hard / rumbling engine stop and start during idle

This just started happening recently, for the record the car otherwise performs very well and to specification.

As we all know, the Prius engine starts and stops as needed. Typically these starts and stops are well spaced — the engine should wait at least a few seconds between restarts.

My problem, however, is this: when idling in park, the engine will start to rumble like it’s about to shut down, but doesn’t shutdown (the car shakes) and keep going. It keeps going in a cycle like this UNLESS I shift to drive and hold the brake.

I thought this was an idle/park only thing but today noted this hard restart issue occurred when I was on an incline (in Drive mode) and was holding the brake, letting go occasionally to move down the incline (a driveway ramp into a building).

I think this is all indicative of the same problem and before I got into it I wanted to ask you guys what do you think? I’m thinking that it might have something to do with a coil or spark plugs or something like that but I wanted to check with the group first.

Notes:

1)This doesn’t really happen when my foot is on the brake and the car is not moving / in drive gear, like waiting for a red light or in a drivethru line except in that one isolated instance I noted above on the incline.

2) It has been very hot in Chicago the last week — 100 degrees here and there so the AC has been working hard

Thanks

 

Thanks, yeah the battery is old, though I still get about 40 MPG.

About a year ago I replaced spark plugs with factory Densos, and cleaned throttle body. Changed the inverter coolant as well. All traction battery vents have been cleaned out, I change the oil and filter regularly. And I put in a new 12v battery last week (this issue was happening before new battery went in)

What’s puzzling about this is that it seems to 95% only happen in park.