Sudden Burst of Speed

[jreinstedler]

I have had this happen twice so far... Running a 2010 IV... each time it happened, it was making a right turn and had started to slow down by letting off the gas and starting to apply the brake. The second I felt the surge in the engine, I hit the brake harder because the car started to accelerate as if I had slammed the gas down. No warning lights illuminated.

I will be advising the dealer of this when I am there next week.

[donee]

Hi All,

I do not have a 2010, but this sounds allot like regeneration disable due to a slippery surface. When your foot is off the gas, all the way, like the time when its going from the accellerator pedle to the brake, the car is regen. If you hit oil, or loose gravel at the bottom of a pot hole (common places for pot holes are in the ares just before where stopping occurs - due to heavy loads placed on these areas by trucks and SUVs) one of the front wheels might slip. In which case the Prius disables regeneration and changes to friction brake ABS action. But, if your foot is not on the brake yet, what it feels like is an accelleration, because before the regen disable, you were decellerating. The Prius has a built in no-peddle decelleration setting by HSD regeration. So, you subconciously tense your neck and body against the decelleration, and then all of a sudden the car is not decellerating, and your neck and body react as if they were thrown back, but in actuality it was your own muscle tension that is creating the effect.

This can even happen when the foot is on the brake, if the brakes are dirty/rusty/wet. Which can happen if they have not been used much. Because the transition to the friction brakes is a transition to brakes with allot less force than the electric brakes in this circumstance.

Here is how to avoid it. Avoid lifting the accellerator, or pressing on the brakes when your car is running through pot-holes/gravel/shinier spots of asphalt on a rainy day. That is, see a pot-hole while braking - lift for just that instant until you are past the pot-hole.

There is another actiong going on here. Friction brakes in traditional cars grab between 5 and 0 mph as your braking - creating a jerk to a stop. The Prius is switching to friction brakes at 7 mph, and which typically have less force than the electric brakes. This creates a slight jerk forward rather than backward.

Additionally, if its been raining, or your car has not been driven for a while, its a good idea on the first shallow downhill to shift into neutral, and brake slowly down that hill. This will force the car to use the friction brakes, and in so doing clean up the surface of the friction brakes. Which will result in less of braking force change during the electric to friction change over.

[jay_man2]

I was going to mention that I experienced something similar just once, going over railroad tracks, similar to how donee described things above. Great description, btw.

There's a similar GEN II discussion:

Help Diagnosing Unintended/Uncommanded Acceleration Issue

[mgb4tim]

Originally Posted by moner

I sometimes get this surge at a certain point on my commute. It happens going downhill, on a bumpy road, and going about 5km/hr with foot on the brakes slowing to a stop sign. I will feel the car suddenly try to accelerate and I need to push the brakes alot harder to stop the car. I called the dealer and he said it was "normal", something to do with the engine modes transition or regenerative braking? I also noticed a slight speed surge once when I hit some gravel accelerating into traffic. These surges don't feel dangerous yet, more startling, but on a slick road or with pedestrians around it could be scary.

Mine did this over a bumpy area just yesterday, I put it off as wheel hope while braking

[barbaram]

I've had more power than expected when going from park into reverse..... not the pedal to the floor for sure in this case.

[WaterbedWarrior]

I had a sligthly different experience this morning on my way home. Cruise control was on and set to 60 mph, transferring from one interstate to another, right side exit offramp with fairly sharp left banked curve elevating to merge onto different interstate.

The offramp was a bit rough and the car started to accelerate midway through the offramp.
It hit 62mph before starting to slow down. No tire squeal.
Just felt all wrong as the car was continuing to accelerate past my cruise control set point while going uphill. I often drive this route as it is between my girlfriends house and mine. This is the fastest I've taken this offramp as there is usally traffic slowing me down.
Will try to reproduce later this week.
Environmental conditions: ~8AM, 83 degrees F, extremely high humidity (about to start raining) but road was dry.

[hobbit]

This sounds partially like the known problem of bumps while
braking confusing the regen system enough that the car gives up
on regen entirely and falls to physical brakes for the rest of
the stop. That produces that little "sag" feeling and may
require foot pressure adjustment, but most second-gen drivers
are used to that and as mentioned, have come up with clever
avoidance strategies to avoid it when possible.
.
I wouldn't be at all surprised if the same thing exists in the
third-gen, and I've just posed a related question about what
one sees in the HSI in my ongoing document about same. It is a
protective mechanism -- bumps cause wheel speeds to change rapidly
and thus regeneration current to experience high surges -- the
Prius system sees those pulses and drastically limits regen
current to avoid sending too much of that through the inverter
into the battery.
.
However, if the OP is very sure that the event only stemmed
from backing off the go-pedal with NO involvement of the brake
pedal and WITHOUT the cruise-control enabled, then that sounds
like a different and more serious problem.
.
_H*

[goingreen23]

I'm one of the statistics. I was slowing down at an intersection and was applying the brakes, went over a pothole and the car just accelerated with my foot still on the brakes lightly. Weird and scary though.

[spiderman]

I noticed a subtle surge this morning as I was in the parking garage, heading slightly downhill, around 5-10mph and I took a sharp right turn into a parking spot. As I started to turn and let off of the gas, it seem the car was free rolling (no regenerative braking) and to me that felt like a surge as I am use to the regenerative braking slowing me down once I let off the gas. I didn't panic but I sure got on the brake faster.

I don't know if this has anything to do with what others have experienced but it was new and odd to me and thought I would mention it.

Peter

[ksstathead]

Originally Posted by goingreen23

I'm one of the statistics. I was slowing down at an intersection and was applying the brakes, went over a pothole and the car just accelerated with my foot still on the brakes lightly. Weird and scary though.

What was your speed before and after this acceleration? Any chance you mean that you felt a surge as when regen is cancelled but the car did not actually speed up?