Previa (supercharged) will intermittently, without any apparent correlation to temperature, humidity, outside air temperature, etc., hesitate in acceleration. But is is not the classical type of hesitation: oh no, if one sets the throttle say about 3/4 of the travel, the engine will hesitantly, like it was too rich, accelerate to a point, then hesitate again, accelerate again and repeat the cycle until it reaches the car speed determined my the constant throttle setting. On other days the car is a relative rocket ship, for a van, and accelerates normally without problems. Steady state in cruise is fine and the car gets 24 mpg with a full load so it would appear a constantly rich mixture is not happening. This behavior does not produce any check engine lights. If you graphed the engine acceleration, it would look like a set of stairs. This behavior is worst under load, as in going up a steep hill and often the cruise control will kick off as the engine is unable to supply the necessary power within the cruise control limits. You can not regain cruise control again unless the engine is shut off. The transmission shifts fine. Suggestions?
You may have other problems as well but the throttle position sensor is the link between your foot and the ECM which regulates the fuel injection system. I would have it checked by a good independent mechanic. Have them check the fuel pressure and fuel filter as well.
Thank you Missleman,
Mechanic has replaced fuel filter, pressure is good. OBDC2 readout says throttle sensor working correctly. The trick/problem here is the lack of codes. Sometimes the hesitation is such that it is actually pretty dangerous, for example trying to accelerate onto the highway and getting nowhere fast.