Idle rpms go up then drop back down over and over

[Drachnem]

I just bought this mustang and I'm not huge into mechanics so maybe you guys can help me out.

My mustang when Idle after a few mins starts to go up in rpm then back down over and over.

What is doing this and how can I fix it?

 

[Kampfer03]

It could possibly be the idle pulley.
I had the same problem on my 98.

 

[Ogre]

I would look at your throttle position sensor (TPS) or idle air control valve (IAC). I had a 94 coupe that when started up when cold, it would idle up to around 2000 rpm then slowly idle down to around 700 rpm, where it should be. Also, driving it down the street it was like the cruise control was set to 35 mph.

I changed out the IAC, and was good to go.

 

[Drachnem]

I cleaned out the IAC and let it idle for about 10 mins, then blasted the heat while going from park to drive seems to have done the trick, I haven't got the chance to take it for a road test yet either. I also cleaned the MAS