Either Justin Bieber's van had some serious mechanical problems in L.A. today ... or his crew was blazing a METRIC TON of Mary Jane inside -- because when the singer slid the door open and stepped out, he was followed by a Hiroshima-level cloud of smoke.
You gotta see it. It's like something out of a stoner movie.
Bieber -- a noted weed enthusiast -- was arriving for a radio interview at Power 106. Here's hoping he can still form complete sentences.