Replaced Spark Plugs Bc Shuddering; 2 Months Later Shuddering Again + Dead Hybrid Battery 2010 Prius

TLDR: Oct '24 took car into shop because check engine light on & car shuddering. Shop replaced spark plugs. Dec '24 check hybrid system light on & car shuddering again. Shop diagnostic P0A80 replace hybrid battery pack. Next steps for a total newb? Worth repairing the car? Head gasket, EGR, intake manifold?

Car history:
My sister's 2010 Prius (it's my ex-car ) has 235k miles on it. Bought in 2016 from original owner with 115k miles. I drove it through 2019, she's been driving it since. Other (relevant?) info: inverter failed in 2020, Toyota rebuilt on their dime. Small fender bender in 2021 (rear ended the car in front of her at low speed). My reliable mechanic shop inspected the vehicle afterwards: minor cosmetic issues (body shop fixed dent in hood, repainted); check hybrid system light on, ran diagnostic & got code DTC P0A85 indicating battery pack cooling fan malfunction, cleared the code, said it's all good. Those two scenarios are the only time the dashboard's ever lit up like a Christmas tree until now. Replaced engine oil & filter July 2024. Regular oil changes (well, up until this year, so I just discovered).

Current issue:
In October 2024, the dashboard lit up, "check engine light", car shuddering. Took it into the shop (different one than previously), diagnostic showed starter battery was bad and spark plugs needed to be replaced. Got new starter battery. Replaced spark plugs. Car ran like normal.

Then mid December 2024, the dashboard lit up, "check hybrid system", car shuddering. There's also a knocking sound with the shuddering. Did hybrid diagnostic, P0A80 replace hybrid battery pack. Tech noted the engine(?) shuddering in his notes, shop did not investigate engine further because check engine indicators weren't on and the first order of business is dealing with the battery pack. The shuddering is most pronounced when the car first starts, especially in this colder weather. Struggles to accelerate. Test drive today got to 55mph (didn't want to push it any further), but not a smooth ride. Shuddering calmed down with driving, but the car was straining. Later restarts of the car had lesser and lesser shuddering. Was not present for first start up today; will observe that tomorrow.

Also just learned she got behind on her oil changes, oil is 1.5L too low. She's fixing that as I type this lol

The dilemma:
She commutes for school & work. Initially started researching buying a used car because shop quoted prices that totaled the Prius. After dumping $1k into it back in Oct for the spark plugs, she felt like she was at the break point where it made more sense to start over with something newer vs keep throwing more money at the 16 year old car.

Unfortunately, prices for anything decent & reliable are obscene (and out of her college student budget). Can't find private sellers, only people flipping cars. Debating what the best option is. On the one hand, swapping out to something newer has the potential to last longer and get her through the next few years. At the same time, anything she buys will still be several years old, we won't know the history of the car, could end up dealing with same issues. With the 2010 Prius, we know the history and what maintenance has/hasn't been done.

No one in my family is mechanically inclined, so not sure what's really going on and thus don't know what the cost to repair the 2010 Prius is. Hard to tell if it's better to repair or get something new. Currently have a company scheduled to come to us & replace the hybrid battery with a refurbished hybrid battery pack (in the $1.5k range; shop quoted $4.5k+). Will probably do this whether she keeps or sells the car; necessary fix if keeping, improves sell-ability if going that route.

What I've researched:
Be forewarned that this represents hours of research by someone who does not understand cars lol I have learned a lot though! But I'm sure my knowledge gaps are wide and I don't know what I don't know. I've watched lots of the videos y'all have shared on other threads and read through tons of helpful info. Struggling to apply to my circumstances though.

The three issues I've seen pop up all over this forum and elsewhere is the Head Gasket, Intake Manifold, and EGR components. To my knowledge it's the original head gasket, and neither the intake manifold or EGR components have ever been cleaned. From my research, all signs point to one of these three causing the shuddering issue. I've reached a base level understanding of the three. Mainly not sure how to proceed.

What makes sense as far as order to investigate?

Do we clean the intake manifold, EGR components (this is the EGR tube and EGR cooler?), and replace the PCV valve for good measure to start? DIY might be an option, I've been watching lots of videos y'all have shared, and my sister has a friend who is willing to help us through the DIY process. However, my sister and I have no car repair experience, and I'm not involving my dad because mechanical issues make him throw wrenches across the yard and expand our vocabulary with colorful new words.

If addressing these parts is the way to go and we realize we aren't cut out for the task, what do I need to convey to a mechanic that I want done? Do they do their own investigation and clean/address these parts as they feel necessary? Or do I need to communicate here's specifically what I want done?

If we clean those components and the car still shudders, is the next step to investigate the head gasket? How does one determine if the head gasket has a problem? I've read about head gasket issues leading to compression issues, which lead to an arm (I cannot for the life of me remember the actual part name lol) bending and eventually punching through the side of the engine. Is there a way I can verify there aren't holes in the engine? Obviously I would look at it lol but what would I be looking for and where? And if there aren't holes, how do I tell if there is an issue but there's not *yet* holes? If there are holes, is the engine a goner? If the arm has already bent and has been knocking against the side of the engine without holes, how do I tell if it's fine or if it's too late to save the engine? Also if an arm is bent and is knocking against the side of the engine without holes, does replacing the head gasket make everything okay then? (I don't need specific answers to each of these questions lol Just wording things in different ways to give an idea of where my knowledge gaps are)

As an aside, my sister's friend with car rebuilding experience says the knocking we hear with the shuddering could be a rod knocking in the engine, which he says means it's game over for the engine.

Does it make more sense to start with head gasket and then move to the EGR & intake manifold?

Sorry this is so long. Again, I'm not sure what's relevant to the current problem and have a lot of questions. I think I understand the concepts of how the three things (head gasket, EGR, and intake manifold) can cause the shuddering, but I don't understand how to properly troubleshoot to get to the core of the issue. Also there's still the question of whether these repairs (replace battery; fix shuddering) make sense financially or if it's time to let this car go. I appreciate any direction!

 

Absolutely! The previous shop owner was great- fair and honest, always did right by customers. Sold it when he retired, and we've been less than impressed with the new management.

Thank you everyone for the input and advice. Sounds like it's time to let this one go. Now back to trying to find something reliable...

One other thought adjacent to this topic- is the EGR, intake manifold, head gasket issue specific to gen 3 Prius, or is it advisable to clean the EGR and intake manifold as a matter of upkeep for newer models too? Other sister has a 2021 (maybe 2020...) Prius. Just curious if it's something she should do to extend the life of the car.

No coolant issues as far as I'm aware. Not sure on oil consumption rate.