Ewwwww... fun one. My blokes have your year of car and we tried the same thing. I think the OBD2 goes crazy at a point during the timing advance. I have OBD1 and I have the same problem. We had a shop try it and no luck. I have been advancing my timing (93 NX) with a timing gun and it reads as advanced without going through the whole "unplug this and rev that" stated in the shop manual. Get your timing light out and just try using that as you adjust. See what happens. My MDM-100 says a whole differant story. I read that you must "warm up car, unplug TPS, rev engine past 3000 RPM 3 times, then adjust timing". Don't know in who's world that works. I'll call my friend at Nissan soon to see what he does. Boy I miss the old days of just "turn this, your done".