Weird "jerking" while stopped at light

18962 Views 19 Replies 7 Participants Last post by rebop, Sep 29, 2015

munkeyluv

Discussion Starter · Aug 16, 2015

So this has only happened twice, once with the 2015 Q3 loaner and once in my 2016. When the car was in Drive and I stopped for a light (with my foot firmly on the break) I felt this rhythmic "jerking" like the car either wanted to accelerate or was about to self-combust! At first, i thought is was the bass as I had my music on pretty loudly =). But when I turned the stereo off, it continued to jerk. Both cars had minimal mileage (< 400 miles) at the time. Has this happened to anyone else? Is it something I need to have checked out? Thanks in advance!

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rnlabas

· #2 · Aug 16, 2015 (Edited)

munkeyluv said:
So this has only happened twice, once with the 2015 Q3 loaner and once in my 2016. When the car was in Drive and I stopped for a light (with my foot firmly on the break) I felt this rhythmic "jerking" like the car either wanted to accelerate or was about to self-combust! At first, i thought is was the bass as I had my music on pretty loudly =). But when I turned the stereo off, it continued to jerk. Both cars had minimal mileage (< 400 miles) at the time. Has this happened to anyone else? Is it something I need to have checked out? Thanks in advance!

Well, first the car CANNOT accelerate in this scenario - fact is IF your foot is firmly on the brake as you say, then you have about 3 times the stopping force fully applied than the engine can make at full throttle. So at least you have the laws of physics well on your side. btw, this is almost universally true with ALL cars. Unfortunately folks in the past could not relate the laws of physics with their own psychological infallibility of mind and came up with "unintended acceleration" as both a phobia AND a misnomer of fact. I hope you're NOT using your left foot for the brake and right foot still on the gas? Please don't EVER do this.

btw, only the very HIGHEST performance cars have brakes that are only close to their power, and very few race cars but dragsters have less brake power than engine power. All this means that as long as you REALLY have your foot REALLY on the brake, your engine will stall (shutoff) cuz the brakes are stronger than the engine.

Second, as for the jerkiness, if you have the brake firmly applied for at least 3-4 seconds, try just taking your foot OFF the brake and NOT on the gas and see what happens. Do this in a safe location such as an empty parking lot. This should engage your hillhold function (even on flat ground) and your car should neither creep nor roll in either direction until you apply throttle. If the vibration or jerkiness continues, yup - I'd get it checked. If it does not, you may still need to have it checked as perhaps your ABS controller has a fault and it would be coincidental that the loaner had the same issue.

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munkeyluv

Discussion Starter · #3 · Aug 16, 2015

Thanks for your reply. And yes, I only use my right foot for driving =) During these occurrences, I had the break pedal down as firmly as one would deem appropriate. Also, the car DOES slowly creep forward after releasing the brake after a timely stop - even when on a slight incline? Is this not supposed to happen? All of my other cars gently creeped forward post release of the brake.

rnlabas

· #4 · Aug 17, 2015

You have a Prestige Q3 which has the hill hold function, which engages the ABS controller after a brief (3-4 sec) firm foot on the brake. Which after engaging should hold the car still either on a hill or flat surface when your foot is removed from the pedal, UNTIL you apply throttle. All of this is electronic so if it's not working correctly may have a fault, or you have manually turned your hill hold off...

Most (older) cars with an automatic will creep as they don't have this function -

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Otto

· #5 · Aug 17, 2015

I would assume this is the ABS system. You used to have to pump the brakes to ensure your car stopped instead of locking up the wheels and skidding. ABS pumps for you by rapidly engaging and un-engaging the brakes, that would be the "jerking" feeling that you are describing.

Ourob0r0s11

· #6 · Aug 17, 2015

rnlabas said:
You have a Prestige Q3 which has the hill hold function, which engages the ABS controller after a brief (3-4 sec) firm foot on the brake. Which after engaging should hold the car still either on a hill or flat surface when your foot is removed from the pedal, UNTIL you apply throttle. All of this is electronic so if it's not working correctly may have a fault, or you have manually turned your hill hold off...

Most (older) cars with an automatic will creep as they don't have this function -

I'll have to test further to be absolutely sure, but I tried this today while at a stop light and on my driveway and I could not get my car to "hold", it would always start to creep even before applying any pressure to the accelerator pedal. Any way way I can check to see if its disabled by default in the system without going to the dealer?

rnlabas

· #7 · Aug 18, 2015

I re-checked mine today too - and while it holds fine on a grade (even with the SprintBooster on full) it did NOT on a flat grade. It had before, so I turned off the SprintBooster and it held for a sec then crept forward. I had the A/C on and I think this self-adjusts the idle up a little, perhaps enough to allow the creep on a flat surface.

This can be adjusted by the dealer, but I'm not going to do that just for flat surfaces - it's hill-hold function designed for grades and it works, not a lazy foot function for whenever ...

rnlabas

· #8 · Aug 18, 2015

Otto said:
I would assume this is the ABS system. You used to have to pump the brakes to ensure your car stopped instead of locking up the wheels and skidding. ABS pumps for you by rapidly engaging and un-engaging the brakes, that would be the "jerking" feeling that you are describing.

I didn't think he was talking about "locking" the brakes in a panic stop, where the ABS does as you say - but rather, just sitting at a light, already stopped. The Hill-Hold feature ALSO uses the ABS controller to apply the brakes without your foot on them (cuz it can) whilst stopped on a hill. This was/is intended mostly for folks with a manual gearbox (most of the RoW) so the car would not roll back on an upward incline - sometimes a challenge for folks with a manual gearbox car.

