Well, you never actually need to clear Canadian Customs, but you will need to carry your passport to get "back" into the US.

I'm sure US CBP would give you a strange look upon your attempt to clear into the US at Pearson without actually leaving the country.

And I assume you're doing this on AC/UA, because if you do it on another airline that operates out of Terminal 3, you will need to clear Canadian Customs, and then Re queue for US CBP.