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Discussion Starter #1
I believe that there have owners to have replaced thermostat. From the youtube, it looks that the access space is very tight.
Just wander it should send to dealer/Garage to do the job, or DIY?
 

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I think you should do it. Who knows? After a few minor successes you might become unstoppable. No longer an easy mark.
 

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It is not that bad a job. Need to remove things out of the way to access the three nuts/bolts. Take pics as you go along. Use toyota thermostat, red coolant and change the rad. cap as well .
 

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I haven't looked at rovi's link, but just to share my 2 cents worth...

After having removed all the necessary piece-parts to get to the area and draining the coolant, loosening the two bolts for the starter and sliding it back slightly will make life MUCH easier if you remove the black plastic inlet pipe. You might not need to remove it, but if this is the original inlet pipe on your 2004, do yourself a BIG favor and buy a new FACTORY pipe (part no. 16321-0A040) and replace it. They have a nasty habit of cracking with age.

The best place to mess up is to not pay attention to the proper placement of the jiggle valve (shown in the attachment here).
Also, you don't want to put the Hulk Hogan on those three bolts. 71 in-lbs is not a lot of torque.
For what it's worth, the seat for the rubber gasket on the aluminum side was not completely "clean" after removing the old gasket on mine, so I took a Q-tip and some acetone to clean up the residual bits of the old gasket and get it nice and smooth before re-installing the new gasket. Having the inlet pipe out of the way was very helpful for this.

Highly recommend getting yourself one of these before refilling the coolant:

Had to do some minor plastic trimming on the green adapter to get it to fit right on the Sienna, but SO totally worth it.
You can certainly fill, run, burp, fill, run, burp, several times, and get it done, but this device just makes it easy.

NOTE: The factor thermostat does NOT come with the gasket. Here are all the part numbers for the 2004.
Thermostat (90916-A3003)
Gasket (16325-62010)
Cap (16401-62090)
Toyota_Sienna_Thermo_Placement.jpg
 

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You can easily replace the Thermostat. but you need to remove some things out of the way first. Like Battery, Air Cleaner Box and I think I also moved the starter out of the way can't remember . You SHOULD replace the plastic housing tube while you are at it. That thing if it has not failed yet - is going to..

I believe that there have owners to have replaced thermostat. From the youtube, it looks that the access space is very tight.
Just wander it should send to dealer/Garage to do the job, or DIY?
 

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Discussion Starter #8
Thanks very much for your reply.
The torque is only 71 inch lbf? on one online, it has mentioned about 15 to 20 ft⋅lbf, which is corresponding to 180 to 240 inch lbf.
Normally, when I change oil filter, I also use 120 inch lbf. Should I use 120 inch lbf for the thermostat?
 

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I admit that the first time is tricky when getting the thermostat housing (long black tube thing) out of the way of the thermostat and when trying to feel your way to the backside bolt that you can't see. The first time takes a while but it's really easy once you know... I've done them 4 times now.
 
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