Vibration on regenerative brake - both at speed and idle

Hello,

I recently bought a 2004 prius with 216k miles. I'm an experienced DIY'er/shade tree mechanic & working through a variety of minor issues, but I'm having trouble with something that might be a bit more significant.

When regenerative braking kicks in, there's a deep, resonant vibration inside the cabin that sounds like a low grumble or growl. This happens at both startup (with foot on the brake), and when driving (to a somewhat lesser degree, possibly because the vibration is masked by other driving sounds). Since this happens at startup while the car isn't moving, I don't think it's an issue with friction brakes or traction control. So far, the vibration doesn't seem to change when the car is heated up vs cold.

Removing my foot from the brake stops the vibration, and there's not any problem with ICE vibration when regenerative charging isn't engaged.

There's no error codes on the car, and I've recently replaced the spark plugs, air filter, cleaned the throttle body, and did an oil change to reduce total engine vibration and as general preventative maintenance. I also replaced the transaxle oil with the toyota spec fluid, since the previous owner threw in something aftermarket shortly before selling it. I took a look at the 'dog-bone' lower engine mount, which seems to be good. I haven't looked at the upper engine mount or transaxle mount yet.

If it's not an issue relating to engine mounts, I'm worried this could possibly be an issue with the damper assembly flex plate or an internal transaxle issue (I've heard there's a pinion bearing that tends to wear first), but I don't have a lot of other leads right now, and haven't found a lot of info on this specific issue online.

Does anyone have thoughts on possible causes or ways to try to diagnose this issue? I hate to take cars in for service, but I might want an experienced mechanic to weigh in on this if I can't pin it down.

 

Update: this is pretty unusual, but this seems to be some kind of resonant vibration that's only noticeable within the cabin - there's not a significant noise, vibration, or evidence of an issue outside of the car or looking around the engine bay. Planning to fiddle with a few things to see if I can move the vibration into a different frequency that doesn't cause the issue.

I'm thinking this probably isn't a major problem unless other signs/symptoms crop up.

 

Update - the problem definitely improved after replacing the dogbone/lower engine mount, but I still notice the resonant vibration while driving (much less obvious at idle/when starting up) when the car is switching between gas & power regen - I may try replacing the upper hydraulic engine mount but will also keep reading up & thinking about this. I also replaced one of the CV axles (which needed replacement based on noticeable play & might have been related to a mild vibration around ~39 mph while accelerating), but I don't think that had an impact on this issue.

Paul - thanks for the suggestion - I'd originally been thinking the friction brakes were unlikely to be in play, since I also notice the issue without activating the friction brake (e.g. switching from acceleration to coast), but it's a good thought & I may consider it more seriously now.

 

Updating for posterity.

This turned out to be an issue with fuel injectors causing a slightly rough engine at idle or 'foot of the gas w/ engine still running' speeds. The injectors weren't bad enough to cause stumbling, misfires, hesitation, variable RPM, or poor throttle response, but definitely caused a light engine vibration at idle speeds. The fact that they weren't 'bad enough' definitely complicated the diagnosis process for me. Not 100% sure why this was more noticeable when regen braking kicked in, but it might have been extra vibration & engine load vs a pure idle in neutral.

I've noticed a lot of folks on this forum talking about a cabin vibration at a specific throttle/engine speed that they can avoid by applying a tiny amount more or less gas - I think I was basically in this condition during standard regen braking/engine idle speeds.

I originally didn't notice this problem at idle or in neutral, but the issue became more consistent/reproducible after swapping out the lower engine mount with a different one.

Was able to figure this out by doing a DIY injector cleaning by pulling the injectors & applying battery voltage to the terminals while spraying carb cleaner through them - the spray pattern got noticeable better (more atomized & wider spray), and the engine vibration issue improved by 70 or 80% when I reinstalled them. It might be possible to pick this up by looking at fuel trims on an OBD2 reader, but mine didn't look problematic when I checked.

Not sure if anyone else will run into this, but hope it helps someone.