1985 Chevy El Camino Bad Idle

My El Camino doesn't run properly at idle. It has the original 4.3L fuel injected 6 cyl engine. We hooked it up to a diagnostic machine, which told us it runs perfect at any range of RPMs other than idle. It said that something is causing the computer to make it run too rich. My mechanic told me to change the MAP sensor and coolant temperature sensor and to check all the vacumm lines for leaks. I found no leaks and installed the new sensors, but it didn't help. I got some advice about the charcoal cannister. I picked up a new one from a salvage yard, but it still didn't help. I don't know if it was a bad part to begin with or not. I was also told it could be that an electrical ground wasn't working. All of them I could find were working properly. I want to know if I missed something or if I am way off base.