Weird Problem

My service engine light comes on when my tank is near empty.
Water in the gas tank?
Ive ran Heet with a full tank of ethonol free 93 octane the last time . The light went off on its on for a while. It came back on today when the fuel level was just north of empty.
Thanks for any suggestions.

Pull the code. A CEL can mean anything from a small evap leak to a broken O2 sensor to a misfire. Even with the light off, you should still be able to see the previous codes. If it isn't flashing, then you not going to damage the engine, you are just not running as clean as you should.

No flashing, I would park it if it did. Funny thing is a it runs smooth as silk at normal driving speeds (45 to 85mph). It idles a little rough when stopped.
Thanks!

Definitely need to hook up a code reader. need to find out what that CEL code is for.