Maybe you don’t go down many mountain grades. There’s absolutely a reason...

Regarding temps, I encourage you to try it yourself sometime, same grade once downshifted once not, you’ll get cooler temps. Not sure what speed fan clutch engages/disengages but whether it’s fan speed or water pump flowrate

You've clearly never played with highly modified vehicles with cooling issues.

Higher rpm always = more heat unless your cooling system cannot properly cool the engine when stopped/idle.

Overheating at speed because of lack of airflow from the fan isnt a thing. It just isn't. Overheating at idle while stopped (when a fan matters - as in, the fan is doing all of the work to pull or push air) due to lack of CFM is a thing.
Likely, higher rpm (so long as your cooling system is up to snuff) does not translate to higher cooling capacity.

Fan clutches are mechanical based on temp and/or engine speed (disengages at higher speeds and engages at higher temps).

The ONLY caveat i can come up with would be that your cooling system is struggling, the tstat isnt ever closing, and the root problem is masked by the higher coolant volume moving through the radiator at speed. However, that means theres another problem.

Lot of bad information here...

For starters... No, I’ve replaced my radiator and thermostat, I know how the cooling system works and no, the fact that I’m seeing cooler temperatures is not a sign of a greater problem. My car runs a cool 186-188 during normal operation (thermostat is 82C). It makes perfect sense, you’re just being stubborn.

The fan clutch engagement lessens with increased rpm (more on that to follow) and yes, you will generate more frictional heat at a higher rpm. However, when going downhill in gear, especially a higher gear, your ECU will cut fuel from the injectors if the throttle is not engaged and the rpm is above a certain speed. That’s why it’s more fuel efficient to go downhill in gear than in neutral. In neutral the injectors must continue to burn fuel to keep the engine from stalling. So yes you get more frictional heat but you get less combustion heat. So no, higher rpm does not always mean more heat generated, only more frictional heat. Also, your cooling capacity overall does increase at higher rpm. Ultimately your cooling capacity is what dictates whether or not your car will overheat (talking extremes here). You don’t blow head gaskets because you’re running a car at 2500 rpm lol. The engine is always generating heat, the cooling system is what saves it. Obviously this is the extreme end, but you get the idea. Increasing rpm on the water pump will also help cooling as you’re passing more mass flow through the block and the radiator (even after the thermostat closes there’s slight bypass via jitter valve). Finally, the fan clutch is not an on/off switch. At higher rpms your fan still spins faster, the slip just increases. You seem to imply that above a certain speed my fan will not go any faster. Simply not true.

Anyway, whether or not the engine is getting cooler is a totally moot point, maintaining control of your vehicle and avoiding warping your rotors is the real reason you do it. Going down a grade, especially on a one lane road with a full load or in icy conditions, you absolutely can not ride your brakes. The people on here claiming there’s no reason to do it clearly have not experienced these conditions. If there were no reason to do it or if it were bad for the trans, do you really think the engineers would let me select a lower gear? Don’t you think the FSM would say something about it? C’mon lol. There’s absolutely no harm in downshifting with an automatic, try it sometime.

So you agree that engine braking will create more heat then? "However, when going downhill in gear, especially a higher gear, your ECU will cut fuel from the injectors if the throttle is not engaged and the rpm is above a certain speed. That’s why it’s more fuel efficient to go downhill in gear than in neutral. In neutral the injectors must continue to burn fuel to keep the engine from stalling. So yes you get more frictional heat but you get less combustion heat. So no, higher rpm does not always mean more heat generated, only more frictional heat."

Higher flow rate does not always equate to better cooling. If this were the case then a cooling system without a thermostat would stay cooler because there is no restriction. Half of the job of the thermostat is to regulate the flow of coolant to allow the coolant enough time to absorb the heat from the motor. Coolant/water mixture doesn't absorb heat as quickly as pure water does. "Increasing rpm on the water pump will also help cooling as you’re passing more mass flow through the block and the radiator (even after the thermostat closes there’s slight bypass via jitter valve)."

You will reach an rpm range where the fan clutch will slip at a higher rate than the engine rpm increases effectively limiting the fan clutch speed. "Finally, the fan clutch is not an on/off switch. At higher rpms your fan still spins faster, the slip just increases. You seem to imply that above a certain speed my fan will not go any faster. Simply not true."

It's safer in low traction conditions to use your brakes. The reason behind this is your brakes have ABS/Stability controls and your engine does not. If your engine braking exceeds your traction then there is no automated controls to help regain traction by increasing wheel speed. "Going down a grade, especially on a one lane road with a full load or in icy conditions, you absolutely can not ride your brakes. The people on here claiming there’s no reason to do it clearly have not experienced these conditions."

If all else is equal, more flow always makes better cooling. Always. It’s given by the equation Q=mcdT. m is mass flowrate, Q is heat. (Again, all else equal here). The thermostat will regulate flow rate rate of the coolant along with the water pump. You always have a very slight bypass of the thermostat with that jitter valve, then once it cracks you get a lot less dP across the valve (more flow if the pump is running). So the rpm on that water pump matters to coolant flow rate (along with thermostat).

Do you have some fan clutch data? Everything I’ve read says even fully disengaged you get 20/30% of shaft speed, which tells me fan rpm is never truly capped.

You won’t have any brakes to stop you in the first place if you ride them down an entire grade! That’s the entire point lol. And my point still stands, it’s even safer to be in lower gears in low traction scenarios. Why do you think your transfer case further gears you down upon engaging 4Lo lol.

I'm not suggesting the fan clutch is on/off. In short the purpose of the fan is to move air when natural flow is reduced (ie, driving at speed). Otherwise it IS a hindrance to the cooling system (drag on the engine and resistance for that natural airflow).

