What causes idle to drop slightly then back up? - 2000 B4000

[Jm2576]

The engine idles normally for a while then the idle drops slightly for a few seconds, then idles back up. What causes this? Is it the AC clutch kicking on? Normally I would not be concerned as it seems like normal engine operation, but recently the weather has changed to cold and damp and now it idles a lot lower than it did when the weather was warmer.

 

[rubydist]

Sounds like the iac may be sticking.

 

[Jm2576]

Thanks. I've never messed with The IAC. I will take it off and have a look & clean it up. Is it possible to test it somehow?

 

[rumblecloud]

only tning I can recommend is to start the vehicle, let it warm. Then unplug the IAC. If the idle drops and or stalls, it is working.

Pulling and cleaning may help. There is an article on how to do that in the tech section HERE

 

[Jm2576]

Thanks for the info! After reading that article I definitely think this is the issue. I am also getting that occasional howling noise as mentioned in the article.

 

[Jm2576]

Update:
I unplugged the IAC while the truck was running and the idle dropped down a little but it didn't stall. I took it off and cleaned it keeping the motor upright. I'm not ready to replace it yet as new ones are $75. It seemed to run a little better and the idle didn't drop down at regular intervals like it did before. Does anyone know what the big black knob on the side of it is for? See attached Pic.

 

[rubydist]

That is simply an air vent cover.