2010 Revving Before Engine Turns Off?

I have a 2010 Prius package 5 with ~153k. I have owned it for three years and it's had no problems until now. Forgive me if I don't know all the proper slang as I attempt to explain my issue, but any help is *tremendously* appreciated.

In the last week or so, when the gas engine is about to cut out to switch to the hybrid battery at low speeds, just before it goes quiet from the battery switching over, the engine revs high (without corresponding foot input from me). The engine will be running fine, but then instead of just falling quiet gracefully like it normally does, it sounds like I'm revving the gas hard for about 5 seconds before the engine turns off. It's a very noticeable change.

- Doesn't matter if it's hot or cold out
- Doesn't matter if the car has been driven for 2 minutes or 15+
- Doesn't seem to matter if a/c is on or not
- Possibly related, I'm not getting that great of gas mileage for a while now according to the trip computer...on trips at highway speeds, I get up to 52 MPG or so, but once I start driving at lower speeds and around town, it drops my MPG average down to 38-42 or so. So it seems like my city driving is causing me to get crappy gas mileage (by Prius standards), but city mileage is supposed to be where the Prius shines.

All I've done since I bought it at 125k is oil changes with inspection at the Toyota dealer...I've asked them if I'm due for any maintenance before since I'm into high miles territory and they've never told me anything, just that my car looks new...surely something ought to be wearing out or failing by now?

Any ideas?

 

I'm pretty sure the 12v battery is due for replacement, I haven't replaced it since I've owned the car and I've had to be jumped off a few times from leaving the lights on inside. That said, I've driven it for ~2,000 miles since the last time that happened and this just started within the last week or so.

Could an aging 12v have something to do with my symptoms?

 

Thanks for the advice...if anyone knows of other maintenance stuff I should be doing or if there's anything that commonly fails between 150k-200k, I'd be interested to know. I am considering keeping this car for a few more years, but I'm a little nervous to considering the miles getting higher...I know Priuses are designed to last a long time, but I've never had a car with this many miles.

 

You're awesome! I had a Lexus IS350 a few years ago and knew they did the owner's site / records thing, but I didn't even think that Toyota would have one....durr >.< .....I just pulled it up and it's pretty much as I remembered from the Carfax when I bought it...previous owner was super meticulous and did everything exactly on schedule and always at the dealership, including the 90k and 120k mile factory services.

Don't tell me 150k is trouble territory! *fingers in ears* I wanna keep this thing forever! From what I understand a perfectly maintained Prius is a pretty safe bet even into higher miles. I expect some trouble here and there but hopefully nothing costing many thousands of dollars all at once, knock on wood....