Engine WTF...changed IAC valve, Platinum spark plugs, fuel filter & PCV valve-still idling rough.

[99 FLA PONY GT]

I have a 4.6L V8 SOHC MOTOR w/101K miles on it. The car is now running rough @ idle 400-900 rpms @ idle.
I have a Steeda tune/chip from 1999 when I first picked up the car which adjusted the shift points on my automatic & also adjusted the speedometer as I changed the rear end gears to 3:73s. I have no catalytic converters & put in MIL eliminators to trick the computer & put in long tube headers an H-pipe & Flowmaster mufflers. Car has run great & smooth w/this setup for over 13 years.
I replaced the original platinum spark plugs with Motorcraft platinum plugs, also changed fuel filter, PCV valve and the IAC valve & the car still idling & taking off rough.
What can be the problem...wished I still lived near Steeda, don't know any good Mustang mechanics in Brooklyn, NY.

 

[Three50won]

Any codes? Are the plugs gapped properly? Are the coil boots connected properly?

 

[trinity_gt]

Has the MAF sensor been cleaned lately?

How old are the front O2 sensors? How long has the car been doing this? Does its onset correspond to the installation of the LTs?

 

[2000fordstanggt]

whats the condition of the EGR system?

 

[99 FLA PONY GT]

To three50won, no codes at all. Plugs and wires ok.
To trinity gt, never cleaned the MAF sensor. I'm the original owner so I don't recall changing any o2 sensors. Its been doing this since mid January when I took the car from NY to Florida. What's the LT's?
Don't know about EGR system.
Is changing to o2 sensors expensive?

Any good Mustang mechanic on here that can pinpoint my problem?

 

[Three50won]

To three50won, no codes at all. Plugs and wires ok.
To trinity gt, never cleaned the MAF sensor. I'm the original owner so I don't recall changing any o2 sensors. Its been doing this since mid January when I took the car from NY to Florida. What's the LT's?
Don't know about EGR system.
Is changing to o2 sensors expensive?

Any good Mustang mechanic on here that can pinpoint my problem?

I don't think anyone will be able to pinpoint the problem. We can all narrow it down based on the symptoms. But ultimately, you will have to check a few different things to get to the bottom of it. It can literally be any of the issues listed above. And there are other issues that could be the culprit. It might even be that a part you replaced may be faulty. Trial and error my friend...rule some things out.

 

[Jhussey1280]

the other thing you way want to look/listen for is a vacuum leak. A vacuum line may have torn during some maintance. Could even have cracked the plastic manifold, but that is probably a long shot.