Patrick Schwarzenegger -- Involved in L.A. Car Chase ... Cops Launch Investigation

Patrick Schwarzenegger got himself in one of those famous L.A. car chases -- but this one started with a simple splash of water ... and ended with a 2-car collision.

Law enforcement sources tell TMZ ... Patrick was driving in the San Fernando Valley last week when some guy drove up on him and threw a bottle full of water on Patrick's whip and then bolted.

We're told Patrick got pissed and started chasing the guy, who at some point banged a u-turn and ended up ramming Patrick's car. Although the damage was minor ... the guy fled the scene.

Patrick drove on to hook-up with GF Miley Cyrus for lunch at a nearby restaurant and called cops from there. Officers showed up, took a report ... and opened an assault with a deadly weapon investigation.

Bonus for cops -- they got to meet Miley, who we're told was wearing a sheer top with star pasties underneath.