in need of a tip

[jrand]

This is a common mistake, people think because they are done emitting smoke/fuel/velocities from their source that the source is no longer doing anything, when indeed it is. If you don't move/remove the source from the grid still acts like a source by continuing to influence the grid either with velocities or temperature or even a void. Sources always affect the grid.

Move it out of the way if you are done with it

I maybe assuming that your source is encompassing or close to your emitted smoke, as I have no idea what your scene looks like.