Thanks for query,
There are few causes of bad breath
or foul smell from mouth,
If local hygiene of mouth as it may be with you,coated tongue you have,
Do 2 times brush,morning and evening,
Clean your tongue with tongue cleaning steel rode,
Clean your mouth every time you take some thing orally,
Take plenty of water,
in your case less amount of saliva keeps your mouth dry,so take more water,
There may be some chronic infection in the nose or sinus
You have some gitract problem so go for stool test
and get treatment done.
ok and bye.