code45

I have a question for all you mech-a-niks out there. My car is a 90 Z24, 3.1L engine. I changed the spark plugs and wires just before the summer began. Ok, my problem, I noticed that I wasn't getting the same gas mileage that I was use to getting and the engine sort of vibrates when you start to give it gas. Anyway I checked the computer codes and found that I have a code 45. One of the things it says to check is the M/C solenoid, what is that? And where is it? As you can tell I know very little about cars. I would like to know what you think is the most lilkely problem?

Any help will be appreciated,
Joe