One thing you can do is to ohm test the TPS ground. Typically the ECU half of the TPS can achieve a "0" ohms, less than point five (0.50) is marginally acceptable. You can jump the ECU half of the TPS straight to battery ground for a test.
The TCU half of the TPS typically has a little resistance inside the TCU that you may never get to "0" ohms.
There is a built in adjustable bypass for the throttle plate. The adjuster is covered with a lead slug. Most times it comes from the factory fully closed, but some have it opened a little. If it is adjusted slightly open from the factory, clean out the passageway. Trying to adjust the idle from this bypass is futile, the IAC will just re-adjust again and you are back where you started.
I typically end up with 0.6 volts on the ECU side of the TPS and 3.8 volts on the TCU side of the TPS. If yours is way out try splitting the difference and see what happens.
Vacuum leaks can also mess with your idle, enough of a vacuum leak and it will eventually idle high, but a small leak and the motor management may try to compensate and sometimes over compensates and lowers the idle. I usually start out checking for vacuum leaks when troubleshooting idle issues and re torque the intake/exhaust manifold.
The throttle plate should be adjusted until it just barely clears the TB neck. My guess is so it doesn't (over a period of years) wear a groove into the TB neck. I typically adjust the throttle plate so I can slip a small piece of paper through the gap.
Also check the valve cover CCV vent tube coming from the rear of the valve cover. That engineered leak is part of your idle.
If all else fails you may have to use that throttle plate adjuster, clearance about the thickness of a Nickle is a good place to start.
I eventually got mine to idle right, it took me around 2 years and I finally found the solution by accident. The catch is I repaired numerous cable faults at the same time and I am unsure which one was actually causing my issues.
Side note, it took me awhile to figure out my harness had the ECU half of my TPS wired to ground through my TCU and the TCU half of my TPS wired into the ECU ground circuit. Somebody got their wires crossed. Made in Mexico by the lowest bidder.