"The motor is not good."
Translation:El motor no es bueno.
I've been told that ser generally refers to static state... soy un gringo, soy blanco. Estar can be used for condition to refer to temporary, unusual, scenarios. Yo fui a la playa, estoy rojo. Happily i think whoever you were talking to about this motor would know what you meant but a little context would help if Duo wants you to go a specific route.
Could be either, depending on the meaning you want.
"El motor no es bueno" could mean something like, "This motor is not good; that model of motor always has issues and I never recommend it."
"El motor no está bueno" would mean this particular* motor is broken, for example.