If you search estar in the dictionary, you find the following meanings regarding the verb followed by the prepositions mentioned:
- intr. Denota la disposición próxima o determinada de hacer algo. Estar PARA testar, PARA morir. No está PARA bromas.
- intr. No haberse ejecutado aún, o haberse dejado de ejecutar algo. Estar POR escribir, POR sazonar.
- intr. Dicho de una persona: Hallarse casi determinada a hacer algo. Estoy POR irme a pasear. Estoy POR romperle la cabeza.
As you see, there are differences among them. For instance, if you say
Aquí estoy para servirle a usted.
This translates, according to the 20th meaning, to:
I am here to serve you. (Google Translator choice.)
This case varies slightly when the sentences is a negative one:
Ya no estoy para andar saltando tapias.
This could translate as:
I can't jump over walls anymore.
I am not going to jump over walls anymore. (Google Translator choice.)
Regarding the por preposition, you have two possible meanings:
Esta parte del código aún está por escribirse.
This would be something like:
That part of the code is yet to be written. (Google Translator choice.)
Another example regarding the 22nd meaning:
Estoy por irme al cine yo solo.
This one is very subtle, and could be translated as:
I am thinking about going to the movies alone.
I am about to go to the movies by myself.
I am going to go to the movies alone. (Google Translator choice.)
This example expresses the idea of being about to do something, as stated by that 22nd meaning.
Note that every example has been properly translated by Google Translator (well, the last one lacks some subtleties of the sentence, but it could be a valid one). That's the importance of well-elaborated examples: the more elaborated the sentence, the better Google Translator works.