The check engine light was one a few times duel to misfire (I guess since Autozone can’t read code for older cars). After I replaced the spark plugs, the engine light was still on a few times. After I pour some fuel injector cleaning liquid to gas tank, the light is no longer on anymore so far. This engine light problem was only in the past few months. However, I don’t feel it is related to the engine shake because this shake problem has been there for many years. Air filter was replaced and is clean. I didn’t check fuel filter recently but it was replaced in 2003 and I think should be fine as well. Well, the problem is I really don’t want to put more money in this old car so as long as it runs fine I guess this minor shake issue is tolerable to me. I was just puzzled/curious what causes the problems though.