So yeah, drove about 30 minutes to work and at the last stop light I gave her some gas and it went nowhere at first then finally took off and got me down the road as normal. I have an aftermarket TruCool transmission cooler installed (not in series with the one inside radiator, completely independent) and it seems to be doing it's job, transmission was at 147 degrees when I parked it so that's good. I just replaced the transmission last year with a good used one with 170k miles. The shop that put it in probably used Dex III if that matters but i'm thinking maybe I should flush that out and put in some Type IV that I have at home...and maybe toss in Lucas additive for the heck of it.

I've been researching transmission issues so much it's giving me a headache so what do you guys think? Should I be nervous? What can I do now? Surely the transmission shop used the torque convertor that came with the good tranny and didn't reuse the old one.

Doesn't sound good. I had a former 99 I installed a used auto with moderate miles and only got 3 months out of it. Junkyard auto is a roll of the dice. Hopefully it was just a fluke.
I have a 5 speed in my current one.

Double check your fluid level.

It wouldn't hurt to drop the pan and see how it looks. Might as well replace the filter if it hasn't been done. You could then do the quart by quart flush with new fluid and hope that it was just a fluke.

Good luck either way

Check your fluid level. Be sure you read how to PROPERLY check it. It can be finicky.

I had a trans that slipped between gears and eventually threw a code for a shift solenoid. I dropped the pan, changed the solenoids and within weeks it started slipping everywhere. trans went out completely. I'm going to assume it was beat and fluid never changed so my new fluid just rinsed out the clutch pack.

I put in a trans that had 100k+ MORE miles than my original for less than $200. I filled it completely with new fluid and 30k+ miles later it runs like a champ. It's pretty hit or miss with these.

As others have said, check your fluid using the proper method.
Dex III is perfectly fine.

I'll drop the pan and do an inspection, I have a filter and pan gasket at the house already so i'll use those too.

I think I'll run the Valvoline Maxlife ATF, this time I haven't heard anyone say anything negative about it. Kind of concerns me to put a synthetic atf in when I've already witnessed slipping but i'll give it a go. It's amazing how much info comes up when searching for fixes on these trannys, lots of people with problems and recommendations on how to fix them...sorting out the correct recommendations is the challenge.

I appreciate the responses, I trust your judgement and experience

THIS!

I've owned my 4Runner since 2001 and at least 3 times it has slipped slightly. Each time was starting from cold and upon further inspection the fluid level was about 1/2 to 1 quart low. Usually this happened after someone else had changed the fluid but also my fault at least twice. When I added the trans cooler I learned the sort of hard way that now drain and fill meant 3.75Q not 3.0.

What can you do? Check the fluid level, then drive it till it dies. Leave the trans alone otherwise.

I'm going to check this first thing this morning, thanks for the reminder!

You know, after all the research I've done I think you're on track with your last statement: "What can you do? Check the fluid level, then drive it till it dies. Leave the trans alone otherwise." Seems like the problems don't happen until people try to service it

Those transmission flushes shops always try to sell are crap. Stirs up debris and creates issues. I think its one of the worst thing you can do to an auto.

To State the obvious, when checking be sure to pay attention to the cold level mark and the hot mark. Also don't overfill by much.

In the morning (cold) even being 3/4 a Q low can cause my trans to slip a tiny bit. With the proper level it drives completely fine.

Well, the fact that your transmission started slipping and it wasn't directly following a service, I'd say this doesn't look good for you. I hate to say it, but your first mistake was paying for all that labor to install a used transmission with 170k on it. Sure, you could have gotten lucky and the vehicle that the used transmission came off of was owned by someone who takes care of their vehicles. But, it could have been off a vehicle with an owner that abused their vehicle and rarely did any preventative maintenance. And, even if the used transmission was taken care of, these transmissions have been known to experience an early death, especially the 01 and 02 models. The bearing for the OD planetary gear is the weak point and they implode. There's 3 people I know with an 01 and 02 model that the planetary gear bearing took a shit on them with fairly low miles, under 150k.

Replacing a transmission is a labor intensive job, and unless money was seriously tight, I would not pay to have a used one installed. Now, if I was swapping it myself and I wanted to roll the dice getting a used one to save money, I would only be risking my personal time and not hundreds in labor cost. Like the saying goes "Pay once, cry once."

OP, for your sake, I hope I'm dead wrong, but a transmission slipping out of the blue and you don't have some sort of leak that lowered the fluid level, I have a feeling you'll be replacing this transmission sooner rather than later. Good luck!

If you already have a filter and gasket, drop the pan. If its full of metal shavings, your tranny is on its way out and plan accordingly.

If it all looks fine, replace the fluid (Max life ATF) and keep on trucking.

It may never happen again or it may start slipping like crazy. Not much else you can do but time will tell.

I swear these 01-02 transmissions are made of paper maché. Planetaries implode or clutches wear out fast. They don’t take the abuse that a 96-00 can, if you can even call many of them abused. These are about on par with 4L60Es in Suburbans. My advice is get a rebuilt unit and run Amsoil or Redline in it at factory intervals.

Thanks for all your responses, it's much appreciated. Been a little too hot lately to mess with it but i'm gonna do a pan inspection and filter change soon and install some Valvoline Maxlife. I'm kind of impressed with how compatible this fluid is with other fluids, seems like it will work with just about anything with a Toyota automatic tranny:

https://www.valvoline.com/our-produc...ti-vehicle-atf

https://sharena21.springcm.com/Publi...2-ac162d889bd1

I'm thinking the synthetic fluid may help out the people that run an external tranny cooler as well. Since the fluid is no longer heated by the radiator, the synthetic fluid may flow better than conventional in the winter time. I don't know about you but my 4runner does NOT like to shift when the transmission is cold out

Hate to cause even more overthinking, but Maxlife is pretty odd stuff. It meets Toyota WS spec, but not Dexron VI. You’d think it would meet DVI because they are both thinner fluids, especially compared to Dexron III. Either way I think it is too thin, and got better shift quality from Castrol Import MV.