Does my feeling of pleasure during the Istinjaa’ (cleaning one’s private part after answering the call of nature) invalidate my fast?
If I exaggerate in making Istinjaa’ because I enjoy it, would my fasting be invalid?
Am I required to make up for the days of fasting when I did so?

All perfect praise be to Allaah, the Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

Exaggerating in making Istinjaa’ to get pleasure and prolong that feeling is impermissible, because this is considered masturbation, which is prohibited.

Whoever commits such an act must repent of it and ask Allaah The Almighty for forgiveness.

Nevertheless, the mere touching of the private part during the Istinjaa’ and deriving pleasure from this  does not invalidate the fast, as long as there is no ejaculation because this act is not stronger than foreplay between man and his wife, which does not invalidate the fast.

Allaah Knows best.