Good report. As far as what to do next, I think you just have to weigh your tradeoffs. I used to pull my 2500# pop-up from Loveland (north of Denver) to Ouray down highway 285 and across Monarch pass with my '94 3.0 V6. Probably about the same power to weight ratio as you have with your 4.0V6 and a 3500# trailer. Yes, it definitely took some time to get over Monarch - about 30 minutes of uphill at 4000 rpm in 2nd gear doing 40mph, but the little 3.0 just ate that up. Never showed any signs of distress. At the time I also owned a '99 Suburban that would have towed that popup without even noticing it, but then I would have been in Ouray driving the backroads in a Suburban instead of the 4runner - not at all ideal. So, it was worth putting up with an extra 15-20 minutes of travel time getting there to have the vehicle I wanted once I was there. It doesn't really add that much time to the trip - it just seems that way on some of the hills.

I do agree that 3500 lbs or so is probably about the practical limit to tow regularly through the Rockies with a 4th or 5th gen V6 4runner. Leave the 5-6000 lb rolling hotels for the guys with the long wheel base F250 diesels.

@Off2Mtns , thanks for the great detail, it really helped to answer many of my questions, even though I wasn't actually in truck with you (I feel like I was!). Definitely reinforces my preliminary decision to try to stick with a trailer that's around 2500# dry weight.

Thanks again!

Update:

Just got back from a long weekend in Estes Park/Rocky Mtn Natl Park. I installed a Scangauge II to monitor transmission temperatures before the trip and observed the following transmission fluid temperatures from onboard sensors located in the pan (TFT) and torque converter (TQC). I would suggest anyone towing in the mountains spend the minor expense for one of these or a phone app/wireless transmitter to keep an eye on temps. Having a hard time finding anything online from a realistic and credible source re: acceptable trans temps, but I personally am calling 225* for short durations my comfort limit based on this info: Transmission Temperature Gauge: A Brief Guide - Transmission Repair Guy .

4Runner and trailer combo…



Camp…



Disclosure: Note that I have a full set of Shrockworks skids including tranny skid, so I might be running a bit warmer than some of you assuming some reduced air flow and thus cooling.

1. Climbing steep and lengthy grades required S2 transmission position, limiting speed to about 45 mph max, which resulted in about 3500-4000 RPM; observed TFT temps ranged 220*-225*F with TQC temps about 10*-15*F hotter (230*F-240*F); some longer climbs could result in exposure to these temps for 15-30 minutes at a time.

2. Normal driving here, up and down rolling hills with trans in S4 position and downshifting as needed, resulted in observed TFT temps in the 205*-215*F range and again, TQC temps about 10*-15*F hotter (215*-225*F).

3. About as cool as it gets running down the flat interstate with trans in S4 and torque converter obviously locked, TFT/TQC temps are running about 200*F.

Typical sustained temps, climbing grades, trans S2…



Typical temp, normal running around thru town/hills, trans S4, note high ambient air temp…



About as cool as it gets rolling down the flat interstate, trans S4…



Parked, idling to cool, trans P…



(I observed better cooling stopped and in P as opposed to D, as I saw one forum member suggest… ??)

Some thoughts:

1. Really had to keep an eye on the temps climbing the long and significant grades, like most of the pull from Lyons to the top of the hill before dropping into Estes Park. 225* TFT was my max before I was going to pull off to cool down. Never quite got there, but real close. TQC was approaching 240*. At any downhill or flat along the way I was dropping to S4 to lock up the TQC and cooling was fairly rapid.

2. Ambient air temps were approaching 100* in stop and go rush hour traffic thru Boulder and south on Hwy 93 to Golden on the way home. Hard if not impossible to get up to speed to get the tranny in S4 and achieve TQC lock and better cooling.

3. Tranny really needs to be kept in S4 so the torque converter can lock up and cool. It runs decent temps then. With no bottom end power, though, that is a challenge unless you are running flat down the interstate–not a lot of that here in Colorado.

I submitted a query with the observations to Toyota online and will see what they say about the temps and whether or not they will give the green light for a aux tranny cooler install. From what I have seen/read though anything like that will void powertrain warranty coverage which I hate to do on a brand new vehicle–not that burnt ATF is a good alternative.

Tranny aside, I was pleased with the 4Runner’s performance and it is winning over this hard-nosed critic. It towed well despite the lack of low end grunt and having to really rev this thing to find the power. Admittedly, I had pretty low expectations and I think it exceeded them. Had thoughts of regearing, but think power in the 45 mph/3500-4000 RPM range is critical for most CO passes and not sure I want to lose that.

Next trip: South to the Sand Dunes in August. That will be an easier route save for La Veta Pass. Coming home will be mostly uphill albeit gradual.

Wow, you got your Scanguage II set up exactly the same way I do!

@Off2Mtns , thanks for the update, very interesting. Can you remind me how much you think your trailer weighs when you are actually towing it? I just scanned your last post, but didn't see it noted (but may have missed it).

We just got back from RMNP last Thursday, stayed in a cabin near Allenspark. Not a bad place in the world, but jeez there were a lot of people in the park!