Fuel isn't cut completely at deceleration, otherwise the engine would shut down. It does, however, have a deceleration function/table that reduces fuel injector duration. Engine rpm decreases due to the throttle body shutting and effectively giving the engine little to no air.

Fuel and spark continue to be delivered- only exception to this is the rev limiter, which depending on how the sytem is set up, can be a fuel and/or spark cut.

Try removing your thermostat and driving around for awhile to tell me how well your higher flow rate works for you.

You completely missed my point regarding low traction scenarios. I also know how to brake during long grades and don't engage them the whole time. Let's say you're using your "engine braking" while going down an icy grade and your engine braking exceeds the traction of the road. All of a sudden you're doing 40 mph on an icy road with the rear of the vehicle sliding (possibly all 4 sliding) what do you do? It's a curve so you better think quickly! Too late you forgot to put it back in drive and instead panicked and hit the brakes further worsening your slide, hit a guardrail, went tires to Jesus a couple times, and now you're having a really bad day. Am I exaggerating? Yes. Is it still possible? Also, yes.

While we're at it, there is no such thing as warped rotors. What is misconstrued as warped rotors is really an uneven application of brake material caused by heating the rotors/pads up too much AND not letting the cool properly before parking the vehicle or stopping for an amount of time. Don't believe me? Take a trip to your local google and research it yourself. Enjoy your weekend and I'll be back on Monday

Do you know that for a fact on our 4Runners?

Most modern cars do cut fuel for efficiency and emissions (though I don’t know about our 4Runners specifically):

automotive engineering - While going downhill, does my car consume less fuel when in neutral or in gear? - Engineering Stack Exchange

Engine won’t stall if it is being back driven by the movement of the car (i.e. through the trans and tc). If you back drive the engine via wheels turning and the transmission there is no need for fuel.

I drive a lot of sports cars and am used to downshifting.. that being said I have an automatic 4runner and will use it to downshift, probably higher rpms than I should, but it works well.

I live in an area with lots of hills and my gtr required a brake job after 7 months at the cost of $4k ��. 4Runner needed a brake job after 3 months..

No. Toyota ECUs are locked down. While you can make some internal component changes (like reclocking the crystal) you can't modify them like with an Epsom based system. It's also why if you boost the motor you have to use a 7th injector and external system to control it or go full standalone.

However I can deduce it doesn't cut fuel. This is based on a typical 90s Toyota ECU system. Then only "upgrade" is that i doesnt utilize an additional injector for the cold start sequence (something they used on popular engines like the 4a, 5s, etc) - but in short it's a pretty rudimentary EFI system. I haven't torn the injector wiring on one but I'm willing to bet it's a batch system (not all at once - which they have used as well, but they 'graduated' from that in the early 90s). Toyota uses a 4 wire TPS on these which is actually unique (most manufacturers use a 3 wire system - ground, 5v ref, and a potentiometer signal - assuming the lowest signal is the TB being shut... Toyota has a 4th wire that registers the TB shut and tells the ecu ro, possibly, be in idle mode there's more ro the idle circuit, but still). The ignition system I'm guessing is a wasted spark as 2 cylinders share 1 coil each.

In short - this system is pretty simple. Newer advancements like direct injection, sequential injection, COP (and the control those give) just weren't available or used on these rigs.

I'm done, we're way off topic here.

My $0.02.

I downshift all the time. Its second nature in my 5speed but I also do it on the steeper switchbacks in my auto. As a rule of thumb the steeper and slower you are going, the more you should use engine braking. I personally have never warped a rotor but in tourist season I smell burning brakes at least once a week. I run engine load on my ultraguage. Under engine braking once the rpms get over 1700 the ecu goes into open loop and the engine load drops from 14%(idle) to 7%. It does it in both the auto and the 5 speed ecu

I practice downshifting whether in my manual transmission Tacoma TRD or automatic transmission 4Runner or Subaru Outback. And many other vehicles. I do so to stay off the brakes on grades.

I’ve done so for so long I never give thought to RPM, it just comes natural and my engine speeds are not excessive. I use the engine/transmission to brake on pavement, dirt, in 2WD, 4WD and low range if necessary.

Never had engine problems, transmission problems. Brakes on all my vehicles have lasted the life of the vehicle, most in my possession five to twenty years, up to 300,000 miles.

I didn’t realize this was an issue. My truck has 315K on it. I tow this short distances. Should I be worried?

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I upshift it at 6500...

Living in Colorado, downshifting on steep grades is (or seems to be) mandatory.

Some of the posters in here are way overthinking it. Downshift to whatever gear you need, generally try and keep your RPMs below 3,500. I mean, what's the alternative; riding my brakes all the way down i-70?? That's crazy talk ...

I live on a plateau and there are two ~mile long very steep hills that drop me down towards town. One has a speed limit of 45 MPH, and I can either ride the brakes all the way down or click off O/D to keep speeds around 45-50. The other is steeper, and has a limit of 35MPH. I general drop down into second on this hill.

Both hills have a stop light immediately at the bottom...so there is no room for the brakes to cool before needing to fully stop. If cars ride the brakes all the way down, then sit at the light at the bottom, it's a great way to warp the rotors and/or leave pad deposits which cause brake pulsing.

I've also been on some wheeling trips where engine braking is mandatory due to 5+ mile constant grades dropping 4-6k feet. That will cook the brakes in a hurry.

Like you, I don't let my engine braking RPMs get above 3k RPM.

I try not to engine brake unless I really need to though. Brakes are certainly cheaper than a transmission.