Avy, over in I & A, used a new idiom, (new to me!) but emotions are running so high over there, i have pulled it over here..
Avy:...letting the steam out of your mouth...

i would have said, blowing hot air, or maybe even full of hot air.. but i wonder if there is an idiom even closer to the meaning..

Sounds like dragon's breath to me!

letting the steam out of your mouth...i would have said, blowing hot air, or maybe even full of hot air..

And I though of letting off steam ???

"breathing fire" or "snorting fire" (each following consuelo's lead)? Seems to me the concept here differs from "blowing off steam".

Maybe not. Fiery rhetoric is akin to "letting the steam out of your mouth" wouldn't you say? And aren't the ones most guilty of fiery rhetoric often portrayed as blowhards?
post-edit What is the context? I will LIU and get back.
"letting the steam out of your mouth (meaning it does not lead to anything)..."
This seems very clear to me that Avy meant letting off steam or venting. Shall we invent a new word for it? How about geisering.

Consuelo, I find this very interesting.
To me (and this may just be my personal use of the terms):

blowhard or windbag or hot air or blowing hot air imply ineffectual, empty rhetoric.*
letting off steam implies rhetoric primarily to vent one's feelings, though the rhetoric may seek actual persuasion as well.
breathing fire implies "fighting words": the swords are drawn.

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*as in the fact that the sobriquet of "the windy city" for Chicago was originally coined to refer not to the city's meteorology, but to its aldermen.

and here, for what it's worth (and because it is my wont), is the dollar-three-eighty word for all of this venting:

bloviate - to orate verbosely or windily; thus, bloviation