This site explains it well:
http://www.exportainer.com/import_requeriments_en.htm
(It doesn't look like it's possible to import a vehicle without paying a very high duty.)
ALIENS ENTERING ARGENTINA WITH A TEMPORARY RESIDENCE VISA: These may bring into the country one or more shipments (while they have a temporary visa) of personal belongings and used household goods under temporary admission according to the present guaranty system : The belongings must be guaranteed to the local customs authorities for the duration of the visa by means of a signed guaranty with a registered company signing as guarantor, or by a security policy, local bonds, moneys, etc.. The guaranty may be renewed for the same extension as the visa. - If at the expiry date of the guaranty the temporary visa has not been renewed or the permanent visa obtained, the goods must be nationalized (import taxes paid on them) or be re-exported.
Once the Permanent Residence Visa has been obtained this must be presented at Customs. The guaranty is thus annulled and the goods brought in under it are nationalized, tax free – These aliens are also allowed to bring in a shipment of used household goods within six months of date in which the Permanent Residence Visa was issued.
When shipper leaves Argentina, if no permanent residence visa was obtained during his/her permanence in Argentina, he/she must either ship out all items originally brought in (they entered under a temporary permit) or nationalize them (import taxes paid on them). Once this has been complied with, the required documentation is presented to Customs and the Guaranty is annulled.
ALIENS WITH A PERMANENT RESIDENCE VISA AND/OR RETURNING ARGENTINE CITIZENS who have been residing abroad for over one year and not visited Argentina for an aggregate stay of over 60 days during the 365 days prior to his/her entry to Argentina (to reside) may bring a shipment of personal belongings and used household goods into Argentina free of duties. – They should obtain a Certificate of Residence from the nearest Argentine Consulate in the country they are moving from. This is especially important for shippers moving from neighbouring countries (Chile, Bolivia, Paraguay, Brazil and Uruguay).
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AUTOMOBILES AND MOTORCYCLES:
a) Argentine citizens returning to reside, and foreigners coming to reside with a Permanent Resident Visa, may import one (1) automobile and/or one (1) motorcycle. These must pay duties of approx. 80% (for petrol engines) and 100% (for diesel engines) of C.I.F. value (cost, insurance and freight). These may not be sold for one year.
b) Passengers with a Temporary Residence Visa may bring in one (1) automobile and/or one (1) motorcycle under a temporary permit for 8 months, renewable."