That's an... interesting question!
Unfortunately, it's not possible for a poop to propel a car. Poop is a biological waste product that doesn't have the physical properties or energy density to generate the force and motion needed to move a vehicle.
Cars require a significant amount of energy to move, typically in the form of gasoline, diesel, or electricity. The energy released from burning fuel or electrical energy is what propels a car forward. Poop, on the other hand, is a relatively low-energy substance that doesn't have the capacity to generate enough force to overcome the friction and inertia of a vehicle.
Additionally, poop wouldn't