Shudder: need help diagnosing

Hey guys, I tried searching around the forum about a "shudder" similar to mine, but came up empty handed so I figured I'd check with a new topic to see if members have some ideas for me to check...

I have a 2012 Prius V that we bought as a lease turn in with 32kmi on it in 2015, and she now has about 189kmiles. We had the infamous leaking head gasket issue about 15k miles ago now. At that time I replaced the head gasket and the water pump (the impeller plastic covering on the magnet on it was cracked and binding slightly in the housing, so I replaced that at the time even though it was still working) along with the typical stuff you'd change in a head gasket replacement job (lap valves, new valve stem seals, new gaskets everywhere, etc). Ever since and even before the head gasket was replaced we sometimes experience a "shudder" at low speeds, say 20-40 mph while light on the throttle. It kind of "feels" like a misfire, but it doesn't sound like one plus I have on occasion noticed this phenomenon while in EV mode. This is different than the for sure misfire that would happen on cold startup due to the head gasket leaking coolant into Cylinder 1. No codes are being thrown by the computer and no flashing check engine light or anything when it happens.

The other symptom I have noticed is lessening fuel economy. When we first got the car it would reliably give us 42mpg regardless of driving style (45ish when we first got it the first year). Over the course of a couple of years this has dropped and now we are down to 35-36mpg. I think the fact that I've observed this situation shudder a couple times when ICE engine not running indicates it's something related to the inverter, or hybrid drive rather than the gas motor. The gas motor is buttery smooth at idle and startup or if you put it in service mode and give it gas.

Has anyone else had similar experience? I am wondering if it could be related to maybe a winding arcing in one of the motor generators or something like that. It doesn't happen frequently but it does happen probably 3 or so times a week, and only when hot. It is about time to change the drive unit oil again so I'm tempted to do that and see if the problem gets better (i.e. thinking if there's enough material floating around in the fluid maybe that makes the problem worse). I'll take any suggestions though of places to look.

Apart from the head job and the (thankfully) leaking brake booster/ABS system TSB issue (that was fixed with 1000 miles and <30 days left on the warranty!) the car has treated us well.

 

Oh yeah... when the shudder does happen, if you press the accelerator down just a little bit further, that's usually enough to get it to stop, and let back off just a little and sometimes it'll come right back. Sometimes if you goose it pretty good and then drop the pedal back to where you were, it'll stop too when it does. It's very random.

 

Aalso considered maybe it could maybe have to do with the damper between the motor generator and gas engine perhaps a bit weak? Don't hear any weird rattles or anything though, so I do not know. I'm grasping.

 

Yep

 

I don't think so... see comment about it doing this while engine isn't running.