It really sounds like a vac. line issue to me, the only other thing I can think of is if the throttle cable got moved around some (but really shouldn't have changing the plugs).
Some of the 4.7's have unused vac. ports that are capped off, it's quite easy to knock the rubber plugs off, they are on the drivers side of the intake, just behind the throttle body (where the other vac. lines connect).
If they are OK, I'd check the PCV hose, it connects to the passenger side rear of the intake, I've knocked these off a few times changing plugs.
If that's OK, then I'd pinch the vac. line off one at a time where they come out of the manifold, it's possible that one knocked loose on the other end, if that makes sense to you. If the RPMS drop when you pinch the line off, follow it and see where it may have come loose.
Also check to make sure that the brake booster valve didn't get partially pulled out of the brake booster, that is also easy to do.
So basically , double check it for any type of vac. leak that may have happened while doing the spark plugs.
Let me know how you make out, and we can take it from there. Please don't click accept unless this answer is helpfull to you.
The slot at the bottome of the throttle body is the passage for the idle air control motor, the computer opens and closes the motor change the amount of air getting to the engine (and therefore the idle speed).
When you have the resonator chamber off the top of the engine, it always sounds loud.
If the idle speed was OK before you did the spark plugs, and now it's to high, the engine is getting extra air from somewhere. The most logical place is one of the vac. lines.
You can try unhooking the battery for 5 minutes, this will reset the computer and force it to relearn it's idle speed, but if your running in the 2000 range, it's not going to help.
I'd put the resonator / intake assembly back together and on, it'll make it eaiser to hear the vac. leak
If that doesn't help let me know and I'll opt out and see if another expert has any advice for you.
I'm sorry, I really can't think of anything (other than a vac. leak) that could have happened during spark plug replacement that would cause a high idle condition...
I'm going to opt out and open the quesiton up to other experts, hopefully one will think of something that I'm missing.
HiCustomer welcome to Just Answer!
Leo is right about finding the source of airflow. Your fuel injection system simply adds fuel to whatever air makes its way into the intake system. So air leaks can compute into higher engine speeds.
I'd suggest blocking the throttle body air intake with a piece of cardboard or the like. Whatever you use, it needs to be fairly stiff to stand up to the pressures it's going to experience.
Pull the air cleaner off and start the engine. Plop the cardboard on the whole face of the throttle body intake. If the engine stalls, you will know that the airflow is getting through this point.
So narrow the focus. The bottom of the throttle body inlet is where the idle speed air is metered. Block off the upper portion where you can see the air valve. If the engine slows considerably (but doesn't die) the throttle blade is being held open. Check the throttle cable for tension. There should be a little slack at idle to allow the blade to fully close. If it's held taut, something along the way is preventing the throttle from closing.
If blocking the idle air passage kills the engine, pull the idle air control (IAC) motor out. You may have a piece of debris preventing the IAC from seating and slowing idle air control. A typical source in some vehicles is this crummy little bridge that protects the intake air temp sensor element. The IAT sensor is located in the air cleaner body to the right of the throttle body. Regardless, any object found in the IAC passages shouldn't be there.
Last, Leo's concern about an open vacuum port is still a possibility. It's not always obvious. Sometimes there are dummy ports that can accept a large vacuum line and look like it's properly connected while the actual open port is still open to atmosphere. If the throttle body block test still allows the engine to run, this is where you need to focus your attention. The ports in question are mostly (if not all) on the vehicle's left side of the area just back of the throttle body.
Give that a try and let us know how it works out.