So I have been trying to help out a friend with her 2004 4-Cylinder Hyundai Santa Fe. About 4 weeks ago it was running very poor. I changed the spark plugs and discovered that the number 4 cylinder plug had a closed gap. I put some additive down the cylinder to break up any carbon build-up and replaced all the plugs and it was running great. About 3 days ago it started running poor again. I pulled the plugs and sure enough the number 4 had a closed gap. I opened it back up reinstalled the plugs and it is running fine again. This makes no noise at all.. no knocking or anything, and it takes several weeks and several hundered miles for this to act up. Any thoughs on what is pushing the gap closed. I dont know much about Hyundai's. any help would be outstanding.