Originally Posted by loepke72

EGR stuck open will cause obvious idle quality problems, and car may not even run at all at closed throttle if bad enough. Doesn't take much vacuum to open a normally operating EGR valve.

EGR stuck closed (or EGR tube/passages blocked) usually doesn't cause any drivability symptoms at all, possibly some spark knock under load, but WILL cause an emission test failure for high NOx. The emission test failure is about the only indication for most of us that the EGR is staying closed (at least on pre-OBDII cars) since it doesn't noticeably affect engine performance when closed.