I would thank all the posters and confirm that cleaning the idle air control valve worked for me.
I have a 99 ES300 with 140K miles. I have had all major service done at the dealer and stalling on starting still happened to me. I do confess that I use mid grade gas. I am not sure if this has anything to do with the fouling of the IAC but mine definitely was fouled.
I took off the air intake to the throttle body and took off the solenoid to the IAC. I removed the air assist line to the IAC to see the cleaner come out and also I did not want the cleaner in the engine. The valve stem was VERY difficult to turn. I sprayed GUMOUT throttle body cleaner ($4.50) into the air bypass in the throttle body and into the stem as much as possible while turning. After a minute of two the valve stem turned easily.
Make sure you do not lose the metal ring and the rubber o-ring when removing the IAC solenoid. Also make sure you put them back exactly as they came out. I must have put it back together wrong or lose because the first time I drove it after I put it back together it was revving up and down. I took it back apart and made sure it was together correctly and it works great. If it comes back again I will replace the IAC.
Thanks again to all that have posted! It save me a lot of money. The so called mechanic at a new shop I took it to wanted to replace the whole throttle body!!! Can you believe that? He said a new one with labor might be as much as $1000. I solved it with the help of you all for $4!
