Known EGR Problems 91 B250 318

I have had people tell me that the EGR system is a known trouble spot on these vans. Has anybody got any info on it? Like what are the symptoms. Mine will poof some black smoke when you hit the throttle and let off. It has had the normal tune-up stuff done to it and I replaced the O2 sensor, map sensor, tps sensor. It has had this rich running problem for many years. Its getting 13 mpg. It has the 318 TBI engine.

Blaze