99 Mustang GT, 5-spd

When I come to a stop at a red light, the idle is fine, right around 800. After a couple seconds, it dips down to around 500, then back up to 800. This was annoying and slightly concerning before, but I've recently performed the IAC rev mod (https://mustangforums.com/m_2625353/tm.htm). I've since have the revs drop below 500, and the car's even died a couple times (fires back up no problem).

The behavior is similar to a radiator fan kicking on, only much more extreme. Any ideas on what this is, and how to correct it? I'd love to keep the mod, as I love the sound and nature post-mod.

Thanks guys!

 

try opening the hole on the gasket slightly until it runs right, mine dips as well every now and then I know a few guys where the same thing happens, not sure if its normal but I'm assuming so.

 

I have the same problem with my car, started happening after the s/c and tune, only fix ive come up with so far is to set the idle at 1100, anything less it dips and stalls out.

 

ya'll don't think he's idle air controll vavle going ****ty??

 

So while this may be a common problem, I have a hard time imagining that this is supposed to be. What/where is this idle air control valve, and how do I test it? Is there some other reason for the drop in RPMs?

 

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what the ****...

 

SERIOUSLY WHAT THE ****

 

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