That makes so much more sense! I'd searched online quite a bit but was never able to find a clear answer on exactly how the transmission cooler was integrated into the radiator. After seeing that pic, it all makes total sense now. I was incorrectly thinking that some section of the radiator fins were dedicated to transmission fluid and the rest to engine coolant. Now I see the transmission cooler is suspended in in the engine coolant and thus, is basically liquid cooled.

That would lead me to think that since the engine coolant is staying at the proper temp, the lower bumper airflow must not affect the transmission temps very much. I'm guessing the transmission is simply producing more heat than what the stock cooler can "shed" to the engine coolant before the trans fluid returns to the transmission. It still seems weird that this whole thing cropped up after the new bumper was installed so maybe that airflow is still a factor in some other way or something else is going on...

Thanks also for the tip about the winch sizing. I guess it makes sense that a full-size footprint would be necessary for the loads of full-size vehicles.

Mostly joking about Warn. $$$$

When I was winch searching I went to each brands site looked at the measurements.
Smittybilt XRC 9500 was the one that fit the limits of AOR bumper might be what your looking for.
Synthetic rope for weight savings.

Can't with the CO bumper. It's so tight already that the only way the fairlead hardware will fit between the winch and the inside of the bumper is to use no washers on the inside, bolt head inside. The nut and lock washer are too tall.

For sure skinny lip profile things are tight winch mounted above frame rails.
And what steered me away from that profile. Winch sets between frame rails mounting foot flush with bottom of rail.
Before there were choices like now and a tight budget makes you do chit 2005.

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Having thought about this a bit more, I'm wondering if airflow through the lower bumper is necessary when the thermostat has shut off the flow of engine coolant? In that scenario, the coolant flow through the lower tank would have stopped and if there is also no airflow to that area, heat from the transmission fluid could build up there reducing the effectiveness of the stock transmission cooler. That might explain why Toyota designed a good amount of airflow in the stock bumper and why I experienced higher transmission temps after the aftermarket bumper restricted that airflow.

On a related note, I also wonder if one of the reasons that turning OD off reduces transmission temps on inclined freeways is because it raises the engine speed. Higher RPMs would result in more heat generated in the engine causing the thermostat to open more. As I understand it, higher RPMs would also cause the water pump to run faster. Both factors would cause more coolant to flow over the stock transmission cooler helping its effectiveness. That's my thought anyway...

Your thinking is generally backwards here.

More airflow blocked to the radiator = more coolant flow through the radiator to get sufficient cooling. That said, if the coolant in the bottom tank is warmer than it used to be (say, 160*F instead of 140*F that it was before the swap) then the trans cooler still has less ability to shed heat. The thermostat is also a continuous control, not some on/off switch. There is always (err, almost always - it will close on a long coast down a mountain in freezing temps) flow in the radiator once the engine is warm.

As for the OD thing changing trans temps - it is 100% due to the torque converter lock/unlock difference. Well proven in not just 4Runners. The OEM trans temp sensor is on the outlet of the transmission (before the cooler and not in the pan) and responds very quickly to different running conditions of the transmission.

-Charlie

Ah yes, I left one thing out of my prior post. I think the CO bumper actually provides significantly more airflow to the top half of the radiator compared to stock (as long as there is no winch or aux lights installed). If that's indeed true for our 4Runners, then the extra cooling up top might somewhat balance out the reduced cooling at the bottom of the radiator making the coolant flow rate roughly unchanged (but creating a relative "cool zone" up top and "hot zone" at the bottom).

Ah good point. Although, from what I have read about general automotive cooling systems, they are typically very oversized for most use cases leading a significantly reduced coolant flow rate much of the time. Even if the coolant flow is rarely turned off completely, wouldn't a reduced flow rate reduce the effectiveness of the liquid cooling effect on the stock transmission cooler? Especially at freeway speeds where significant ram air up top provides sufficient radiant cooling to greatly reduce flow while also producing high transmission heat output? (both of which would exacerbate the lower "hot zone")

I know this is all just theories and guesses on my end without data from real world testing but I'm still curious to hear what you think and am grateful for all the input you've already shared.

