This has happened twice now. My 2000 toyota sienna runs fine until I went to add gasoline. Upon startup, my idle was immediately much lower. I can rev throttle and it sounds smoothly until I release the throttle. After shutting off the engine and restarting it, I had normal idle and was fine afterwards. This situation only happened after adding gasoline. I'm speculating if there might be a vapor pressure sensor issue or something regarding the emissions to cause this. I;m considering taking it to a shop, but wanted to see if anyone had this kind of issue.