River Reese

Alright, so a buddy of mine bought this 2006 mustang 4.0. Being a personal mechanic for friends, he brought it to me to work on and what not. Well, after doing a tune up on it, which included cleaning the MaF sensor and throttle body, and boring out the cats, I have a idle issue. This car idles just fine from cold to hot. But when you start driving, whether the car is Cold or hot and you put the car in neutral to coast, the engine revs from 1k rpm to 2k rpm and back down and so on until you either completely stop or you engage the clutch. Does anybody have any idea what could possibly be doing this? As far as what I can tell, this car has no Idle Air Control Valve and I get good response out the Throttle position sensor and Throttle actuator, though the values are a little high from what I've seen on other vehicles.

Dino Dino Bambino

Look for a vacuum leak.

River Reese

I have yet to find any on it. Though I did notice that while driving and it's in gear, if you're going along and just let off the accelerator, it will stick at the rpms it is at for about 5 seconds then start to decelerate, doesn't matter if youre going uphill or downhill