If the cars do not grow, then why did Marty see them in different sizes?
Those must just be different models! A living thing needs to be made of cells to grow! Marty could have used his scanner to detect if the cars had cells!
NO CELLS DETECTED! NOT ALIVE!
Besides that, some of your arguments don't make sense either! So what if they use a black liquid for energy, that could just be a power source! As for the release of gas, it must be simply a biproduct of this energy source. not homeostasis which needs cells! You said the cars seemed to respond to stimulus, yet they are doing nothing at the moment! Marty could and should have done better.
Come out here for a demo!
There's controls in here! The movements you saw were from another life form controlling the car from inside!
Well Marty, you could have literally seen life forms driving the cars from the inside. You really could have done better to determine whether the car was alive or not. There were no cells in the car so it is not alive. Therefore, I charge you with negligence of your duties and incompletion of your assignment.