3rd Prius problem when driving constant speed

I think you're overthinking this... If the speedometer is constant than the car is not accelerating or slowing down. And yes the gauge does look a bit weird the way it goes up and down like that but I can't imagine that's a sign of a problem unless your actual speed changes.

 

I have the same issue on occasion - primarily in cold weather - and it’s definitely NOT normal. The engine is surging up and down even though the speed remains constant. If I change the accelerator pedal position it seems to resolve itself after a short period of time. Like the OP I’m interested in knowing what causes the anomaly and if there’s a reliable fix.

 

Pretty sure there's more than one sensor that are combined via a calculation to keep the car at the same speed in cruise control and at a specific frequency/sensor readings that calculation causes too much fluctuation in power provided by electric and gas engine sequence. Probably the best person to better explain this system and if anything can be done for a remedy would be @Elektroingenieur

Hopefully he sees this notification and can provide a response.

 

When I encounter behavior like that, it is often when I have the cruise set at a speed (somewhere in the 40s, miles per hour) where the car would be about equally happy driving electrically or with the engine, and sometimes it will alternate the two more often than I'd like. Your 76 km/h is around 47 mph. I'm not sure I have ever seen mine hunt like that quite so repetitively and rapidly as yours is doing though.

 

In my case it happens at freeway speeds - ie 55 to 65 mph. The constant is cold weather (<25 F) with the car fully warmed….

 

I have never had that problem.
What you might try is to increase your speed 1 or 2 mph/kph, let it level off for 5-10 seconds,
then press and release the cruise lever to lower the speed 1 m/kph.
Maybe it will smooth it out.

The cruise controls I've work with on for non hybrid car, when it does that, the actuator is usually bad.
Is the car noticebly speeding up and slowing down, or is it just the indicator?

 

That video was taken on what appeared to be a bridge which was far from a flat. You can see the car bouncing up and down with the expansion joints. It's normal for the car's traction control to struggle to handle the lurching in that situation.

It bears repeating that the engine speed has nothing to do with the tire speed. The speedo clearly shows that the speed is remaining constant. It is quite curious that one would use KM for the speed in a US city while using mile per gallon for the fuel consumption.

 

Well, what the (digital) speedo clearly shows is that the speed variation is staying within a km/h.

The difference in delivered power when that MPG bargraph lurches between 50 and 100 like that is pretty much guaranteed to be noticeable.

I had a high-school classmate who drove a lot like that. It was as if her foot only had the much-gas and the little-gas position. With her driving, the car stayed on the expected average speed, but your head would be constantly bobbing back and forth. As the OP said, it was uncomfortable. (She, as far as I could ever tell, was not even aware of it. Sometimes, when you're the one causing the motion, it's easier to tune out.)

 

To clarify from my experience - this can happen with or without speed control on/engaged, but with the throttle in a steady state. The actual speed in NOT varying, but the engine RPMs are surging up and down.

 

normal for prius, unfortunately

 

No, it is not normal for a Pruis, or any other car....

 

My Mom used to drive like that... It's like they're allergic to their car staying at a constant speed.

 

This is the best answer to this thread...

 

I've only driven 3 Prius models for about 200,000 miles over 19 years. I've been a passenger in a dozen or so different models. It's not normal. I've never seen the MPG meter jumping up and down while the car kept a steady speed.

I've also never seen the car bounce up and down like that without external influence such as a bridge with an uneven road bed. My best interpretation of the video is that the car was rocking on an uneven road surface, causing one or more sensor to interpret it as loss of traction which led to the engine surging.

If the video were a bit longer it might help show the real situation.

 

In my classmate's case, she did drive the car at a pretty constant speed. She just had this not-what-you'd-call-modulated way of doing it.

 

The bouncing may be coincidental to the porpoising engine speed. I can’t speak for the OP, but when my 2011 does this, it happens at freeway speed on smooth roads and is independent of even the expansion joints.

 

Ordinarily I'd agree with you, but BIsco runs the show here... Didn't you notice he has 94k comments on here, as well as 43K likes?

 

When yours does this, does it bounce like that?

 

"The answer, my friend, is blowin' in the wind
The answer is blowin' in the wind"

As load changes, the hybrid system has the ability to change torque applied to the wheels. Even changes in the wind could do it. Certainly changes in road grade. The standard Prius cruise is too good at maintaining the desired speed with little hysteresis programmed in to smooth out variations in load. I find cruise a major annoyance on any increase grade when the engine and transaxle immediately react causing higher rpms and changed gear ratios. Allowing the speed to drop slightly keeps things smooth and quiet. As a result my 99% go to speed control is a calibrated foot. Better modulation for mpg, noise and comfort.

Sometimes freewheelin' is the way to go....



* In electronic control circuits, beneficial hysteresis is often programmed or mechanically designed into the process. Clearly modern radar based Adaptive Cruise Controls are better at balancing the variables. What is Hysteresis in Electronic Circuits - Homemade Circuit Projects