When Justin Bieber's in trouble -- and he's in a ton of it right now -- his go-to person is still ex-girlfriend Selena Gomez ... cause he paid her a quick visit Wednesday afternoon.

Justin's chauffeur-driven party van was spotted parked behind the gates of Selena's driveway for more than an  hour. The exes reportedly left her home separately.

Selena's home isn't too far from Bieber's Calabasas house ... where a neighbor says Justin spit on him during an argument.

As we first reported ... the neighbor has filed a battery report against Bieber and wants to see him prosecuted for the incident.

Hmm ... wonder what Justin and Selena talked about?