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GMC Acadia 2010 When the engine is cold, I get fluctuating idle when I start the car. Rpm fluctuate From 1500 to 1100 then get back to 1500 this happens till 40 seconds
But once engine is operating temperature all idle symptoms go away only very mild fluctuations
 

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Maybe it needs a new air filter, and/or a good cleaning of the throttle body.
 

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you mean on a cold engine start?
THe engine runs at high idle till about 46 seconds or so.
THere is dip in rpms when it does what is described as a double injector pulse to a single injector pulse... its a significant dip that you can hear and see on tach.

But here is a cold engine start for my 2010.
note that this is about 46 seconds of high idle.
it starts at 1419 RPMs and then remains around 1277-13XX RPMs.
at -68 seconds, you see the switchover.

But you say yours is 1500 to 1100 over and over for 40 something seconds?
I would try cleaning the Throttle body 1st.

Also- is this vehicle new to you?

(mod edit: click on the figure to enlarge it)
 
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When cold engine starts at high rpm 1500 and remains for 120 seconds and after that comes to 700-750 rpm but the first 40 seconds rpm fluctuate between 1500-1300—1100, and remaining time i mean 80 seconds rpm is stayed at 1500
 

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I had checked vacuum leaks with home made smoke machine No leaks found out and TB clean but still same problem
 

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I had checked vacuum leaks with home made smoke machine No leaks found out and TB clean but still same problem
Other things to observe would be correlation of data from the IAT (intake air temperature), ECT (engine coolant temperature), and MAF (mass air flow) sensors, or any other sensor which would be used to control cold idle engine rpm. You'd need a good scan tool to do that.
 

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GMC Acadia 2010 When the engine is cold, I get fluctuating idle when I start the car. Rpm fluctuate From 1500 to 1100 then get back to 1500 this happens till 40 seconds
But once engine is operating temperature all idle symptoms go away only very mild fluctuations

I know this is kind of old. These my solutions to keep my Acadia from rough idle issue. I have had a same issue but was able to resolve it. An issue with rpm fluctuation happened when the engine was starting at cold or after a night or at stop a red light.
1) Applied a 10-12oz bottle of Chevron's Techron Complete Fuel System cleaner at least once year. Many people have used Techron but I found Gumout All-In-One Complete Fuel System cleaner also produced a same result as Techron or even better. Best applies before you head on a long trip on high way. It helps to clean the cabon build up in the valve system that mostly cause the rough idle.
2) If the rough idle issue still exists or having a trouble code related to crankshaft position sensor. There are online video show how to replace this sensor. It is not hard to replace but you have to lift and secure, so you can crawl underneath the engine. Just remove and wipe the sensor with a clean solution like TB cleaner sometimes it work without replacing a new sensor.
3) If your Acadia has issue with losing oil as my case, I installed on my Acadia with two oil catch cans. One in front and one in the back connected to PCV orifice or breather. This keeps oil from getting into the air intake tube and top engine. This is also a main source of carbon built-up.
 
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