Fixed transmission overheating issues!
 

I have a 2019 5th gen with a supercharger, 34's (soon going to 35's), 4.65 gears, and I am EXTREMELY heavy. I also have full skids (which don't help with tranny cooling). Since forever I had my transmission overheating like crazy. 250 was normal Offroad temps for me. 220 on highway I didn't bother to care anymore...

I posted a few threads about this. I have also met people on the trails that have similar issues. So I figured I'd share my experience and solutions:

1st I installed the CSF radiator. My engine temps are good but it promised to increase tranny cooling. It didn't work.

Then I drilled a ton of holes in thought out areas in my skids to increase airflow. That helped by a few degrees.

Then I installed a tranny cooler with a fan (to keep fan going when crawling). That helped a bit.

At this time I was still averaging 200 at highway flats. 220-230 on steep mountain passes. and 210 - 250 Offroad.

I would flush the tranny fluid after every 3-7 day trip (monthly). Half the time it would be burned only after 500 miles of [hard] Offroad driving.

*I flushed to spec (waited till 110 degrees on flat surface and drained excess out).

I then installed a second tranny cooler (with no fan). That helped substantially by lowering all the above specs by 10-15 degrees. However, I was not able to maintain normal temps all the time and once I got to 220+, it wasn't coming down until I stopped.

Magnuson SC gives you a tune for the SC. I contacted them a few times and always got the same answer... they will not customize the tune. I reached out to some tuners that are familiar with Magnuson and our industry and a custom tune was over 1.5K. Plus there is no guarantee it will work.

I was recommended modifying/installing aftermarket torque converter but the cost was over a few K. And no guarantee that it will work.

I purchased a cooler pump that would force the oil to move through the coolers faster and help cool faster. I was warned it may ruin the stock torque converter so I held off on installing it. I figured once my tranny get's worse (as it was already acting funky with only 18K miles)... I would see if the Cooler pump works. If not, I will need a new tranny anyways and at that time can install the aftermarket torque converter for cheap as the tranny will be out anyways.

Then I talked to a friend who works at a cement factory and told me how they have a ton of HD machinery that runs extremely hot. The way they keep operating temps for the oil is by putting in less oil than recommended...

So... I figured why not. I did a full flush (my 6th time!) and put in the exact amount of oil recommended by Toyota (110 degrees, flat ground, remove excess oil...). I drove around for a day and then went back to my tranny and removed a about 3/4 of quart (I was aiming for 1/2 but why not...).

Then the weirdest thing happened... my tranny is now running very cool. I did about 900 miles to Baja and back home after this experiment. I did 500 Offroad miles in the mountains that normally I would be running 250 degrees.

On flat pavement now I am running around 160. On mountain passes on pavement I can't get it about 190. Offroad I am running 180 on average at high speeds where before I would hit 230+. When pushing her hard Offroad I touched 220 once in Baja. Whereas I would have been at 250 1/2 that day in Baja before.

Now I think I may have too many tranny coolers!!

IDK if this will help anyone or is useful information. I don't know exactly why this worked for me or if it will work for you. However, I figured I'd share this as I was at a point where I already made peace with getting a new tranny soon...

FYI... all my tranny clunks and weird shifts also went away...

Just forked out for the URD supercharger and was thinking about the CSF radiator. I think I will hold off on the radiator until I see a problem. I am mostly stock so may not have any issues.

Did you try removing the armor to see if that made a difference? I could imagine less fluid would mean the existing fluid would be circulated more and be less exposed to the heat trapped under-body.

I’m thinking either the low fluid level does something to prevent this “flex lockup” mode on the TC or there was some kind of bubble stuck that was either restricting flow or causing turbulence in the fluid as it was going through the coolers.


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more fluid means it stays hot longer once it heats up. so the theory checks out right?. i just wonder how low can you go??? lol



sounds like you mob the sh1t outta this thing. props.

When you added system capacity, thermal expansion of the fluid increased but had no where to go. You were probably running much higher fluid level when hot than the transmission was designed for…

Yes on armor. It helped a bit. That’s where I got the idea to drill holes in the armor.

Once holes drilled… I’d estimate it was close to the same as without armor…. But without armor had same issue… once heated up it stayed hot.


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If I get 50K out of this tranny… I’m stoked. I am probably way over GVWR when on 5 day expeditions. Something has to be the weak point. Can’t have it all! I’m getting the RCLT kit installed, have a Dana 60 on order. I basically made the tranny the weak point knowingly. I figure a new tranny every 50+K is OK with me. At least it’s not a part that fails and can’t get me off the trail as usually a bad tranny can still get you home.

So if this “fix” gets me over 50K…. That’s just bonus.

I WISH 4runners had a manual. This would be a much more manageable problem then. The overdrive and gear shifting still does its own thing.


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You sound wayyy smarter than me. My hunch was something like that. The coolers I added somehow messed with the system. However I had this problem before adding the coolers….


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ohhhhhh the RCLT kit is baller