It depends on how well tuned the engine is. If NH3 and O2 are burned in the perfect mixture, it would produce H20 and N2. If the mixture is to oxygen rich, it will produce NOx's. If the mixture is too oxygen lean, the exhaust will contain unburnt ammonia which is even more toxic. The ammonia enthusiasts will tell us that all cars will be perfectly tuned all the time. I have no data.

It's the same as for an engine burning petrol or natural gas. If it's a nice hot well-tuned engine, petrol gives you H₂O and CO₂. In practice, when the engine's starting up, and when it's not well-tuned or when idling at traffic lights, or if it gets revved too much, combustion is imperfect, and you get lots of CO (carbon monoxide), which is the deadly poison some suicides rely on when they put the exhaust into their car and run the engine.

So ideally an NH₃ engine would produce just H₂O and N₂. In practice, a significant amount of nitrous oxides (NO_x) will be produced. This will contribute to acid rain and the greenhouse effect. In fact, 800 million cars burning NH₃ (ammonia) will most likely have a greater greenhouse effect than 800 million cars burning petrol or natural gas; less NO_x will be produced than CO₂, but it has a much greater greenhouse effect.

The other aspect is producing ammonia for agriculture. Something like 7-8% of greenhouse gases (in CO₂-equivalent terms) is caused by too much nitrogen fertiliser being added, and this leaching into the soil, reacting and being processed by bacteria there, and becoming NO_x gases. However, this is a problem with both artificial and natural fertiliser (animal manure, etc). So that's a problem not with how you get the nitrogen fertiliser in the first place, but how it's applied. Anything which offers cheap and plentiful nitrogen fertiliser will tend to increase the amount farmers use; the typical farmer is more concerned with having a fertile field this season than climate change in a generation or two.