I have a 2004 mustang 3.8 with 66k miles. a few months ago i noticed my car was running rough at idle in the mornings, id rev it a little bit and then it would be fine. I figured no big deal. in the last few weeks its gotten progressively worse, i would try to smooth the idle by reving the engine bit but this time no luck and it continue running rough at lower rpms. and every few days its been getting worse and worse. i use fuel injector regularly, change oil every 3-4k miles. do all normal maintenance etc.. i changed the spark plugs and wires last week thinking it would help. it seemed to at first but now its bad again and the check engine light came on about 5 days ago. i unhooked the battery to reset the computer, check engine light went out and it ran fine for 2 days and now the check engine lights back on and its running terrible. it would run good after a bit of driving but not now. Im really at a loss.. any help would be much appreciated!

 

You need to have the code read. It will probably point you in the right direction. Also, use the search feature of this forum. Odds are someone else has had your exact problem.

 

When they check engine light comes on for the heck of it take it to Auto Zone and they'll check it for free. They check a lot of other parts for free. If it's the EGR, It could also mean bad hoses coming from or to the EGR. Not necessarily the EGR. I have a 2002 V6 and it's pretty frustrating to figure out things sometimes. Auto Zone says on the website they can test Throttle position Sensors, ONLY OFF the car when I asked at the store. Well I have Loctite'd screws on mine and they ain't coming off without being cut off or whatever. LOL Max is right, Go get a code read, Or it'll drive you freakin nuts. Try Auto Zone, Unless you're in California. They don't do it there.
I had a 2008 Dodge Avenger that did symptoms like that and it was the fuel pump.

 

Vacuum leak? Its one of those things that gets worse w/ time b/c the leak gets bigger? of course a code read is required before much can be said.

oh, im in california and auto zone pull the codes for free all the time, so does pepboys. i guess some part of cali are different.

 

Well I guess what I read is a bunch of bull. Sorry about that.
I have a 2002 V6 and I sympathize with you. I bought the car 2 months ago and the idle is better. BUT which thing made the idle better? I don't because I did all these little things in one day. HAHAHAHA I have the jump forward when I shift to drive or park still.
Ok Mine was revving high when I started it in the morning. I have a flowmaster and it was louder then it should be. I'm sure my neighbors loved it. Lol
This might not help but this is what I did and now my idle settles down better when cold. Like I said it may not be a solution.

I changed the hose under the IAC and also past the T. That's the one that goes in the passenger side wheel well and goes to the purge valve. It was rotted REAL BAD. I didn't bother taking the tire off, I just unscrewed the shield that goes around the tire and slipped my hand up there. You'll need about 3 feet of hose for that one.

It's called an "EVAP RETURN TUBE. It's a tube that fuel gases go from the Canister to the Evaporative Emission Canister Purge Valve, THEN THE MANIFOLD. I looked up how to take off the Purge valve. So that led me to find out I need to take the tire off and remove the right front splash shield when says it has three screws and a five pin-type retainer.

I cleaned the IAC with carb cleaner. I'm STILL iffy about it. I did find a cheap Motorcraft one on Amazon for $58.00.

Cleaned the MAF.

Checked the grommet,Changed PCV hose, and PCV

Check your plugs for a bad gap. Mine were spaced out bad. I had champion plugs and I switched them to all copper Autolites.

I ran some Chevon Techron through it.

See if your oil smells like gas. My old plugs were misfiring and since we changed the oil and plugs it's a little better. My husband put in 5-30 this time instead of 5-20.

I have a new Throttle position sensor but we have to cut it off. They the factory Loctite'd the screws. I got a Motorcraft one from Amazon for $40.00

I am going to install a new Engine coolant sensor which I was told this. This is a post I made about it. This effects your idle also. It costs $14.00 and free shipping at Latemodel Restoration.

"CHT sensor should be located on the back of the drivers side cylinder head. It will be a 2 wire sensor just above where the block and head meet. If you locate the last spark plug and follow the head towards the firewall and reach around you will feel it." I was told that the Engine coolant sensor is the same part as the Cylinder Head temperature Sensor. Only the Engine coolant sensor is saturated by coolant, The other no."

I Haven't replaced the other vacuum hoses yet, Those plastic ones.