Something is just not right about your statement that they can not read any codes or data but the computer is working ok. If you used the paper clip and got the codes then some of the wireing must be ok.
scan tool that reads all data. Is it at a GM dealer then they should have a Tech 1 scan tool.
Have you bud pull up the ecm wireing specs and then post them here so we can help to find the problem
1989 2.8 does it have a cold start injector?? Cant remember when they stoped that. Now this was a while ago and it was in a w-body that failed e-test but the cold start injector was bad and it caused a rich mixture all the time.