"Typically, fog is created by vaporizing proprietary water and
glycol-based or glycerin-based fluids or through the atomization of
mineral oil. "

Which is poisonous to the catalytic, and may have a deleterious effect on the MAF if you go directly into the air intake. Newer diesels MIGHT have a MAF so we are back to the ether. I think it is still less than four bucks a can, which should be enough to troubleshoot the intake system on about ten cars if you know what you're doing.