Gas Additives Cause Knocking?

Our 2000 Mazda 626 ran great this morning! We put in a gas additive and it is running rough and the engine light stays on!!! Anyone know what is going on and how to help?

What was the additive and why did you put it in if the car was running well? How much gas was in the tank? Usually, these additives don’t do much to make an engine run any better, but at the same time, don’t cause the problem you are having. The only connection that I can see is that the additive may have caused a spark plug to foul out, or an injector could have plugged up. At any rate, have a scanner check for the trouble code. Additives purport to have a special purpose–e.g. cleaning fuel injectors. If your engine is running well, don’t use them. If you don’t have a cold, you don’t take cough syrup, do you?

You really expect help when you don’t tell us what the gas additive was?

Stuff called NuEnergy Super GAs Treatment to improve Mileage. Main ingredient is Methanol.

Dumb question: Did you properly replace the gas cap after you added this stuff?