Is a spouse responsible for a car loan in her deceased spouse's name only? no estate or will was done. voluntary reposession?

I am sorry about your loss... The surviving spouse is not responsible for the debt if they are not a co-signer to the loan. The lender is entitled to take possession of the vehicle, and auction it off. I'm assuming that the car is worth less than the balance on the loan so there is no reason to try and preserve any equity value in the car. There is no real benefit of a voluntary repossession in most cases from the borrower's point of view, versus an involuntary repo, it just tends to make it easier for the lender, and may reduce any deficiency balance, but that does not appear to be a major consideration here.

No, not unless the car was in both names. Otherwise, the estate could be found responsible for any shortfall. Why not sell the car for a profit and pay off the loan and keep any overage available. Good luck.