Loud idle noise, IAC? Please help!

[Alec]

Hey all, maybe you can help me with a problem (and yes I've used the search function and read a whole bunch of posts).

At idle RPMs (~700) or more specifically, idle vacuum levels (~18-20 psi), there is a very loud drone coming from the engine. If you were on Top of the World with me at Moab you might have heard it. It used to only happen intermittently, but now it happens all the time. Basically whenever the engine is at around 18 psi of vacuum you hear this noise. This can be at idle or while coasting down the road at idle RPM. The noise is a very loud drone, almost like an airhorn. Other drivers look at me very funny and at lights I tend to double foot it a bit to keep the RPMs above 1000 and thus the vacuum around 10 psi. Gotta love my vacuum/boost gauge!

I cleaned the IAC and now my idle fluctuates from 1000 to 500 and back, about every 10 seconds it dips. If I unplug the IAC when the engine is on, the truck dies. Does this mean the IAC is bad or good?

I don't think its a vacuum leak b/c my vacuum stays steady at idle (700) and adjusts accordingly to engine load and speed.

Could it be another component?

Thank you for your help, my eardrums appreciate it!

 

[Alec]

If the new IAC I just got doesn't work, then I'll see about the wastegate. That's a good thought Gimp, thanks! I'm letting the engine cool now so I can put the new IAC on. Hopefully that will be it. I think that's what the problem is, and the supercharger resonates the sound so it's louder than a bad IAC on a stock motor.

 

[Alec]

Replaced the IAC, all better! Splat purrs like a kitten again (a kitten on steroids that is!).

 

[SCEXPLORER]

Originally posted by Alec
Replaced the IAC, all better! Splat purrs like a kitten again (a kitten on steroids that is!).

Is this sound a loud whine sound like a horn. IT only happenes sometimes. What did u do to fix it again? Whats the IAC?

-Ray

 

[Alec]

Yes, the sound was a very loud air horn type drone. I think it was very loud on my truck due to resonance from the supercharger, large intake tube, and large KN.

The IAC is a valve that controls intake air after the TB. Use the search engine to look for "IAC" and you'll find many posts about it.