Will.i.Am -- Back to the Future Car IMPOUNDED By Cops

Will.i.Am has temporarily lost his customized "Back to the Future" car -- a tricked out DeLorean -- after cops pulled him over in L.A. this week ... and IMPOUNDED the vehicle ... law enforcement tells TMZ.

According to sources, cops pulled the rapper over Thursday night for having no license plates. When they looked at his documents, we're told cops also discovered a registration problem ... so they kicked Will.I.Am out of the car and impounded the custom vehicle.

Will was cited for the registration SNAFU as well as the no-plate thing -- but we're told he was very cooperative throughout the entire process. Will then had a friend pick him up from the scene.

The DeLorean will be held until Will.I.Am gets his paperwork in order.