[shorod]

Welcome to the forum!

How much smoke would you say you're getting from "underneath"? Approximately which area underneath? Does it have an odor? What color is it (gray, white, blue)? Are any of your fluid levels dropping? Does it only happen when you are experiencing a lack of power?

Also, just because the check engine light is on does not mean there are no stored or pending diagnostic codes. It would be worth the effort to connect a scan tool that is capable of checking for pending diagnostic codes.

Did the issue start following the tune up and oil change, or was the tune up a result of the lack of power? If it started after, the issue could be a spark plug wire that is not fully seated or just plain bad (somewhat common with "economy" plug wires). It's also possible a spark plug could be cracked, but that often would cause a misfire even at idle and start immediately after the tune up.

-Rod