Could be your intake manifold....but if your were using up antifreeze through the intake like that you almost always would have the strong antifreeze smell out of the exhaust, and oil contamination. I know from experience that even the smallest headgasket leak gives you a very definate sweet smell from the exhaust, so I would assume your UIM would be the same deal. Check and make sure there isnt exessive steam and sweet smell from the exhaust (note, alot of steam is normal under certain weather conditions I.E. humid & cool)