My idle runs 900 to 1200 depending on engine temp (I think). Putting in neutral (automatic) jumps idle up to 1500-1600 and then slowly drops back to about 900.
Cold idle of 1200 to 1500 rpm is normal, decreasing as the engine warms up. Per the GM/Helm FSM, hot idle for the '88 V6 should be 950 +/-50 rpm. Factor possible tachometer error and 900 rpm (on the tach) is reasonable. Idle speed is controlled by the ECM and is not field adjustable. DO NOT attempt to adjust idle speed using the throttle stop screw.
On the other hand, the abrupt jump to 1500 rpm when placed in neutral and the slow drop back down are not normal. My '88 V6 w/auto momentarily jumps to ~1100 as the torque converter load is removed but it quickly returns to 950 within a second or two. The first thing I would check is to see that the idle air passages are clean and unobstructed and that the IAC (idle air controller) is clean and working freely,
Looking to build a dash mounted display of ECM data, so diagnosing will be easier ...
For inspiration, see Oliver Scholz's (fieroluke here on PFF) Dash Scan II. I have had one of these installed in my Formula for more than six years and it's still working flawlessly. These were very professionally engineered and constructed (surface-mount components, etc.), but, sadly, Oliver didn't sell enough of these to make the venture worthwhile. (Cheap Fiero owners!) I don't think Oliver even makes them any more due to low sales and limited parts availability.
[This message has been edited by Marvin McInnis (edited 09-28-2012).]
I'll post information when I get that dash mounted ECM display done. A young man named Luke did a version for his senior project for his Camaro, but he didn't do the work to make it usable while driving. If you Google Luke and ECM you will find his work.