2000 528i

I just purchased this beautiful 2000 528i last Sunday. I started the car Tuesday night and the Service Engine Soon light came on followed by an annoying rough idle. I called the salesperson whom I bought the car from (Certified Pre-Owned, still under warranty, thank God) who asked me if I put any gas into the car. When I told him I did, he asked if I were sure that I sealed the gas cap correctly. He told me the system is pressurized and if there is any leak, the car will idle rough. Kinda scratching my head here. Why would this make such a difference? Has anyone else had a similar problem? I am thinking about an extended warranty for the car. Is it worth it?

If you didn't tighten the gas cap, it could throw a check engine light (CEL), it's part of the emission system that monitors leakage.

Have the code read, it might be something else also. The CEL usually get turned off after 3 successful starts of the engine, and if the system doesn't find anymore problems.