Toyota Repair: Use of fuel injector cleaner, fuel injector cleaner, freeway driving

Question
I just bought a 98 Toyota Camry with the 2.2 engine and 113,000 miles on it. The car has had excellent care with all regular maintenance done, but on the way home from purchasing it I decided to put in some fuel injector cleaner. The trip up to that point was all freeway driving and I checked the mileage. To my pleasure it was mid to upper 30's. After I filled the tank and added the STP injector cleaner I had another 2 hours of mostly freeway driving. After an hour as soon as I pulled into a parking lot the engine died. I restarted and it was running really rough and would die if I took my foot off the pedal. I finally got it started again and kept driving home. After that I could feel surging once in a while. When I filled the gas tank the next time I had lost 10 miles to the gallon. Just before I got home the Check Engine Light came on and has stayed on. I asked another mechanic what to do and he thought that there may have been an excessive amount of buildup in the system and recommended putting Seafoam additive in the fuel. Have I ruined something?

Answer
Forget about adding any kind of additive which is just wasting your money, you said the check engine light came on, the one thing that I need to know is why the MIL is on and you can help me try to diagnose the problem by finding out what the diagnostic trouble code is that is stored in the onboard computer, you can do this by finding an autoparts store that will hook up the diagnostic tool at no charge and then let me know what the codes are or if that's not feasable have a repair shop or a toyota dealer retreive the codes and get back to me and I may be able to help.