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Why do certain cars momentarily speed up instead of slowing down when you step on the break pedal?
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Why do certain cars momentarily speed up instead of slowing down when you step on the break pedal?

I really need an opinion/advise/information on this one: The family vehicles (an '02 Nissan car and an '08 Toyota SUV) both speed up instead of slowing down in the first few seconds when we step on the break pedal. The car has conventional breaking system and the SUV has ABS. This doesn't happen every time, though. But it does happen most of the time. What's the deal with that?
Additional Details
Dani G: I often use flat-soled, non-bulky shoes that are most conducive to (safe) driving, so I don't think my footwear touches or hits the gas pedal when I step on the breaks. I

Maranatha!: The acceleration happens almost instantaneously when the break pedal is depressed. As for the coasting condition, I will now try to note my speed when I step on the breaks to see if this is causing the pickup in speed.

Thank you.


    




Petey
I have over 35 years experience with automobiles and currently hold a position of moderate to high responsibility regarding them. To hear this is alarming. No vehicle should accelerate during any course of braking. The only possible malfunction in this manner that is concievable is a faulty check valve for the brake booster vacuum feed, but this would only occur at an idle. Could you perhaps add some additional details so that I may better understand what you are saying exactly. Is this before you touch the brake or as soon as you do begin to depress it? or something else?
.
No additional details?
All that comes to mind is a slightly newer characteristic known as coasting condition. This is when the vehicle is going over 40 MPH and you release the accelerator. What will happen is the ignition timing will advance to the greatest amount possible to maintain the current speed without applied throttle. However, the instant the brake pedal is touched, the timing drops and the car is out of coasting condition. and slowing. The design was intended to add mileage to vehicles and does.


Saturday Morning
Rating
Actually, I've experienced this condition before as well and also on a Nissan – a very subtle increase in speed when the brake pedal is first depressed, You're not causing it and you’re not crazy. I looked into it and what it is – is the automatic unlocking of the torque converter when the brake pedal is depressed. Here it is as best I can explain:

The condition being described is that when the brake pedal is first depressed on a long downhill slope the vehicle would exhibit a very slight increase in speed. The condition would occur when the brake pedal was pressed just a very small amount - enough to activate the brake light switch, but not enough to generate any measurable stopping force. For example: the Nissan Xterra I experienced this on is a conventional rear will drive vehicle with a normally aspirated V6 engine, 4 speed, automatic transmission with OD, and rear deferential. While driving downhill the engine actually performs a measurable amount of engine braking when the gas pedal is let up. This engine braking is what holds the vehicle at speed without letting it accelerate on the downhill.

When the brake pedal is first depressed the switch at the brake pedal sends a signal to the engine computer (ECU) that is then sent to the automatic transmission to unlock the torque converter. Without this unlocking of the torque converter the engine would stall at a stop light or any time the vehicle speed was reduced significantly.

For example: picture a vehicle beginning down a long downhill slope. The driver is applying the gas and the engine is running at an rpm based on the pedal position and the drag/slope/etc. as the slope increases the dynamics change from the engine pushing the wheels to the wheels pushing the engine but the change is so subtle and occurs over time so that the driver may not even notice it on a slight to moderate slope. At this point the engine is actually providing a small amount of braking and holding the vehicle to a specific speed based on the gas pedal position and the gear selection and all this is happening without the driver even knowing it.

Now as the brake pedal is depressed. The switch at the pedal sends the signal through the ECU to the ATS to unlock the torque converter. The direct coupling between the engine and the drive train is now lost and with it the engine braking. At this point, the wheels and the drive train are allowed to overrun the engine speed. The engine retunes to its idle speed and the vehicle is slowed by the brakes. All this is perfectly normal and can happen on any vehicle although low geared vehicles make it most notable.

I should add that vehicles with a lot of brake pedal travel often referd to as 'soft brakes' may be more prone to this than a vehicle that has 'tight' brakes as there is a greater time delay involved between the unlocking of the torque converter and the actual application of the brakes. You may want to have the brakes checked to ensure that the pads are not excessively worn and that there is not any air in the brake lines. If everything checks out ok, just simply exercise a little more travel when applying the brakes to get to that part where the brakes actually start applying in measure to slow the vehicle..

-Saturday


Dani G
Rating
This is not at all normal and the fact that it is happening to both of your cars suggests that you are unknowingly causing it.

Are you quite sure that your foot is not also hitting the gas pedal while you're braking ?

This would explain what you are experiencing: the car speeds up when you first start pressing on the brake because you are accidentally giving it gas while not braking very hard. As you press down harder on the brake the car starts to slow down because there is not a car made that is capable of overwhelming its own brakes once they are fully applied.

You can test for this. Let someone unrelated drive your car and suv and see if they experience the same thing. If they don't, it really might be you.


Vince M
Rating
Either both vehicles are malfunctioning (doubtful) or this isn't really happening. You may be observing some other condition that, to you, "feels" like it is accelerating.

But no way should the car speed up by pressing the brake pedal.

UNLESS, the driver is one of those that uses BOTH feet, one for the gas pedal, and one for the brake, and, for some reason, the gas is being pressed while the brake is also being pressed. This is one of the reasons it is NOT advised to use both feet when driving an automatic.





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