1990 1500 check engine light

I have a 90 K1500 that is stalling at idle and running rough. The ck eng light was coming on. I took it to the GMC dealer and they replaced the vacuum motor that goes to the EGR valve. The engine ran much better, but the light was still on. After some looking around on my on, I found that they had plugged my vacuum line at the EGR valve. When I plug it back in, the engine acts the same as it did; trying to stall and running rough. Also, since the repairs at the dealership, I have had to burn 93 octane gas. Without the higher octane, the engine knocks like crazy. By the way, this truck has 342,000 miles on it; every major driveline piece original. The engine and tranny have never had a wrench turned on them. I am going to try for at least half a million miles. If anyone can help me with the the problem on my truck, please let me know. Thanks. Steve

Take it back and thell them to fix it right on there dime or ask for your money back.
Was the code cleared?
What is the code it is now setting.
Ground pin A and B on your diagnoastic connector and post back code or check engine light flashes.
MT