In Michigan, paying on 18 year old that doesn't come. Demands full tank of gas to come. Do I still pay

You pay for the child support until such time as the Court orders otherwise. You need to re-read your child support obligations and see when your payments end. In some states you must petition the Judge and have the Order for Child Support reversed. As far as a tank of gas - is your ex that cheap that mom can't afford a tank of gas out of the child support money you send her so he can go and see you? Or are you financially unable to come up with the tank of gas cost to see your kid? Either way someone is a cheapskate here and without more information I can't point the finger at just who it is.

Do I still pay

Certainly.

Duty to pay CS is independent of visitation enjoyed.