My 1996 Celica GT runs rich. Please help!!, Otherwise the car runs great. Options

[JohanFer]

Hi everyone, I write to see if I can get some insight into what could be wrong with my car. I bought a BlueDriver scanner to see how then engine behaves and get information from the computer, get 0 to 60 data, and learn more from the data the car spits out, I am a scientist and am crazy about data handling. Anyway, I noticed that the short term fuel trim (STFT) goes negative, making the Long Term Fuel Trim (LTFT) push it to the negative limit of -14%. I have learnt what it means and realize that the car is running rich and therefore the computer is trying to compensate. The thing is that I don't know what could be wrong.

I was told that the problem could be an exhaust leak, since the car had some work on the engine head for it had overheated, this was also supported by a light click-clak sound when the engine was cold at idle and by a small cloud of white smoke when the car was started. So when I took it to the mechanic, he replaced the seals with new ones and brushed the exhaust manifold to clear the leak. The sound is gone, but I still get a now smaller cloud of white smoke when I start the car in the morning and after it has been off for a few hours. The car continues to run rich and I don't know what it could be. I get about 24 MPG and drive conservative.

Any ideas on what it could be? Are the any test I can do to check for possible problems. I know the O2 sensors work fine, since the voltages work as expected. Please help!!!

Oh, and I have a somewhat unrelated question: Is the 3SGTE engine OBDI or OBDII?

Thanks.