Am I obligated to pay the remaining amount for the car, knowing that I discovered later a defect in its engine that was present at the time of purchase?

If it is proven that the item was defective before its transfer to the buyer, the buyer has the option of defect [khiyar al-'ayb]: either to return the item and reclaim the price, which is permissible by scholarly consensus.

Or to keep it and reclaim the "arsh" (the difference between the value of the sound item and the defective one). The majority of jurists prohibit this except with the seller's mutual consent, and they say that the buyer has the option to return it or keep it without arsh. However, the Hanbalis consider it permissible to keep the defective item while taking the arsh.

The most preponderant opinion is that of the majority, which is that the buyer is given the choice between returning the item and taking the full price, or keeping it without arsh, unless the seller was fraudulent, in which case he can be compelled to pay the arsh.

However, if the buyer was aware of the defect and used the item, his right to return it is forfeited, because using it after knowing about the defect is evidence of consent to it.