You said you had a code for the IACV. Has the code returned since you replaced the IACV?

Can you let us know the actual code? A code doesn't necessarily mean a component like the IACV is bad. There are other causes for an IACV code to set and may be the source of your problem.

 

 

Start with the simple stuff first. Bleed the air from the coolant again. See video below for a good DIY.

I'd take of the snorkel to the throttle body and clean the bore of the throttle body with throttle body cleaner. Clean both sides of the throttle plate to make sure it shuts completely. You can get a buildup of carbon that blocks the plate from closing.

Do you have a scanner that can read live data?

 

You said you did the reset/relearn process, but did you adjust the base idle to 550?

 

 

With the IACV unplugged from the wiring harness, it should be basically closed (even if it's operation is flaky). You can check whether there's any suction from the small port in the wall of the throttlebody. If you cover that port and still can't reduce the idle, then the problem isn't the IACV.

If you can't get the idle down low enough, then I'd look for vacuum leaks. I'm guessing that you've already done that. One difficult vacuum leak is the brake booster - people don't always look that far away from the intake manifold.

Another thing is checking whether the throttle plate actually closes fully. Just in case someone in the past has adjusted the mechanical stop-screw on the throttle crank. That should never be used to adjust the idle. It's only supposed to prevent the throttle from wedging itself stuck closed.

 

Vacuum leaks...
Try these one at a time, to narrow down your search. If your idle drops by disconnecting & plugging any hose, then you've narrowed down the vacuum leak to whatever system you plugged off.

Disconnect & plug the hose going to the brake booster. NOT for driving that way, because you won't have any boost for your brakes.

Disconnect & plug the hose for the EVAP valve. I think that valve is on the firewall hidden behind the intake manifold plenum. Probably will throw an EVAP code.

I'll try to think of other possibilities, but 1999 didn't have the miles of vacuum hose that you'd find 10 years earlier...

 

Try Rock Auto first. I believe they get a lot less for it.

And I agree with Jim, look for vacuum leaks as well. Could be something simple as a split hose, or a bad hose end. Did you split the intake manifold and clean out the EGR passage? If so look for leaks at the split in the manifold too (gasket leak?).