I have a 2004 Toyota Sienna the has a slight hesitation when accelerating. It is most noticeable from idle and/or low speed. There are no DTCs indicated on my scan tool.

Welcom to 10w40.com.

There could be several potential causes that ill need to be tested and replaced if faulty. Air flow meter and contaminated fuel might be the cause. Dirty fuel injectors and a faulty ignition system would be the last thing to check.
When accelerating the injector on time should increase and the calculated load reading should go to 85 percent or more. Use of the correct NGK spark plugs is a must on this vehicle.

Tests/Procedures:
1. Monitor the airflow meter signal using a scanner. On wide open throttle check to see if the calculated load reading is 85 percent or more.

2. Check to see if the vehicle is going lean on acceleration. Use a gas analyzer and dyno if available.

3. Monitor the injector on time to see if it increases when the throttle is opened.

4. Add an external fuel supply to the engine to see if this helps the acceleration.

5. Check the fuel quality fuel pressure and fuel injectors if the vehicle is going lean on acceleration.

6. Check the spark plugs to make sure they are the correct type of spark plugs.

Good luck.