Mfr. sticker lists right about 2900 lbs dry, so I figure its about 3500 lbs running down the road, give or take. I've never put it on a scale.

We came home Wednesday. Saw a white TRD Pro badged 4Runner w/ kayak or two on the roof running Old Fall River Road on Monday... that wasn't you was it, lol? Park was crazy crowded, have never seen it so bad, but I imagine the 4th is probably busiest weekend of the year aside from maybe Elkfest in Sept.

Awesome writeup and trip report. Please keep us informed what Toyota says. I still have not installed the trans cooler I bought due to this bs warranty crap. Honestly I don't understand how something that helps the vehicle last can cause it to void the warranty??

Good information. Just to give you an idea, should you choose to add a cooler, you'll see around a 40F drop in temps. I did a 1400 mile road trip this past weekend. Outside temps around 95-100F (south Georgia), 70-75mph on the highway, I was showing a steady 157F trans temp. I've never close to 200F pan temp since I've had the cooler installed, towing or not.

225F max really isn't that bad though, especially for only having the oil to water cooler. Still hotter than I'd want it though. Good luck on Toyota admitting you need an aux cooler. They will probably just go back to the owners manual which states not to tow when it's over 80F outside...

Nope, I had a cargo bag up on the rack and a 2x Kuat on the hitch. I asked a ranger if it was the busiest weekend of the year, and she said it's pretty much the same from now until elk season (I presume because of the limited capacity of the park). We didn't get into town, but our friends who did said it was literally elbow to elbow trying to walk down the streets of Estes.

Yep. We try to avoid Estes and RMNP anymore during the summer. However, we were there in the beginning of June. It was still packed then. We will be headed back up to CO next month, but it will be for family stuff. Hopefully, I will get some time to do some exploring.

Sheesh, useless...

I'm not surprised at the boilerplate answers you got from Toyota. As they said, the customer service people know nothing, and the engineers are not accessible. They're not going to incur extra liability by putting anything in writing beyond what's in the manual and that the lawyers have reviewed.

Here are some engineering based thoughts on this, however. Keep in mind that, while I think the math is good, there may be some errors in the underlying assumptions and there are no guarantees for what follows, but here goes.

- It seems generally accepted that transmission fluid is good for ~100K miles at 180 degF and below, and that the degradation rate doubles every 10 degC (18 degF) above that temperature. So, if you drove 50,000 miles at 198 degF, or 25,000 miles at 216 degF, that would be the same as 100K miles at 180 degF, and so on. So, how can we use this to tackle your problem?

Another way of looking at it is, each mile driven at an elevated temperature is equivalent to a greater number of miles at 180 degF. So, we can construct a sort of a worksheet table here.

For each mile driven at 180 deg, subtract 1 mile from the 100K life.
For each mile driven at 198 deg, subtract 2 miles from the 100K life.
For each mile driven at 216 deg, subtract 4 miles from the 100k life,
234 deg => subtract 8 miles/mile driven
252 deg => -16 miles/mile
270 deg => -32 miles/mile
288 deg => - 64 miles/mile
306 deg => -128 miles/mile.

So, if you drove 10 miles uphill at 234 deg, that would subtract 80 miles from your 100K life. Drive 100 miles at 252 degrees, and you subtract 1600 miles.

Drive 781 miles at 306 degrees and you've used up the entire 100K mile life of the fluid. (781 x 128 = 100,000). That correlates well with what various sources say about the fluid lifetime at elevated temps.

Looking at it this way, you can see that short distances (<100 miles) at moderately elevated temperatures (<250 deg or so) are not really all that much of a problem. It's long distances at extreme temps that will get you into trouble.

This only addresses fluid life. Other issues, such as increased wear or component lifetime at high temps may also be an issue.

@RonJR , I like it... thanks for taking the time to put your thoughts on paper!! Kind of brings it all back full circle to the 50K ATF exchange recommended in the manual if you're towing on occasion as an additional point of confirmation that your logic is sound for more most normal users like myself.

Okay, long overdue update.

Had two more trips around the state last summer in August. One up La Veta Pass to Great Sand Dunes NP, the other to Fairplay for the weekend. Gear I am running: stock motor & tranny, no additional cooling, stock size P265/70 Grabber AT2s, full steel skids and sliders. Monitoring temps off a Scangauge II. Trailer is a few years old 19ft 2900 lbs dry/guessing 3500 lbs wet Forest River MicroLite 19FD.



