rough idle/poor gas milage

I have a 94 jimmy with a 4.3 w engine CPI,I had most of the problems you describe.My check engine ligth was on,Took it to a shop to have it scan but the tech said that there migth be a short from the ligth to the ecm and he would have to find out what was wrong with the system.so I left and deciced to change the ecm($80.00) cost.was told I could check codes just by using a jump wire on the inboard connector.When i did it flash a code 12 (meaning it was working)then it flash a code that told me the IAT sensor was bad.(its located in the Air intake tube on the bottom ).I changed the part cost 12.00.after that the car ran great no more hard starting ,no more like the engine was missing.If you haven check this it can be check.a very simple job.hope this helps you,Good luck