2001 Other Nissan Models Black Smoke

Hi, Originally purchased and white smoke would appear first thing in the morning when started, then go away. It was using oil too. Finally got the oil valve seals replaced and the heads machined plus everything that goes with it (reconditioning). Now after a couple of months the check engine light has come on, took it in and they just said reset the sensor. Another week and the light is back on. Also noticed that when I drive around town, too and from work (mainly first thing in the morning) black sooty smoke comes out. Though when I floor it it seems to clean out, then reduces this. It runs less black smoke when really hot.

Thinking by your answers to some of the others, maybe fuel pressure, oil pressure (it is a direct injection model), or air flow meter/sensor?

Oh the model is STAGEA 250 RS station wagon, rear wheel drive.

Much appreciated for your comments.