Buick/Olds: 1990 Buick Century High Erratic Idle, vacuum leaks, air control valve

Question
I have a 1990 Buick Century, V6, 3.3 and it runs well except after you run it for over an hour or so and shut it off. Once it's warmed up and you shut the car off and then start it again it has an extremely high erratic idle. The idle stays high while driving and the car jerks to stop because the idle is so high. The idle will sometimes drop back down to normal after driving and then idling in park for couple of minutes. We are afraid that the transmission is sticking in gear. It seems to me that it happens more frequently when the car is low on gas.  I've heard others complain of this problem with buicks also. Could you tell what you think it might be?

Thank You,
Megan Malone  

Answer
Its not a transmission problem and its not a common problem on Buicks either. The most common problems for a high idle are vacuum leaks or a faulty IAC(idle air control) valve. It could also be caused by an ECM(computer).
Since it only does it after it has been running and then hot soaks, I would say that it is probably the IAC. But that is only a guess at this point. You should probably take it to the shop and have it properly diagnosed.