Emission test failure.