Chevrolet Repair: chevy truck idle, vacuum leaks, vacuum leak

Question
I have a 1990 chevy K3500 4x4 454 tbi, It cranks and runs good but when stopped in drive it will idle up to its  upper limit. i have to put it in neutral to idle back down. i took it to 2 diffrent shops 1 changed the base gasket and said the idle controller is working. the other thought it has a vacuum leak but could not find it. i have replaced the fuel pump,trans modulator,o2 sensor, and all basic tune up stuff. thanks for any ideas

Answer
Hey,

 Someone needs to try and force the idle control motor closed and see if it will still idle.  This will tell you if there is a vacuum leak.  With a Tech2 scan tool the IAC motor can be actuated and told to extend or retract.  A defective winding in the IAC 4 plug stepper motors can cause this problem.  Check closely for vacuum leaks and have the IAC motor tested properly.

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