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Discussion Starter · #1 ·
Hi guys, I am new to the community. I have a 05 sienna have 255K miles, and recently (over the past 3 month), the car won't shift to 5th on highway probably 70% of the time, so I am basically cruising at 4th gear. Also notice some harsh shifting, and sometimes won't downshift in local driving.
I have been doing the trans fluid service (drain and fill) every 2 year since day one using OEM fluid. Went to the dealer, they suggested to replace the trans. Wondering if anyone have some insight about what should I do...

I live in the rust belt, a little bit hesitate to replace the filter cause the bolts are very likely to break and snap and end up getting into bigger trouble; since currently don't have an alternative car to drive.
Not sure if this type of issue is well unknown to 2nd sienna (sorry for my ignorance)
Maybe it's time for a new trans?
Any suggestion will be greatly appreciated.

Thank you!
 

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Unfortunately, you've done all the low hanging fruit. The only options now are an independent mechanic assessment, a specialty trans. shop that isn't a scammer, or firing the parts cannon replacing solenoids, hoping it fixes the problem.
 

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I live in the rust belt
Might be worth checking some of the electrical connections for corrosion associated with the transmission.
Isn't there one on top ? think it is a tranny speed sensor of sorts? Might be others too.
 

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The two items that I would suggest to solve this problem are both inside the transmission pan. The filter and possibly solenoids in the valve body.
Perhaps clean off the transmission pan bolts with a wire brush to notice air condition? They are not in there very tight, so it probably wouldn't take much torque to get them moving. They are, however, difficult to access in a couple of spots. I have found a 10 mm offset wrench to be handy, or a very low clearance ratchet wrench such as the quarter inch gear wrench tiny tool set for about $20. If the bolts are in bad shape, you can try heating the head of each bolt with a torch to harden the underlying metal so that you can put a wrench on it. The remaining bolts that are easy to access can always be turned out with a big vise grip. You'll just need new bolts to go in to hold the pan and gasket in place.
 

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Discussion Starter · #6 ·
update on this, may sound stupid but just want other folks realize this,,, I noticed fan runs really hard during some local driving recently, and the temp sensor seems showing normal temp. Think might be the coolant level is low, checked the coolant, and the overflow tank is empty., I end up adding a good amount of coolant back to the radiator.
After some detailed investigation under the car, I noticed the radiator is leaking slowing and come to a point where significant amount of coolant has been lost. Since the transmission is cooled by the cooler located in radiator, I realized this affect the proper functioning of the transmission. Not sure if the low coolant caused permanent damage to the tranny since I have been driving like this for a lot of miles. Right now, the transmission shifts fine and a new denso radiator just ordered.
 

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may sound stupid but just want other folks realize this,
Nothing is stupid here , at least not for me, we are all here to learn and help others. Hopefully your issue will be totally resolved and thank you for reporting back. So yes the radiator with that many miles is overdue for a change. You may also want to consider preemptively changing the T couplings for the heater system as well as the plastic inlet coolant tube where the thermostat is. You can do a search here on this forum for these items. Here is my post that mentions these. There are others as well.
 
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Right now, the transmission shifts fine...
It reliably goes in to fifth gear in highway driving now? Has the harsh shifting & not-downshifting gone away? If so, that's very good news. Still, with a quarter million miles on the original filter, it should be replaced. Try turning those three hard-to-reach pan bolts out first. If those come out, try tapping on the remaining accessible bolts with a small hammer if they're stubborn coming out. Sort of like a manual impact wrench, shocking the threads between the bolt threads and trans case to give up their hold on each other.

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'04 and '05 are notorious for developing cracks between the radiator tanks and the radiator itself. I ran mine for a year like that. Realize that any leak in the cooling system won't allow the system to develop a vacuum overnight when the engine cools, which in turn won't allow coolant to siphon back into the overflow tank. It's not a catastrophic failure and shouldn't effect transmission cooling since that cooler is placed at the very bottom of the radiator. Install your new Denso radiator and hopefully that solves the low coolant/fans running problem. The van should throw a P0128 code if the thermostat is stuck open, not allowing the engine to heat up causing the computer to not allow the trans to shift into fifth.
 

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Discussion Starter · #10 ·
right now it shift much better, can get into 5th on highway no issues, and most importantly no more harsh shifting/down shifting locally.
Before it always have hash shifting and won't downshift under acceleration some times.
 

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Discussion Starter · #11 ·
Nothing is stupid here , at least not for me, we are all here to learn and help others. Hopefully your issue will be totally resolved and thank you for reporting back. So yes the radiator with that many miles is overdue for a change. You may also want to consider preemptively changing the T couplings for the heater system as well as the plastic inlet coolant tube where the thermostat is. You can do a search here on this forum for these items. Here is my post that mentions these. There are others as well.
Thanks Paul for the info, I will look into that too.
 
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