Wa `alaykum as-Salamuwa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
As long as you have repented form interest-based transaction and the borrowed money belongs to you, you can perform Hajj with the price money of the car.
In his response to the question, Sheikh Muhammad Saleh Al-Munajjid, the prominent Saudi Islamic lecturer and author, stated:
It is not permissible to deal with riba, whether that is by taking or giving a loan. The one who has done that must repent to Allah Almighty by giving up the sin, regretting it and resolving never to go back to it again.
The interest-based loan, despite the fact that it is prohibited and reprehensible, becomes the property of the borrower according to the correct view. So the money that was borrowed belongs to you, and you may make use of it in whatever permissible way you wish, such as buying a car or otherwise.
Based on that, it is permissible for you to do Hajj with what you have of that money, so long as you repent from riba, as you mentioned. It does not matter that you are still paying off installments on the loan or not.
There is nothing wrong with doing Hajj even if you owe a debt, if the debt is to be paid later on, or is being paid in installments and you are able to pay it off on time.
Allah Almighty knows best.
Excerpted, with slight editorial modification, from, islamqa.info