The engine on my 1993 Bonneville SE surges at idle. It idles fine some times, then for no good reason revs up to 1500 tp 1800. Also having trouble getting the tranny to stay in overdrive. It wants to downshift without being prompted by the throttle then it won't go back into OD until you turn off the engine and restart it. Both problems appeared at the same time, not sure if they are related.
Checked for vacuum leaks, seems fine, steady at 20 inches vacuum. Replaced the Idle air control this week. The TPS is about 2 yrs old. I am getting codes 13 ( 02 sensor), 21 (TPS high voltage), 34 (MAF), 39 (TCC), 42 (electronic spark timing)
Looking for sugestions of what to try next. Thanks

did you clean the iac seat when you replaced the iac? you can spray tb cleaner around vacuum joints, tb gasket area, intake man gasket area listen for engine to change pitch.

unplug the maf sensor and see if it gets better as for the tcc code you may be low on trans fluid or have a bad tcc switch

Now what? I have not been able to figure out what is going on. Now I have another symptom. The trans will not engage in reverse or first for a few minutes when you first start it in the morning. It will rev to 2,000 or 3,000 rpm and just barely move the car. Once you get it warmed up it starts to act just fine upshifts and down shifts just fine, except for the issue with it coming out of overdrive and going back into drive. After the first start in the morning when ever you start it it goes right into gear just like it should. I added a bottle of Lucas oil trans fix and nothing changed. Could there be some kind of electrical problem causing 3 seeminly unrelated symptoms ?

Checked my maintenance records and found out that I had purchased the TPS a couple years ago from Aotozone for $19.00. So I decided that maybe a $19.00 part will only last a couple years. Went to Advanced and bought a TPS with a lifetime garantee for $50.00. Sure enough that was the problem. Idles fine and runs good. Disconnected the battery to clear the codes on the computer. All the codes went away except the TCC. The transmission still slips sometimes in the morning when you first put it in gear but, it drives fine from that point forward. It will only stay in overdrive on the highway if you use the cruise control. Once it drops back into third it will never go back into OD until you turn it off and restart it. Is there any way that the problem with slipping in the morning and the TCC being bad could be related? Would it make sense to replace the TCC or would it be time to rebuild the entire transmission? Ole' Nellie Belle has 320,000 miles on the clock. What should my next move be?