I had a similar issue with my Sentra. I think around 1999, Nissan may
have found a way to prevent the consumer from doing timing and other
adjustments anymore. The CPUs in Nissan vehicles from 1999 onward seem
to be a bit harder to diagnose unless you use the $1000 Nissan
diagnosing computer, the CONSULT, only available and used by the
dealer.

In my car, I tried to adjust my timing to +10 degrees with a timing
light. I dis-connected several sensors; the fans, tps, IACV etc. When
adjusting the timing I would slowly move the distributor as the degrees
went up: 3....5.....5....5...5......5....5...7-then immediately BAM! 30
degrees off the scale. Somehow the ECU picked up that I was adjusting
timing and at seemed to just throw everything off. I tried several
times. Same result.

What I did instead was then to play by feel. I set the distributor to
one point and drove for a while. I try to give it a week each time in
case the ECU needs time to adjust to the new setting. While not
perfect, that has worked out ok for me.

CD