Prius surging while extremely cold

If it hasn't set the MIL I doubt that the dealer will find anything wrong. It's probably caused by the fuel flow trim sender not being wide ranging enough to compensate for the nessary richness needed for really cold conditions. There are lots of other reasons but that would be my first guess. The reason that I'll guess that is because you said if you use the block heater it's fine.

 

Possibly this is the traction control hitting when you've hit a slippery spot on the road. When there's potential for slipperly spots Im more likely to feel slight drops and surges in perceived power. However, it's more likely to occur on acceleration. Although you say you are cruising you are still obviously describing this situation with the accelerator still pressed.

Here's another theory, but this is now blind speculation, but this is based upon your observation that the surges go away when the engine is warmed up. You see your Prius has two power sources, one gas and one electric. The gas engine has the characteristic that it can provide smooth power for hundreds of miles without stopping, but it needs to warm up to get maxmimum power. The electric engine, however, needs no warm up to get maximum power, however, it can only sustain this power for very short distances. So when you first drive your Prius your gas engine is a bit anemic in power compared to it's full potential. If your Prius never had an electric engine, you'd start off driving and noticing a bit less acceleration from your Prius, but would experience no surges. As your gas engine warmed up you'd get more power from your gas engine. Now throw in the electric engine. Now the electric engine makes up for the lack of normal power from your gas engine. So for a limited period of time you are getting very good acceleration from your Prius. BUT, if your hybrid battery can't sustain this acceleration long enough you're electric engine will have to cut out before your gas engine is warmed up. So you might experience a drop out in power. Then your gas engine would start driving both the wheels, the electric engine and charging the battery. As soon as your battery gets enough charge, the electric engine might pulse in and now you get a power surge sensation. The goal of the Prius hybrid system is to properly coordinate the amount of power the gas and electric engine will deliver so your acceleration seem smooth and continuous and not jerky as one engine shuts off or turns on. With a very cold gas engine or a drained hybrid battery, I'd imaging it becomes hard to avoid jerkiness.

I recall one experiment in which I used an EV switch to drive my car in cold weather from my parking lot up to the stop sign merging in with 50 mph traffic. I started to drive with nice acceleration with my gas engine just starting up. I then drove up a hill on which I normally get 18 mpg. However, because my gas engine was cold at that time the Prius chose to put the brunt of effort on the electric motor. So for about 20 seconds I was getting 40 to 50 mpg going up a hill with nice acceleration! BUT BUT, my battery went from nearly full to almost drained within 20 seconds! At that time I lifted off the acceleration a bit before I drained my battery further. I suspect that if I kept up that acceleration I would have soon felt a power drop out as my electric engine cut out with a cold gas engine.

 

Is this something that's caused you problems? or just something you noticed? I have seen it too but at higher tempratures than you, in the low 20s F but only a couple of times. It didn't hurt anything as far as driving goes and quit after it warmed up for a mile or so.