Where is it located at on a 4.6 2003 expedition
The Idle air control valve (IAC) is on the back of the throttle body housing on the 4.6 V8, it's half the size of a can of Red Bull. Try tapping on it with a small hammer and if the truck clears up then it's time to replace the IAC.
You can clean it with Throttle body injector but that's just a quick fix for now. Good luck and hope this helps.
Hope this may help.
On a 4.6, it is located underneath the plastic "4.6" cover on top of the engine. It looks like a shiny silver tube with a cast "L" shaped fitting connecting it to the air induction system, has a two-wire harness running from it, and is held in place by two bolts, about 7 or 8mm in size.
If you buy one, make sure to also get the mounting gasket or be prepared to use old gasket.
Disconnect negative terminal from battery. Two reasons, safety, and the engine computer has to be provoked into calibrating the new IAC to work with your engine. Disconnecting/reconnecting the battery will accomplish this.
Unbolt the three bolts (12mm, I think) that secure the plastic "4.6" or "5.4" cover to the top of the engine.
Find the little SOB. It's on the driver's side, looks as described. Unplug the two-wire harness from it. Unbolt the old IAC (two bolts, either 7 or 8mm) and remove the IAC and the old black mounting gasket.
Bolt the new IAC in its place. Don't forget the new gasket! Plug the wiring harness you unplugged, on to the new IAC. Put 4.6 cover back on. Reconnect negative terminal to battery.
Start the engine and let idle for fifteen minutes (Don't touch the accelerator!) to allow the engine management system (ECM) to learn how much voltage the new IAC needs to perform it's job. Note: Turn off the radio before disconnecting the Battery, this will not lock your radio and or read the manuel to disable the radio security codes.
Keep me posted, glad to help.
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Where is the air bypass valve located on a 2004 explorer 4.0 4by4 
A computer-controlled air bypass valve located in the throttle body. It helps control idle speeds by allowing air to bypass the throttle plates as to increase the idle spedle Speed Actuator
Idle Speed Actuator Operation: 
It regulates the position of the throttle valve plate(s) to compensate for additional loads on the engine, like the air conditioner. It measures engine RPM readings, compares them to the reference RPM in the engine-control computer memory and adjusts the idle air by-pass.ed.
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could it be beside of it, instead behind it?
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