Won't hold idle, unless pressing gas pedal.
 

I'm posting this more to help others that may encounter the same problem I've been banging my head with for the past few days. I'm preparing my 99 ES300 for a long drive from Texas to Michigan, so I've been spoiling her the last few days to make sure everything runs perfect. However, after I cleaned my idle air control valve, the car wouldn't start- which made me panic because I start my new job in Michigan next week... so it's kind of crucial that my car starts. Anyway, it would turn over just fine, and then just immediately die. Just would not hold idle at all, I cleaned IACV again, same results. I had kept it idling by pressing the gas pedal to see if it was a relearning issue, but as soon as I let go of the gas pedal- sure enough died again. Since engine was hot now, waited for it to cool down and pulled out both O2 sensors and cleaned them out (desperate at this point). STILL same thing.
So I take out the IAC one more time, and whatever side the little (cylinder/rotor/not sure proper name) is turned to, I turn it to the opposite side. I put it together and go to start the car and she's alive!! the IAC must have been stuck closed, so wouldnt let any air through to idle. But since it's not used if I'm pressing the gas, it was running fine. Granted, now it was stuck open, so the car was idling super high, like 1500-2000, but my baby is back in action, and I couldnt be more relieved. Ordered new IAC for around $120 off RockAuto- Airtex Part #2H1150

TL;DR- If car won't hold idle, your IACV may be stuck closed. Remove the two screws holding it in place, turn the little dial looking thing the other way and try to start car again.

Did you tamper with the TPS?

Nope. Received the IACV this morning, replaced it and car started up with no problem.
IAC should feel springy as you turn it, if it moves whichever way you turn it, need to replace it.

Congrats!

Drive safely to MI.

Congrats on getting the problem solved.

I like how you say you "just pulled the oxygen sensors out" and "remove two screws holding it" as if its that easy.....the Oxygen sensors are usually really rusted in there, and if there's a Philips screw that is worse than a Honda rotor screw, its those damn IACV screws! :)

Hey man im happy you posted the issue you were having with es300, i believe im having the same issue with my 2002 lexus es300. I both the car about 2 months from a private dealer and everything was fine until a month went by where i was experiencing rough/low idle, loss of acceleration, and eventually a failure for the engine to start but it was cranking. I only got a few codes that had come up which were camshaft position sensor, tps, air idle control valve, and air intake temp sensor. I had 2 mechanics check my car and they both could not figure out the issue. They told me the timing was on point, i was getting enough fuel pressure, compression was good, so basically they couldnt figure out why the car wouldnt start. i hope you get to read this message because it's beating my head i cant figure out how to get it started my plan is to replace the idle air control valve, do you know exactly where it is located , i cant seem to find it, is it built in to the throttle body itself or is it somewhere on the side. Thanks have a great thanksgiving!

My guess is something isn't plugged in. I've worked on a couple 98s and every plug and connector is brittle. Check the throttle control motor, mafs and tpms sensor are plugged in.