The function is there in the software already, so it's usable even with an automatic. But the car meanwhile should NOT jerk in this mode - so something else is at play. But still NOT to worry about unintended or runaway acceleration - just NOT possible. NBC had to specially rig a car back in the hoopla days and it STILL didn't behave as people claimed - who were simply in denial about their mistaken application of the throttle for the brake ...

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Otto

· #9 · Aug 18, 2015

I've always expected my car to go forward when the engine is started and there are no feet on the gas or brake. If it has some system to stop it from doing that, I wouldn't even notice because I always keep my foot on the brake when stopped.

Reactions: 2

Quatto

· #10 · Aug 18, 2015

Otto said:
I've always expected my car to go forward when the engine is started and there are no feet on the gas or brake. If it has some system to stop it from doing that, I wouldn't even notice because I always keep my foot on the brake when stopped.

I'm the same way, it's all out of habit, a good habit and due to how common it is I doubt there's a lot of people that have the same experience. Can't recall when last i've seen someone do that. Closest thing was someone stalling a vehicle trying to get it to move from a stand still.

munkeyluv

Discussion Starter · #11 · Aug 22, 2015

Just an update.. i hit 500 miles on the car on Tuesday night. Starting Wednesday, I began to notice the car is not creeping as often after releasing brake, and on my way home last night, every stop but one remained in place after the brake's release. Maybe it was just the break-in period? I love this car by the way. What a joy to drive! I even found a new route to work that has a few more curves to have fun with!

john

· #12 · Aug 22, 2015

Do we have to turn on this hill hold? I have the button on my lower dash that looks like a car on a hill. I have not read this part of the manual yet.

munkeyluv

Discussion Starter · #13 · Aug 22, 2015

Yes, you have to engage the switch, and it goes off as soon as you accelerate (from my "brief" experience!)

Ourob0r0s11

· #14 · Aug 22, 2015

munkeyluv said:
Yes, you have to engage the switch, and it goes off as soon as you accelerate (from my "brief" experience!)

That's not quite accurate. The button under the MMI controls is for a controlled speed down an incline (hill descent assist) which is NOT the same as a hill hold which helps you switch from stationary braking to accelerating without rolling on a hill.. The button on the dash is for while you are moving, slowly, down a steep decline, the engine uses low gears for engine braking and the brakes are applied by the computer to each wheel to maximize grip while maintaining a slow steady speed down a hill. The "hill-hold" to assist from going from a complete stop to accelerating on a hill without any rolling is activated by applying full pressure to the brakes for a few seconds, and only lasts for a second or two (enough time to get your foot from the brake to the accelerator).

john

· #15 · Aug 22, 2015

Ourob0r0s11 said:
That's not quite accurate. The button under the MMI controls is for a controlled speed down an incline (hill descent assist) which is NOT the same as a hill hold which helps you switch from stationary braking to accelerating without rolling on a hill.. The button on the dash is for while you are moving, slowly, down a steep decline, the engine uses low gears for engine braking and the brakes are applied by the computer to each wheel to maximize grip while maintaining a slow steady speed down a hill. The "hill-hold" to assist from going from a complete stop to accelerating on a hill without any rolling is activated by applying full pressure to the brakes for a few seconds, and only lasts for a second or two (enough time to get your foot from the brake to the accelerator).

OK - that makes sense to me now. Thanks for the explanation and differences between the two. I doubt I will ever use the switch for the hill descent assist - just rarely in that situation and I guess the engine and brakes can do the trick. Ohio is FLAT!!

munkeyluv

Discussion Starter · #16 · Aug 22, 2015

rnlabas said:
I re-checked mine today too - and while it holds fine on a grade (even with the SprintBooster on full) it did NOT on a flat grade. It had before, so I turned off the SprintBooster and it held for a sec then crept forward. I had the A/C on and I think this self-adjusts the idle up a little, perhaps enough to allow the creep on a flat surface.

This can be adjusted by the dealer, but I'm not going to do that just for flat surfaces - it's hill-hold function designed for grades and it works, not a lazy foot function for whenever ...

You are right about the A/C. We had a "cool front" (by Texas standards) come through mid week and I had the windows down instead of the A/C - and the brakes worked as they should. Then today, back at 100 degrees with A/C blasting...and car is creeping after braking. As soon as I turned off the A/C to test the theory, the car stayed put after releasing the brake!

munkeyluv

Discussion Starter · #17 · Aug 22, 2015

Ourob0r0s11 said:
That's not quite accurate. The button under the MMI controls is for a controlled speed down an incline (hill descent assist) which is NOT the same as a hill hold which helps you switch from stationary braking to accelerating without rolling on a hill.. The button on the dash is for while you are moving, slowly, down a steep decline, the engine uses low gears for engine braking and the brakes are applied by the computer to each wheel to maximize grip while maintaining a slow steady speed down a hill. The "hill-hold" to assist from going from a complete stop to accelerating on a hill without any rolling is activated by applying full pressure to the brakes for a few seconds, and only lasts for a second or two (enough time to get your foot from the brake to the accelerator).

Ahhhh, Thanks for clarifying. I had no idea the two were different.

rebop

· #18 · Sep 26, 2015

My Hill Hold on Prestige is not working. Just 400 miles on it. I am sure driver door is closed and seat belts on (both required). AC is off. No matter what, I get strong creep forward after releasing the foot brake.

Defective? Anything to try?

~Bob

rnlabas

· #19 · Sep 26, 2015

Try on an uphill grade - be sure you have firmly stepped on the brake for at least 5 seconds before releasing...

rebop

· #20 · Sep 29, 2015

You are correct. Dove to San Francisco yesterday and was amazed how well this worked. Thanks! Glad I pursued this.

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