And that is another good point. I guess the fact that OD off still significantly reduces temps on 4Runners that have aux trans coolers supports your point as well.

Heat rejection is heat rejection. The BTUs have to go somewhere. There is excess capacity for rejection in the system, but why would you want to reduce it?

Look at any halfway modern OME grill or bumper. They almost are never just "holes". They almost always have a venturi element in the design. That changes the velocity of the airflow in the radiator path.

The comment about less coolant flow through the thermostat vs cfm in the stack isn't a thing either. Unless your in a very cold ambient temperature situation, thermostats are almost always open going at varying degree of "open"
@Option01 , What have you been reading to make you think otherwise?

Thanks for the reply! My understanding was that the thermostat tries to keep the coolant at a certain temperature. The coolant going above that temp can obviously be problematic but keeping the coolant too cool is also not ideal. If your cooling system is not allowing your engine to reach or maintain the normal operating temperature then that is when "you would want to reduce it." At least that is what I've concluded. Let me know if I'm misunderstanding something.

Really? I admit I'm no expert on this stuff but it seems kind of intuitive that a high CFM would meaningfully increase the system's ability to cool compared to little or no airflow. My thinking is that if a high rate of airflow through the radiator makes it cool the fluid significantly more, then wouldn't that result in the thermostat reducing the flow of coolant compared to the same scenario with low airflow? Or does rate or airflow simply not have much affect the radiator's ability to cool the fluid?

My apologies if I'm misunderstanding but it sounds like maybe we're saying the same thing in two different ways?

I was talking about the thermostat reducing coolant flow when the full cooling capacity of the system isn't needed. Isn't that the same as your point about thermostats having "varying degrees of open"? Like "more open = more flow" and "less open = reduced flow"?

So some off-brand versions of the LPD49211 transmission cooler have showed up on Amazon. I'm guessing these are leftover inventory from the same Chinese factory that makes them for Tru-cool. There's no warranty but I'm thinking this kind of part isn't one that's likely to fail and this listing has free returns so I'm thinking of ordering it:

Off brand LPD43211 on Amazon

TrueCool was not made in China last I looked. TrueCool before being bought by Dana Corp was a private company that made the coolers for a lot of other brands…..including B&M. One of the reasons I prefer TrueCool over B&M.

I’m guessing the patent is expired so is free game for anyone to make.


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The manufacturing of is likely why it would fail. Having free return, before installation I'd pressure
test it with compressed air.

Thermostats only know temperature. Nothing else. In a gas engine unless you've done some major setup changes or are in very very cold weather, it's going to open and it's going to flow. Even in up to -20 F I've yet to see a car not open the thermostat while running. Toyotas and Subarus come to temp rather quick.

Diesel engine is a little different story. Except the new ones. They seem to run nuclear hot.....lol.

You want CFM. You want pressure. You want corrosion protection. Blocking diesel radiators air flow in winter assuming your not towing is a thing. Not so much in gas applications.

I got a little sidetracked so I haven't posted here in bit but I wanted to share a small update:

I had the bumper modified to add more airflow to the lower portions of the radiator and I've since been able to take it on one trip to the mountains. This trip was not as long nor as inclined as my last tests but I could still tell that there was definitely an improvement. The transmission temps still got high at times but not nearly as high as before and they always dropped back down pretty quickly. On my previous two trips (since I got the OBDII sensor), the transmission temps would rise too high and then at best, fluctuate around that 210-220 range and at worse, spike to 230+.

My next step will be to install a aux transmission cooler and I should be good to go. For anyone who may be interested, here are a couple pics showing the new vents in the lower portion of the bumper:

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Out of curiosity, why did you size your bracket to fit the 698 cooler rather than the larger 699 model? It looks like both would fit and as long as it was one of the thermal bypass models, wouldn't bigger=better?