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Erratic high idle

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My truck started to randomly high idle yesterday. Truck does not high idle on a cold start in the morning but stays high idle between 2000 to 3000 rpm when I get in the truck around lunch time and eventually goes back to a normal idle. My thoughts is that the temperature in the morning been around 30 and around lunch time it's 60-70 may have a role in this. So far I unhook the battery for a hour yesterday to reset the ecm and it still high idles in the afternoon. My next guess is the iac but I'm open to any suggestions. 

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On 10/19/2021 at 3:42 PM, jdog said:

Check the bolts to the manifold

Checked manifold bolts and all bolts were tight.

On 10/19/2021 at 8:29 PM, MiNi Beast said:


Check the tps with the renix tip technique and was in spec.

Inspected vacuum lines for leaks and found no issues. 

Check alternator to see if engine was raising rpms to get voltage.

For giggles I figure I would see if I was crazy in my guess of the IAC going out. So I "borrow" one from my dad's project Comanche and swapped it to see if the issues continued.  The good news is when I went to the truck this afternoon it didn't even try to high idle. Ran smooth as glass. I appreciated the suggestions y'all gave me it did help me out with trouble shooting. Hopefully this helps someone else down the road with the issue that I had. 

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