Eating poo is most often not related to anything missing in their diet. It seems to be more prevalant in winter time...apparently poop is much tastier when it's frozen

And bitches may be more prone to it than dogs (I have no idea why...our old vet told us that, though it doesn't hold true for our hounds. Our boys are the poo eaters).
Why do they do it...who knows! Tastes good...they're bored...Kibble has filler in it. A lot of that filler passes right through the dog and into the poop, so my guess is that it has something to do with that. I've never seen a dog eat a "raw diet" poop.
We are lucky w/the greyhounds b/c we can put a muzzle w/a stool guard on to keep them from eating what we haven't picked up yet. You can try additives. I've heard people have been successful using canned green beans, pineapple or Adolph's meat tenderizer. There are products specifically marketed to make poo unpalatable...usually the active ingredient is papain (also found in papaya, pineapple and Adolph's meat tenderizer

) Mostly I've found that keeping it picked up is about the only way to guarentee a dog won't eat poo.
(If you do add beef or chicken broth, though I've never heard that to stop poo eating, make sure you use a low or no sodium version!)
No advice on the car sickness...sorry
