defiance

So recently my engine has been running a little rough during idle, i didnt pay it any attention because i did not think it was serious. Yesterday i changed the battery in my car, got in cranked it,all good then 2 seconds later rpms drop to about 250~300 and the engine feels like its about to cut off and then in a few seconds goes back up to 500 but does not stay there. The rmps keep going up or down. So i figured maybe its my IAC valve and decided to clean it. The problem is that i cant find its location.

Will cleaning IAC help?If so where is IAC located? Is there anything else that could be wrong? By the way the car has 53k miles on it, all stock and i have NO check engine light yet.

defiance

Is there no one that can help me, or point me in the right direction with this issue?

Mr. Ugly

Make sure your MAF and TPS connectors are tight.

Check/clean your MAF and throttle body. Also, replace your fuel filter if you haven't done so every 24k miles.

Probably your ECU lost all its adaptive when you disconnected the battery, and it's trying to figure out your settings, but everything is too dirty.

Mr. Ugly

4.0 doesn't have IAC if I remember right. It just cracks the throttle open slightly.

Charlie321

This may be a bit of a long shot, but is your AC coolant running low? If so, the AC compressor will cycle on and off, causing the car to suddenly idle high, then low, etc...

Your car's probably too new for this to be the problem, but it may be worth a look.

defiance

The car still idles rough, after cleaning the MAF. yesterday check engine light came on and it ended up being a bad TPS, well that got replaced. The engine is still idling rough and pulsating, and when the ac is turned on the vibration amplifies. The car is not under warranty and i really dont want to pay$ 92 for them to 'try' to find the problem.

Any other help would be appreciated

gmchx4

sounds like a fuel issue, almost like the motor is starved for fuel.
my thoughts:
replace fuel filter
clogged injectors
run a bottle of complete fuel system and/or injector cleaner in the next tank of gas

just my thoughts, good luck ...

Mr. Ugly

If you are still running the original spark plug wires, they may be bad.

At about 30k miles a couple of mine had really high resistance.

Check your plugs too. Owner manual says you can leave them for 100k miles or something like that, but to me that's way too long.

You can use a decent inexpensive Autolite 5143 and change them every 12k miles or so. The X-charger guys use those, and change/check them at about 8k miles.

No need to get any fancy coils. No real gains there. Decent replacement wires can help if the original ones are worn.

I'm using MSD wires. Other people like Live Wires. Even standard replacement carbon wires are fine if you check/replace them every 24k or so miles.

Before even any of that, make sure that your plug wires and connectors are fully seated.

Mr. Ugly

Also, have you cleaned the throttle body yet?

At 30k miles, it's pretty dirty. If you have 50k w/o cleaning, the deposits behind the throttle blade can throw off the idle air flow.

JCON

Not sure if its the same with the Mustang as my old Passat but I had an issue with the engien starving, ethanol in the gas ate through the seal in the EVAP valve, which was used to pressurize the fuel system. So everything I got gas my car wouldn't start for about 5-10 minutes. Also, another cause of rough and fluctuating idle is the idle air tube (don't know it by same name in a Mustang) This is the tube that sticks out of you intake tube and runs to the rear right (when standing in front of the car and looking to the back) of your cylinder bank. This tube provides the air to the engine while idling, if it gets cracked, clogged, or disconnected you will know immediately. You car will sputter while idling and may even shut off. Check both ends of this tube (or hose for CAI's) for connectivity and make sure its not cracked. Other things to check are your TB with some carb/TB cleaner. I'm at 25k on my car and it still looks spotless on both sides.