To adjust the timing on the GM TBI setup you first must disconnect the ESC (Electronic Spark Control) wire.

It's located in the center console, usually you'll find it under the small tray in the forward part of the console. It's the only wire in there that has an inline quick-disconnect.

Just unplug and you'll be able to check and adjust the timing. BTW stock timing is 10 deg BTDC but your 2.8 will run a bit better with the timing adjusted to around 14 deg or so.

You'll get a 'check engine' lite while the ESC wire is disconnected, but no problem it'll go away when you reconnect the wire. The trouble code for the ESC circuit will be logged into your ECM for a while so don't get worried about that if you check for trouble codes and see that one.

Your fast idle problem could be an issue with minimum air setting or TPS voltage so you might want to review the procedure for that:

http://www.s-series.org/htm/how2/tpsadjust.htm

HTH.........ed