My son has a 97 Geo Metro that he's had since new when he was in his last year of high school. He's taken great care of it too. He and I trailered it back East(from California) to Toledo behind a rental truck(his furniture and belongings) two years ago so he could attend medical school. Anyway, the little 4 door, 4cylinder Geo Metro(Suzuki) is starting to show some age, but my son still is using it for his going to school car, and all other things. Recently, his idle started acting up(getting rough) and seemed that it wanted to die at times. He took his car to a AAA diagnostic center in Toledo, and the techs checked out his injection/fuel system, and said all seemed ok, but they did find that one of his combustion cylinders was giving some erratic compression readings. They(techs) presumed that the head gasket was starting to leak. Also the spark plug in that particular cylinder was fouled, as though it wasn't firing very much. The discovered that the spark plug wire to that spark plug was bad. AAA replaced the spark plug wire, and buttoned up what they took apart. They said that the bad plug/wire could have caused some problems that led to head gasket failure. .............. Well, my son got some help from some friends that have a repair garage in Toledo, and they offered to install a new head gasket for zero labor, and just have my son buy a new gasket and needed parts. After taking the head off, they couldn't visually seen anything wrong with the head gasket, but did replace it with the new one. After buttoning-up the engine, it still runs rough as before. Note: He thought that the idle might be improving a little since the AAA appointment a couple weeks back, and the new gasket. ......... Here's my question: Is it possible that the cylinder that wasn't firing very much has bad carbon deposits behind the valves; most notably the exhaust valve? Could this be what's causing the bad idle even now? Could the erratic compression reading be an exhaust valve that's got carbon deposits causing it to not fully seal, causing some compression leak-down? When they took the head off, the valves visually looked good from the combustion side. None appeared cracked, broken, or burned. Could there still be carbon deposits on the inside of the valve/s, on the exhaust exit/port side of the head? ......... Are there any strong additives that my son could buy, to put in his fuel system/gas tank, that might help burn some excessive carbon residue? ........ My son is on a very limited budget. He's a newlywed, as he came back to California to marry his sweetheart last June, and is living pretty frugally to survive. He could never have gone to med school if he hadn't been awarded a scholarship to this school in Toledo.(Medical University of Ohio/ formally Medical School of Ohio/Toledo...now affiliated with the University of Toledo.) His mother and I couldn't have afforded to pay for med school, so his scholarship was a "God-send" for him. He actually paid his way through 5 years at San Jose State University, near our home in San Jose, Ca. Anyway.......as his dad.......I'm trying to do what I can to help out. He's over 2,000 miles away from where he grew up, and I thought maybe someone on the "Garage" forum might have some crucial advice. I'm hoping it's just some carbon deposits that might burn-off with a little help from an additive. Maybe since the defective spark plug wire is fixed the improved combustion in that cylinder will gradually burn-off the deposits.....if that's the problem? Can a bad spark plug that cause exhaust valve deposits to build up and eventually affect compression? Can a good spark plug replacement that's firing ok, gradually burn off deposits, and remedy a rough idle/running engine?