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2004 Sienna CE 97k miles
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Discussion Starter · #1 ·
first time towing with my 2004 Sienna this past weekend, it went okay without any hiccup, I didn't find the van struggling at any point. Trip was about 200 miles with some small section going uphill, took about 4 hrs, boat/trailer combined maybe 2k lb. I was monitoring the trans temp (odb2 reader with bluetooth and Torque app for the phone), have couple questions.

1. the first couple hrs, temp stayed between 180 to 200 F or so, then later it went to 220 or so and stayed there. When climbing hills, it occasionally went up to 235. Is this okay, should I be worried?

2. when going uphill, is it better to downshift to 3rd and on use 4th instead D on leveled road? what about going downhill?

3. should I be looking at external trans cooler, I know the rad. already has an internal cooler.
 

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1. How hot does it get under the same conditions when not towing?

2. Do you find you have to downshift often or does it downshift on its own? I’d use manifold vacuum as guidance. Low single digits, downshift. You might be able to do the same with MAP sensor values through Torque but I don’t know what to look for.

3. Definitely get an external transmission cooler.

4. Do you have enough road clearance with the tongue weight? The hitch doesn’t hit the ground on driveways and ramps?

Sixto
‘04 LE FWD 207K miles
 

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Discussion Starter · #3 ·
1. How hot does it get under the same conditions when not towing?
when doing my normal everyday commute, it's between 180 and 190, never above 200.

2. Do you find you have to downshift often or does it downshift on its own? I’d use manifold vacuum as guidance. Low single digits, downshift. You might be able to do the same with MAP sensor values through Torque but I don’t know what to look for.
It does it on its own. I downshifted only once just for experiment and then back to D after the climb, other than that I didn't touch the shifter at all.

3. Definitely get an external transmission cooler.

4. Do you have enough road clearance with the tongue weight? The hitch doesn’t hit the ground on driveways and ramps? no problem with clearance so far (even on driveway). I didn't launch the boat yet only picked it up from the seller.

Sixto
‘04 LE FWD 207K miles
 

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2. when going uphill, is it better to downshift to 3rd and on use 4th instead D on leveled road? what about going downhill?
Generally speaking, the only reason to shift out of D when towing is if the transmission is shifting a lot. My 2013 Honda Pilot when towing our camper would struggle shift into 5th, then back to 4th, then to 5th. Like once a minute, on essentially level ground. Shifts wear out the transmission, so you don't want it shifting excessively. If it does, manually select a lower gear. If not, leave it in D. If it only shifts when climbing a hill, that's normal, and fine. But if you are on a road with lots of rapid ups and downs that cause it to shift a lot, you can manually downshift to reduce how much shifting it has to do.

When going downhill, the only benefit to downshifting is to prevent excessive heat buildup on the brakes. If you're going down a mountain, then downshift to use engine braking and reduce how much brake pedal you use. Or if you see a sign saying "trucks downshift" you may consider downshifting when towing as well. Otherwise, you can just leave it in D.

when doing my normal everyday commute, it's between 180 and 190, never above 200.
Seems kinda warm for commute driving, but I do not have a 2nd gen myself. Could be the 2nd gen just runs hotter. My 3rd gen gets to around 130 °F for my normal driving, here in Wisconsin winters. My Pilot was around the same, about 145 or so in the summer. When towing our hardsided camper, it would often go to ~225 in the summer. Hanging around 220 is probably OK for towing, though less would be better if you tow a lot.

As a reference, when I switched from the Pilot to a 2020 Silverado 1500, and got an even bigger, ~9000 lb hardsided camper, the truck transmission was usually around 190 when towing. Much, much better transmission cooling on a truck.

Finally if you do a lot of towing, probably change the transmission fluid more often.

Out of pure curiosity, what's your engine coolant temp when towing and when not towing? On my Pilot, normal engine temp was around 180 °F. When towing, once the trans temp got to 180, the engine temp started going up too, and stayed at the same temp as the transmission, probably since most of the transmission cooling was from the engine coolant to transmission cooler. So if the transmission got to 225, the engine coolant was 225.
 

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Discussion Starter · #6 ·
Seems kinda warm for commute driving, but I do not have a 2nd gen myself. Could be the 2nd gen just runs hotter. My 3rd gen gets to around 130 °F for my normal driving, here in Wisconsin winters. My Pilot was around the same, about 145 or so in the summer. When towing our hardsided camper, it would often go to ~225 in the summer. Hanging around 220 is probably OK for towing, though less would be better if you tow a lot.
i think i misspoke. in my commute, the first 25, 30 min, it's usually around 160. Then when i get off the freeway and towards the last couple miles, it then jumps to 180/190.

Out of pure curiosity, what's your engine coolant temp when towing and when not towing? On my Pilot, normal engine temp was around 180 °F. When towing, once the trans temp got to 180, the engine temp started going up too, and stayed at the same temp as the transmission, probably since most of the transmission cooling was from the engine coolant to transmission cooler. So if the transmission got to 225, the engine coolant was 225.
i didn't check the coolant temp on the app, but the temp gauge on the dashboard stayed pretty constant the whole time, slightly below the 3 clock position.
 

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Towing 2000 lbs, you don.t need an auxiliary transmission fluid cooler. Your temperatures are good. You need to get worried if it goes above 240F.
Put the transmission in 4th and leave it there, it lets the engine run in a more efficient RPM and torque range, and it keeps the transmission from constantly down and up shifting (basically the same as the Tow/Haul mode on a pickup). There won't really be any MPG penalty doing this, in fact, it might be better.
If you tow with it frequently you should do a transmission drain and refill every 25k to 30k miles. Use Toyota T-IV.
Put some Lubegard Red Automatic Transmission Fluid Protectant in it. When I did this my transmission temperature dropped about 10F when towing. It also improves the transmission fluid's heat tolerance, heat transfer, and life.
 

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I am not doubting you but how did you derive this temperature ?
From long-term experience in the automotive industry and from my early drag racing experience. It has been kind of common knowledge. It is my understanding that bad things start to happen to transmission fluid when it is subjected to temperatures exceeding 240F for any length of time. I picked this up many years ago, I don't know where exactly.
 

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I’m towing quite a bit more than you and agree with most of @wag3719 points. You don’t need another trans cooler (your 2004 has one already) and your temps are fine for towing.

I would keep it in 5th unless it starts hunting, then drop to 4th. Downhills, drop it to 4th or 3rd as needed for engine braking.

I did install an additional trans cooler on our 2007 but I am towing a small travel trailer that is probably close to 3500 loaded. My temps are pretty similar to yours… I did get to 235+ on a long uphill climb at 9000+ ft elevation.

Do change your trans fluid regularly (drain and fill is fine). I do a drain and fill on mine very 15k-20k miles.

-Mike
 
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