We have a 2002 Highlander that has about 150,000 miles that is having similar problems. In the past 6 months it has happened 3 times to my wife, and today it did it to me for the first time. The vehicle starts but will not idle. After starting if you push down on the accelerator pedal it will run perfectly smooth at various RPMS, but as soon as you let off the pedal it instantly drops to a very low RPM and dies. After starting, reving, and dying a number of times I tried turning the ignition key completely off, waited a minute and then re-started, and that seemed to cure it, although it still idled a bit slow but it did not stall. That made me think this is not a fuel problem, but rather some sort of electical sensor issue that is not reading that the engine is not yet warmed up or otherwise not reacting to a cold engine.
I called two local Toyota dealers and they both told me that unless the check engine light came on, there would not a problem code registed for them to dowload and diagnose. They also both told me that unless it happened to do it to them while in the shop, they would not be able to figure out what was wrong.
In checking my Haynes manual for this SUV, it appears to me there are two likely possible culprits. The screw in coolant temperature sensor, known as the ECT, which runs about $30 for a new one, and is easily accessible...... or the Idle Air Control valve (IAC) which runs about $250 new for a non Toyota brand, and I think I read it costs $350 from a Toyota dealer. I've read a few posts here and elswhere saying you can possibly clean that valve. It is located underneath the throtle valve assembly, which means you have to take the entire throttle body valve off, which appears to be a bit time consuming.
I think I am going to try purchasing the $30 temperature sending unit first, as it is the cheapest and easiest to replace, and see if that cures the problem. If not, I may try the IAC next or just wait until it does enough to make the check engine light come on..... by the way, sometimes I see this IAC valvle refered to as a "motor", but I am thinking they are one and same device. If anyone has any direct experience in fixing this low idle / cold engine stalling problems on 2001, 2002, or 2003 Highllanders let me know.
ajbakaitis@msn.com