Sorry for the delay in replying.
The IAc controls the idling speed and when cold, it would increase the amount of air to the manifold to increase the idling speed.
When engine is warmed, if the IAC is not closing correctly, it would cause the surging as it tries to compensate for low or high idling.
Other cause of the erratic idling could be clogged coolant hoses, high idling speed and insufficient coolant in system.
Thursday, February 11th, 2010 AT 12:24 PM