Episode : Double Vision.
Quite a long way into the episode you will spill some motor oil on young doc! He will then use a 'shower' to clean himself up and will talk to you about how he has made a concentrate of his cleanser.
Take the cleanser (the spray bottle) and stash it in the Delorean. Tell Doc that 'the light is green' and he will then test drive it. When he returns the light ill go red so interact at random then go back to check and it will have turned green - again, go tell doc that it is green, he will test it again and as he explains that the time circuits still aren't fix the achievement will unlock!