Japanese episodes
 

I have seen two "Japanese" episodes of the 1987 TMNT cartoon: One where New York City is floode, and onw where the TMNT go to Japan? Both the TMNT and Shredder use some mystic stones to mutate. I don't like the episodes, but I wonder if there are more Japanese episodes. Are they set in a special TMNT universe, or just unofficial episodes of the 1987 TMNT cartoon.

those where the only two and they where kind of just parodies of the old toon and the anime cliches. They don't fit in any continuity as far as i know, and recently some memebers on the drome did a fan dub of one of the episode in the fan section.

They are just 2 episodes telling the story of super turtles gaining their power by stones. I do not like them much either. Too japanese.
They have a toy line also i think.

Dimensional portal
 

Also, Krang's dimensional portal was wrongly depicted, since it more looked like a door rather than a "hole" in time and space.

Yeah, to be honest i did not like it since first seeing it. It seemed to Power Rangers-like to me. And i never loved Power Rangers since childhood.
Also Krang's japanese voice hurts my ears so much.

Which year
 

Which year were they released?

96 i think

1996
 

If it's 1996, it says much. It was in the Power Rangers hype (I was never into it, just saw some episodes).

The Japanese have a tendency to take American franchises and sprinkle some Sentai/Henshin hero sugar on top of them. The Turtles and Spider-Man are good examples.

Comic books
 

Are there any special Japanese TMNT comic books too (set in an own TMNT universe i suppose)?

Here are some scans of a couple issues of the TMNT manga I bought from Rose:
http://www.flickr.com/photos/22047800@N07/

And here is a thread on the mangas:
http://forums.thetechnodrome.com/showthread.php?t=24754

thats some scarey shiznit! :o