GRSA trip:

4Runner does okay on the flats going down the interstate, but I am seeing sustained temps of nearly 235* (pan) and 250* (torq conv) any time there is a long decent grade, like the climb up La Veta Pass. The 4Runner just does not have the bottom end to keep the speed up and in a high enough gear to get the torque converter to lock and cool things off when climbing.* No amount of feathering the throttle, dropping into a higher gear, etc will “trick” it into locking either; it can sense that it is still under load. Lock only occurs in 4th & 5th gear. I will also deliberately not exceed 50 mph in 2nd gear as RPM is running 4k+ at that point as it is and I really don’t want to push it more than that for a 30 minute climb; level spots in the road I’ll try drop it to 3rd and try to get it to 60 mph or more, but it bogs and slows right back down once the grade resumes. Really frustrating part is just how quick it gets this hot; I’d say I’m nearing these peak numbers, or maybe a few degrees south of it, shortly into the climb. Thus, pulling over to let the tranny cool for 10-15 minutes is not really an option as I’d be back to peak numbers within a few minutes of resuming driving. On the plus side, when I do finally crest the climb, the converter locks and it cools within a couple minutes to a more reasonable sustained temp below the 220* mark.







Also, I am happy to report that we had gale force winds blowing directly perpendicular to the vehicle and trailer for a good part of the drive home and the Equalizer hitch handled it like a champ; as long as I kept my speeds down to a reasonable level (sub-60 mph, don’t recall exactly now) the entire platform was extremely stable and planted. Anyone considering a trailer this large should put a good WDH w/ sway control on the required equipment list.

Fairplay:

For those of you who have made the drive up Highway 24 out of the Springs to Fairplay know that it is not one of the grand climbs of the state like Monarch or WB I-70 out of Denver/EB I-70 out of Silverthorne up to Eisenhower Tunnel. What surprised and dismayed me was the seemingly flat stretch of road between Hartsel and Fairplay that was apparently just uphill enough to keep the 4Runner needing to stay in 3rd gear to maintain speed, and thus, the converter unlocked, which in turn of course caused the tranny temps to rise back into the 230*/250* range. Meanwhile, the 4Runner just feels like it is constantly being flogged and strained under such a “heavy” load, which is truly only light to moderate by small travel trailer standards and only about 2/3 of the 4Runner’s rated tow capacity.



General Conclusion:

The 5th gen 4Runner with a stock 4.0L just does not make a great platform for towing, at least not at elevation in the mountains with a 3500# full size travel trailer. I NEVER felt like this in the 5 years prior when I towed this same trailer with my 2006 4.0L Nissan Xterra, but was instead always pleased how well it did with such a torquey motor and solid auto trans. Really a disappointment. I personally think a person would be nuts to consider anything heavier than about 3500# here in CO. In its current form, I would not be too comfortable taking it on a long out-of-state road trip with the trailer in tow as it is just tiring to drive, so I just acquired a roof top tent for the extended trips, albeit not a great alternative. We’ll continue to use the trailer in-state and I’ll do my best at driving it conservatively, keep an eye on the tranny temps, and change the fluid more regularly. I’ve considered regearing, but I do not want to lose my current power band in the 2nd gear 45 mph-50 mph range, which is good slogging speed for the mountains passes and I’m not sure it would keep in 3rd gear at 55+ mph as would probably need to be the case with a regear to 4.56s. Intake and exhaust won’t help significantly, and a Magnuson is not in the budget.

Also, I chatted with the head mechanic/shop foreman at my local Toyota dealer, showed him the temps I was seeing, and asked them about adding an aux tranny cooler. He seemed very knowledgeable, proved very patient with my questions, and was not too concerned about the temp readings I was showing him. He said Toyota does not disclose the temp capacity of the ATF. In a nutshell, he echoed the Toyota data we know from the manual–don’t worry about it unless the idiot light comes on. Also, he did not recommend an aux tranny cooler due to our cold winter months and the more likely risk of damage based on too cold of fluid, never mind the fact that it will void your powertrain warranty.

PS:
As a point of reference, I was seeing 210* pan/220* tq conv temps running WITHOUT the trailer, just my wife and me plus backcountry gear, for a Labor Day trip over Monarch Pass. So, trailering seems to be raising the temps an additional 20*-30*. I’m sure my tranny skid does not help either.

Easy $100 solution. Just add a trans cooler and be done with it.

But, as you already noted, you are only seeing ~20F higher temps when towing a substantial load. Not bad really. I wouldn't pay as much attention to TC temp. It's relative to total pan temp (as in you won't hit 250F TC temp while the pan temp is still at 170F), and it pretty much just tells you that the TC is unlocked and you're adding heat. The number itself isn't THAT important. Case in point, my pan temp will be in the 120-130s. If i'm at a stop, the TC temp will be at 150F, just because of the drag.

It looks like you are already doing things correctly, you're at least mindful of the trans temps, etc. But just to remind you, how many failed transmissions do you read about on here? i don't think ive seen any...

Read on other forums about over trucks and SUVs and their temps. Jeep JKs routinely run around 230F for coolant and trans temp for normal driving. Ever seen the TFL "Ike gauntlet" videos? Watch the gauges. F150s will be over temp for coolant and transmission by the time they reach the top. They did a Durango with had the trans around 250F and engine oil at 280F at the top??

This is why automaker either don't give us gauges for temps and pressures any more, or just makes the ones they do give us as highly buffered, so they won't get questions and complaints about true temps.

If the temps make you uneasy, do something about it. No one else is going to tell you it's wrong. I'll tell you it doesn't have to be that way though...