New computer for '92 325i (kind of long sorry)

My car wasn't running right so I took it to the shop to get it diagnosed. The mechanic told me the computer was shot and I needed a new one, actually he said he would put a used one in. I said OK do it. I waited FOREVER. Still no computer, My dad said he could get one from a friend, I said OK get it, paid the mechanic for diagnosing the problem and got my new/used computer today. Here's my question, is there any way to tell if this is a good computer? It didn't make a bit of difference when I put it in my car. The car still runs like crap. Do you have to leave the new computer in for awhile and let the car adjust? or should you be able to tell a difference right away? I drove the car for about 1/2 mile with no improvement. I'm beginning to wonder if the mechanic was just guessing that it was the computer. Either way, I'm not happy, because my car still runs like crap. Any thoughts?