There's not any chance to find "porque" as a question tag: "¿porque .....?" Sure not. never
1. POR QUÉ
Explicit Question: We use it for ask the reason of something.
> "¿Por qué llegas tarde?" (Why are you late?)
Implicit Question: Apart from find it at the begining of the sentence, there's another position in the middle of the sentence in that cases when a kind of reason is implicit:
> "Quiero saber por qué llegas tarde". (I want to know why you are late)
We used as the answer for the previous cuestion, that is, the answer for any reason.
> "Llego tarde porque perdí el tren". I am late because I missed the train
Reason, cause. Imagine that someone I invited to my birthday party wasn't there. If I want to know his/her reason we have just known that we can ask in two different ways:
> "¿Por qué no viniste ?" Why didn't you come? (1) ....or....
> "Quiero saber por qué no viniste" I want to know why you din't come (1)
The last one is to refer the problem only by the question tag "why", without saying anything else, without more information, because it is implicit in the situation:
> "Quiero saber el porqué". I want to know why
4. Por que. It's a bit more difficult and long to explain, because it conects with gramar functions.
4a) Por + (los) + que (relative pronoun) [los, las, el, la ...changing it for concordance purpose. We can use it or omit it]
> "Los premios por (los) que competían no resultaban muy atractivos." The prizes for those who competed were not very attractive.
4b) Por + que (starting relative clause) [por que, por donde, por cuanto...]
There's even any other possibility conected in english with the preposition FOR, WHAT, THAt or NOTHING
> "Están ansiosos por que empecemos a trabajar en el proyecto" They are anxious for us to start working on the project.
>"Al final optaron por que no se presentase." In the end they chose not to show up.