2008 Toyota Prius – Intermittent limp mode, loud engine with poor acceleration, no fault codes

Hi everyone, hoping someone here can point me in the right direction because I’m a bit stuck.

I have a 2008 Toyota Prius with about 90,200 miles on it. There are no warning lights on the dash and no fault codes showing on diagnostics.


The issue started suddenly after refuelling. Since then, the problem comes and goes. Some days the car drives completely normally, and other times it struggles.


When it happens, the engine gets very loud but the car doesn’t accelerate properly. It does move, but it feels like driving uphill or like being in the wrong gear in a manual car (even though this is an automatic) — the engine sounds like it’s working hard but the power doesn’t really translate to the wheels. Even when just maintaining speed, the engine stays louder than normal. Sometimes it goes into limp mode, but it doesn’t stay that way and can clear on its own after a while or after switching the car off.


RAC (UK breakdown assistance) suggested it could be a sticking throttle body and mentioned these faults can be intermittent.


The first mechanic I took it to initially thought it might be a wiring or communication issue. He cleared the only fault code that had ever shown (this was when pressing the accelerator while parked, the engine wouldn’t rev). That particular issue hasn’t come back, but the main driving problem didn’t go away. He later changed his view and said it could be the catalytic converter.


While the fault was present, he also looked at live data and noticed odd throttle behaviour — delayed response, the throttle opening slowly and then dropping back down even though my foot was still fully on the accelerator, and the throttle position sometimes showing 0%.


I then took it to a second mechanic, who found no fault codes. He checked a few things and mentioned the purge valve as a possible cause, but said he wasn’t sure — someone he knew had a very similar problem on a Vauxhall, and in that case it turned out to be the purge valve. The 12V battery was showing about 64%, which he said may be a contributing factor (he mentioned being expensive to replace) so he mentioned not wanting to get me to change the battery and that not being the problem.


Since I’ve owned the car, I’ve also had some intermittent electrical quirks, like the screen flickering and the radio or heater turning off randomly, though this has been happening on and off for a long time.


At this point I’m honestly a bit confused about what to do next. Could this be a hybrid battery issue, wiring/grounding, throttle body/sensor problem, or even a catalytic converter? I’d appreciate any guidance on what might make sense.

Thanks in advance for any advice — I’d really appreciate it.

 

These were the findings from Dr. Prius

 

Good question it just sounds loud and feels like it’s lugging when I first start driving, then when I am on the motorway or require to go at a higher speed the noise gets louder like a loud fan almost. It hasn’t gone into limp mode so I have not been experiencing the car struggle to pick up speed.

 

These were the other results of the battery check from the car itself in vehicle signal check mode.
Battery 11.9V with Tail lights off
And 11.4V with them on.

thank you for the help, I will try check out the fuel trims like you mentioned.