Exact meaning of 临+ 时/前

[candie111]

I was wondering lately what the exact translation for the use of 临 is. I understand that it expresses the immanent future, something like "at the time when something is about to happen".

But especially when it is used with 时 or 前, I cant really understand the difference between the usages.

他临出门前，。。。。-->I understand the meaning here is "Just before he went out"
他临出门时，。。。。--> But what would the difference be in this sentence?
他临出门，。。。。Can 临be used on its own like it would be here? And what would the difference in meaning be?
他临时出门，。。。How exactly is 临时used if the two characters are put together like here?

Thanks for your help.