Since 2014 we have had a 'no cars left behind' policy in Mongolia. This was due to changing wishes of the Mongolian Government. Over times things changed and Mongolia no longer wanted rally cars - this is something we complied with and respect.
If your vehicle breaks down in Mongolia, you are not permitted to leave it there without customs tax being paid. This is very expensive. You must not under any circumstance leave your vehicle unattended or abandon it.
Failure to comply with local rules may result in payment of fines, confiscation of property or unplanned delay in travel.
This article written by the UK embassy in Mongolia gives a very good summary of customs taxes. It's written about Mongolia specifically, but we think it's a concise summary of quite a complicated subject. Well worth a look.