I bought my 2006 CTS in November. The engine ran fine. When I bought it the check engine light was on but I was told it was from a bad o2 sensor so I had it replaced and the light went off. In January I went out of state and made the mistake of letting my room mate use my car. He called me around January 10th to say the check engine light was on. I said just keep your driving to a minimum and I'd get it checked when I got back. About a week after that he said it was running rough. Well, I didn't get back into the state til February 4th. On the drive home the car was shuddering badly and the engine light would blink. Unfortunately I didnt have the money to get it checked out. On March 4th I took the car in because I was leaving Michigan and moving to Texas (driving my debilitated car there) and the power steering was going out and I was putting in about a quart of fluid a day in. I took it to a local guy and he fixed my steering line and said he'd throw it on the computer to see what was up with the engine. Cylinders 1, 3, 4 and 6 were misfiring. He said I should just change the plugs to see if that would help. So we did. When he pulled them they were all completely fried and a couple had oil. Cylinder 4 he said wasn't firing at all. I made it to Texas with my engine light blinking the entire time. A couple weeks ago my friend offered to help me with my car. He changed the plugs. They were all fried all but one with oil. It is now running really rough (a bit worse than before). I've put about 2300 miles on since changing the plugs in Michigan. My car overheats when I turn on any fan on in the car or sometimes just because. The coolant and oil are both at normal levels. I've noticed the exaust smells like gas. What can I do to fix this? I'm hoping to get at least another year out of it. I'm also sort of limited on funds.

It has about 158,200 miles on it