Loss of power and rpm drops at 40mph

hi everyone, I have a 2012 tacoma V6 w/ 167k miles.for the past 2 weeks I've notice when the speedometer reach 40mph,the rpm drops and engine lost power. but once above 40mph everything is back to normal.there is no CEL,idle is smooth and accelerates fine. 3 months ago I did some tune up.changed the spark plugs, air filter,radiator flushed and refill, oil and filter change, transmission fluid drained and refill, replaced serpentine belt, rear diff fluid drained and refilled. Last week I changed the MAF sensor and it seem to help a little bit but still happens at 40mph. I'm trying to rule out the possible cause of the problem.has anyone have the same issue.pls. advise. any help is appreciated.

 

Can you replicate the problem when leaving it in different gears?
3rd gear?
4th gear?

 

I think it happens after the 4th gear but I’m not sure? I was also thinking about transmission issues. Torque converter? I hope not cause I won’t be able to do this job.

 

Did you clean the throttle body; may want to throw a bottle of injector cleaner in the gas tank.

 

The last time I cleaned the throttle body was last March 2021 w/ 130k.I'll clean the TB again and hopefully it will fix the issue. I'm afraid to put additives in the gas tank cause i did that to my Nissan Frontier and messed up the fuel injectors.Thank you Sir for the advice.

 

Put it in 4th gear. It the problem doesn’t happen in 4th. Cleaning the throttle body won’t help. We need to isolate the issue before we can start diagnosing.

 

I have an automatic transmission so next time I drive I will count which gear it would happen.

 

I hope I did not overflll or underfill the transmission fluid?I don’t remember how many quarts of fluid I put in.

 

You can put it in 4th gear. Move the gear shifter to 4th.
That will prevent it from going into 5th (OD)
Same thing with 3rd gear.

 

How did you refill it?
Put back in the same amount that came out?
Or did you do it the correct way by putting it in Temp Check Mode and then checking the level?

 

As said above just put the gear selector in "4" instead of "D". This way it won't shift above 4th gear. Around 40 mph is sort of a "no mans land" where the truck wants to shift into 5th gear but it doesn't have enough engine speed and torque until you get more speed. If I'm driving at a steady speed of around 40-45 mph I always just leave the selector in 4 until I reach a place where I can go faster. There may not be anything wrong with the truck. And if there is something wrong this will eliminate one possibility.

 

Thank you guys for the input.I'm learning a lot from your responses. it hasn't got worst and I'll see what I can do and I'll post if there's any changes.

 

I got 187k miles on my Taco. now I'm hearing a humming sound starting at 30mph.I checked the u joints and their tight but the center support w/ the rubber bushing has some play. do you guys think I need to replace to support bearing? I also added Lube gard instant shudder fix 2 weeks ago and it still has a shudder at 40 mph. I don't know what to do?.btw no CEL