2011 Prius Whirring Sound During Driving

Brake accumulater going bad

 

What tankyuong said. Nothing to do with the inverter. You are hearing the pressure pump for the brakes.



The pump is in that lower assembly, but that is not necessarily what's at fault. The reason the pump has to run so much can be in the upper assembly too. In fact, it usually is, because that upper one is where most of the valves are.

If it is having to pump every 10 to 15 seconds, you can probably tell which assembly has the leak, just using a cheap mechanics' stethoscope. Touch both assemblies with it in the interval between pump runs, and listen for which one is making more of a hissing sound.

 

My 2010 is in a similar situation, has been for about 6 months. I was considering, since they are available, obtaining a brass cap the same thread as the pressure line coming from the pump up to the master cylinder. Remove the line at the master cylinder and cap it off. Then a power cycle to see if the pump runs the same amount of time. In theory, that should isolate where the leak is, or am I overthinking?

 

Overthinking. The pump is in the lower assembly, but the accumulator pressure sensor is in the upper one, so if you disconnect and cap, it won't ever know when to shut the pump off.

A mechanics' stethoscope is cheap, and if the pump is running on a cycle of some seconds, the sound of hissing fluid ought to give you a pretty fair clue.

If you've caught it when the cycling is much slower than that, like minutes instead of seconds, that might make it a harder call (or you might need better ears).

 

So the barely audible “sss-sss-sss” while my foot is on the brake may be the clue as to where the impending failure is?

 

Some people see a failure mode where their foot doesn't even have to be on the brake, and the pump will have to continually cycle for as long as the car is on, even when the brakes are not being used at all.

Others will see a failure mode where the system stays pumped up pretty well (and the pump remains off for long periods) when the brakes aren't being used, but the pump runs extra often when the brakes are used. Or it may cycle continually, but only while the brake pedal is pressed.

All of these differences are useful hints. Can you closely describe what your car is doing exactly?

The "barely audible" sound may be quite distinctly audible when you touch a mechanics' stethoscope to the right spot.

 

The only thing that I notice other then the pump rapidly cycling (on 10 sec, off 30-45 seconds) is the sss-sss-sss sound. If I’m light on the brake pedal, it’s not there, but if I go more then about half pedal, it’s there. No lights or codes.

I have tried a stethoscope but probably need to enlist a helper to apply the brake while I’m checking.

 

I have a telescoping snow brush in the car that is my go-to tool when I want to hold the brake pedal down, such as for bleeding. I just put it between the pedal and the steering wheel and telescope it out.

How about the pump? Is it always on 10 sec, off 30-45 sec whenever the car is on, or does it stay off longer when you are not on the brake, but fall into the 10/30-45 cycle when you are on the brake pedal and hearing the sss-sss-sss sound?

 

It’s about a 30 second cycle while on the brake pedal.

 

So I take it the pressure holds up ok when you are not on the brake pedal.

In this case I suspect that (your stethoscope will confirm) the hissing sound is from the booster assembly (the upper thing). The accumulator (lower thing) is pressurized at all times whether you are on the pedal or not, so if the leak were in there, it would be leaking down and repumping all the time.

(The upper thing also has some passages that are always pressurized, so if a car is repumping all the time, either thing could be the culprit. But the upper thing is the only one with other passages where the pressure depends on whether you're on the brake or not. So if you only get pump cycling and sss-sss-sss while you are on the brake, it makes sense to focus on that upper thing.)

 

The pressure seems to hold reasonably well when I am not on the brake. I have looked at a couple of other used Prius to compare and off brake pedal cycling seems to match up.

The local Toyota dealer to me is willing to match the lowest online price plus shipping on either part. Considering how important the unit(s) are in the proper operation of the brake system, unless someone was giving me one completely free, I’m not going thru the trouble of installing one that may be bad already. In fact, I would probably dissect it to see if I could find anything amiss.