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2016 Toyota Corolla - Transmission revs downhill

Driving at posted speed, in town or out, control by foot or cruise control, while moving down a fairly steep hill, the engine suddenly races to very extreme RPM. Hasty response with brakes quickly regains control. Local dealer reacts with disinterest.

Out of warranty, try an independent shop.
The dealer charged you for a diagnosis then stated “disinterested” on your report?

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There was a recall years ago on older Corollas for unintended acceleration but nothing as new as yours. Offhand I’d guess a glitching throttle position sensor.

Could that be a motor mount issue?

This is normal behavior. The transmission is downshifting to slow the vehicle or maintain speed. The high revving is a result of the downshift. You aren’t losing control of the vehicle, the vehicle is keeping you from running out of control. Because your Corolla has a fairly small engine, the engine braking effect is minimal unless the transmission downshifts and the engine braking is done at high rpm.

Let me ask, does your car suddenly accelerate when the rpms jump up? Or does it actually start to slow down or maintain speed? To many people the sound of a high revving engine while not pressing on the gas makes them THINK they are speeding up.

I could be wrong, but I think I have diagnosed your issue and you have nothing to worry about.

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You might have something there but that’s only true when using cruise control. The OP mentioned that it happens whether he’s using cruise control or not. If I’m going downhill in my Corolla without cruise control it just goes faster with minimal change in engine RPMs. The other day I deliberately took a route to work that involved hills to make sure. The OP’s Corolla probably has a CVT transmission, which my Corolla doesn’t have, and that might be a factor.

It is part of the programming of the CVT. It’s all over the internet and Toyota service writers almost universally respond to this type of inquiry “this is normal behavior”. My Mazda has a six speed automatic (not a CVT) and it does not do this unless cruise control is engaged. Different transmission, different behavior.

I once rode with my cousin in her Sentra with CVT.
The rpm rose when applying the brakes going downhill.
That makes sense as braking assist.

Is this new behavior?
It did not rev before on same hill?

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I wondered the same thing. Perhaps the car is a fairly recent purchase? If not, either the OP hasn’t noticed until now or it’s a new development and therefore a potential problem.

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We had an older Corolla, it had a button on the side of the shifter that put the car into “hill” mode. See if yours has something similar, may have been engaged by accident.

Toyota technicians and service managers respond with “This is normal behaviour” because… this is indeed normal behaviour.

The more modern a car, the more programming and sensors are used to control shift patterns in an auto trans - CVT or otherwise.
The first car that surprised by shifting down a gear when I crested a rise and coasted down the other side was a 1996 VW Golf VR6 hatchback. I initially thought it was faulty then quickly realised it did it consistently under the same, or relative, conditions of speed & throttle position.

BMWs from the mid 90s performed like that too. Definitely not my 90’s Toyotas tho’.

It seemed to take the Japanese (generalisation here) about 15-20 years to do the same thing consistently across the brands so it shouldn’t be surprising that some are reporting “my Corolla doesn’t do it”. Technology changes.

I’m one of those and I allowed that CVT equipped Corollas might behave differently. That said, we still haven’t heard whether this is a new issue or a new-to-the-OP vehicle.