Ashton Kutcher -- Good Samaritan, Bad Mechanic [VIDEO]

Ashton Kutcher did a solid Saturday for a stranded motorist ... only problem ... he needs to brush up on his grease monkey skills.

The driver tells us ... his car battery died while he and his group were hiking in the Hollywood Hills ... so they pushed the whip to a nearby street and tried flagging down anyone who would stop.

Ashton was driving down the street when he spotted the group ... pulled over and was asked if he could help jump start their car. Problem was -- he had a passenger and said he had to drop her off at home but he would return. The group thinks the passenger was Mila Kunis.

Shockingly ... Kutcher came back and got down to work, trying to jump start the battery. But first things first ... he opened his hood and had big trouble finding his own battery ... because it's in the trunk! Ashton eventually accomplished his mission.