Do you know if there is a software for full-text search engine working with Informix DB ? I know sphinx but it cant talk to Informix. I dont have basic in Informix so I hope someone can help me ? Thank you.

There was a full text search Data Blade available some years ago that we used in a previous assignment, I don't remember the name of it. Perhaps someone in the DataBlade group at Informix could help you better.

I am also looking for a Full Text search engine for Informix, Sphinx is great for Mysql but it does not work for Informix. Any one can help would be highly appreciated.

Informix Excalibur datablade provides full text search capabilities.
But AFAIK its not free.
In IDS 11 exists also Basic text search engine datablade which allows you to do searches on text data found in char,varchar, lvarchar, blob and clob datatypes. And it's included in IDS 11.

Hope this helps