Last post on Jul 28, 2012 at 2:44 PM
You are in the Toyota Sienna
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Toyota Sienna, Van
#8 of 27 Hesitation
Feb 23, 2009 (10:37 am)
#9 of 27 Hesitation
Feb 20, 2009 (7:58 pm)
My 05 Sienna hesitates usually when accelerating after coming out of a turn. Originally, they said the computer may need to be reset. Later, dealer said they did not find a problem but now it is getting worse. Any advice?
#10 of 27 Re: Hesitation [arodory]
Feb 23, 2009 (10:37 am)
Roll in to the throttle gradually. I think a large part of the delay is the trans hesitating before shifting down.
#11 of 27 Re: 2008 Toyota Sieanna - "The Hesitation" [arodory]
Feb 23, 2009 (12:34 pm)
According to info at techinfo.toyota.com even the latest 2010 RX350 still has the 1-2 second "re-acceleration" downshift delay/hesitation. But the 2010 RX350 has new control firmware that attempts to alleviate the problem by watching the driver's actions on the gas pedal and thereby predicting, attempting to predict, the driver's next move/action.
Assume you are accelerating even ever so slightly and accoringly the transaxle is in a lower gear ratio that it would otherwise be at your current roadspeed.
If you now lift the throttle SLOWLY the system will predict that you wish to simply begin cruising along and the trnasaxle will shift into the highest gear appropreate to the current roadspeed.
On the other hand if you lift the gas pedal FAST/QUICKLY the assumption will be that you wish to slow, perhaps using a bit of engine compression braking, and the transaxle will be more likely to remain in the current "lower" gear ratio.
In this latter case, FAST/QUICK release of the gas pedal, and you NOW wish to quickly return to acceleration, there is no gear upshift pending, in process, nor having just completed. So the transaxle is fully "armed"/prepared for even a downshift if that is what is required for the level of acceleration your "new", re-acceleration, gas pedal position dictates.
In about 1998 Toyota/Lexus commited to a major (as it turns out more MAJOR then they predicted) transaxle design change mostly in favor of improved FE. The design change resulted in their transaxle's INABILITY to support two gear changes in quick succession in some situations, mostly if the gas pedal has been released into a position that results in an idling engine or almost so.
#12 of 27 Re: Hesitation [ateixeira]
Feb 23, 2009 (12:48 pm)
No, the delay is the result of either a pending upshift, an upshift in process, or an upshift just having been completed within the past few hundred milliseconds. In that situation the transaxle simply CANNOT support a downshift and so DBW is used to delay the rise in engine torque. The engine torque cannot be allowed to rise until (presumption of engine at/near idle) the ATF gear type oil pump can (re-)pressurize enough ATF to support the next gear change.
To reduce weight, complexity and improve FE the "old" line pressure accumulator was eliminated back in about '98 in order to allow "real-time" control of ATF line pressure. This was primarily the result of the need to provide the extra space needed build more robustness into the Camry transaxle now that it had to serve duty in the upcoming HEAVIER RX300.
But without the accumulator to sustain line pressure the line pressure collapses to near zero with each gear change if/when the engine is idling or nearly so.
Ford has adopted the same ATF "real-time" line pressure control but also adopted a variable displacement ATF oil pump so line pressure recovery can be quick(er) even with the engine idling. That variability also allows them to improve FE to an even greater extend via reducing the pump displacement volume as engine RPM rises.
Ford has a better idea.......sometimes.
#13 of 27 Re: 2008 Toyota Sieanna - "The Hesitation" [wwest]
Feb 23, 2009 (5:19 pm)
Thanks- it started recently and got worst... does that sound right to you?
#14 of 27 Re: Hesitation [wwest]
Feb 24, 2009 (3:08 pm)
Thanks for the detailed write-up.
Most people just look at the number of ratios in an automatic.
I keep telling them the 4EAT in our 2009 Forester is a whole lot more responsive than the 5 speed auto in our Sienna but noone wants to believe me.
I sense that when I go over a speed bump, I let off the gas slowly, it shifts to 4th I think, then I'm back on the gas and it takes an eternity to figure out it should be back in 2nd, 3rd tops.
So basically the Sienna seeks the tallest gear possible to improve FE but cannot conduct multiple shifts back-to-back without a slight delay between each one.
Sound about right?
#15 of 27 Re: Hesitation [ateixeira]
Feb 24, 2009 (6:05 pm)
Not quite. The problem only arises when the engine is near or at idle and the ATF pressure is being, or has just, "just", been exhausted due to a throttle lift upshift. "Ask" for a downshift for acceleration in the above circumstance and for up to 2 seconds you are left "hanging".
Just as in your "speed bump" example.
#16 of 27 Re: Hesitation [arodory]
Feb 25, 2009 (9:34 am)
i had the same problem for years,including "pinging"when accelerating out of a turn,toyota said i was not using the proper gas octane and that if it's not chevron that usually happens,so i started using only chevron 91 octane but the problem was still there.take note that 2004 and later siennas does not require 91 octane,this is one point to argue with toyta,but anyway there is a recall to adjust the computer settings to correct the engine pinging problem and transmission hesitation problem and siennas are supposed to be covered up to 95,000 miles if i'm not mistaken,search google for toyota "TSB"or toyota service bulletins.i took mine back to dealership and had them reset the computer,now mine accelerates very good and i dont notice the hesitation.
#17 of 27 Re: Hesitation [wwest]
Mar 08, 2009 (6:12 pm)
I have the same problem with my 2005 XLE.
So can it be fixed then? Is there a TSB for this?