Why does your 1999 Toyota Sienna turns over but then die right away If you give it gas it will run but then dies when you let off the gas again?

Sounds like fuel filter clog or inconsistant fuel pump. Have you recently replaced the battery? If a memory saver wasn't used during the install, the computer has to "re-learn" how to run. Here's one method I've used to "teach" it faster... 1. First, (sounds strage), disconnect the battery, and leave it disconnected for at least ten minutes or so. 2. Now, reconnect the battery, then turn the ignition on , BUT DON'T START IT! Leave it on for a minute, (mayber pump the gas a few times), then restart it. 3. You might still have a few problems for a little while, but it should do away. The reason I have had to deal with this problem is because I've replaced a couple hundred batteries as an Auto Zone employee, and it always seems to be Toyota's or cars that have Toyota engines in them that have problems restarting after new battery. Good Luck!!