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Unexpected acceleration in my Fiat 500

Like S. Newman, Benica, CA, US my Fiat 500L on two occasions, one getting ready to park and one while stopping at a red light, unexpectedly accelerated. No there was no loose mat on the accelerator and yes I had my foot fully on the brake. The car both time reved up very quickly and seem to do so like it does when one resumes cruising after a stop except it did so on its own. My foot was definitely on the brake each time. Only twice in the three years I have owned the car (2015 model year) has this happened. Both times the dealer found nothing. The second time cost me $40+ to have it checked. The first time the dealer told me they did a software update. This time no.

Were you using cruise control at the time? It could be a faulty brake light switch.

Whitey

Thanks for the thought. I was not suing cruise control, although it felt like it does when the car resumes while using cruise control. I was in town and had no practical use for CC. I use it only when highway driving. However, I had just slowed in a 70 km zone as I approached a red light. The dealer says there is noting wrong. Clearly, since the car accelerated at the intersection, they were wrong.

Another person reported the exact same problem, I think on your web site, and I have heard of other acceleration-related problems on earlier Fiat 500 models (e.g., 2014).

G. Page

There is a good chance that whatever caused the issue is an intermittent issue making it difficult for your mechanic to duplicate. When this happened to me once, it was a faulty brake light switch, but the failure was intermittent, so it was difficult to diagnose. In fact, it was so intermittent that it only happened years apart between incidents. Of course, I was actually using the cruise control when it happened.

This is something you can test yourself, either with a mirror or with a helper. You can repeatedly press the brakes while looking at your brake lights. If there is the slightest delay, you know the switch is sticking.

If the switch is inexpensive and easy to replace, you might consider doing it just to rule it out.

Another thing you can do, just for peace of mind, is have both the throttle assembly and gas pedal disassembled, cleaned, lubricated, and reassembled. Don’t just have them spray-cleaned and spray-lubricated, have them fully disassembled, fully cleaned by hand, and fully reassembled by a qualified mechanic you trust.

I’d like to know how you ruled out the floor mat. If you previously removed it, so there is no longer a floor mat in place, that’s the only way to be sure. Otherwise, there is a chance it might be a false assumption. There are a lot of logical fallacy pitfalls, such as confirmation bias, that can get in the way of troubleshooting problems like this one.

Since you’ve been through this before, and you apparently didn’t wreck the car either time, I assume you have the routine of what you should do when your car accelerates unintentionally. If that doesn’t give you any confidence, I recommend practicing the routine recommended by Tom and Ray Magliozzi.*

Lastly, I’d keep taking this car to different mechanics, for second, third, and fourth opinions until someone identifies the problem. You might consider doing that before you consider the items I recommended above (replacing the brake light switch and having the throttle and gas pedal taken apart, cleaned, and reassembled).

*https://www.cartalk.com/content/today-what-do-if-unintended-acceleration-happens-you

Were you unable to keep the car stopped with the brake pedal? That would be very unusual. If that’s what happened, check (in a safe spot) by putting one foot on the brake, then flooring the gas.

The safer choice would be to never find out because you shifted into neutral.

Absolutely. But I’m trying to understand what happened. If the car moved during the incidents, but didn’t when tested, I’m inclined to blame it on user error.

Your vehicle has a drive-by-wire throttle system.

This means there’s no throttle cable between the accelerator pedal and the throttle body.

Instead, your vehicle has an accelerator pedal position sensor that sends a signal to the computer, and from that signal the computer operates the electronic throttle body.

The electronic throttle body is also responsible for the engine idle speed control.

The computer controls the electronic throttle body’s throttle plate position at idle depending on the load imposed on the engine at the time.

So for example, when the AC is turned on when idling, the computer commands the throttle body to open to a point so the engine doesn’t stall.

So, I’d be looking at the electronic throttle body circuit for the cause of the unexpected acceleration.

Tester

That’s the natural inclination. Yet, even the best technology can fail: oh, Apollo 13!

I didn’t make any errors. Reviewed them all. Had to struggle to keep the car from going into the intersection. Happened exactly the same process both times. The other customer had the identical problem and my son also had a like problem on another Fiat/Chrysler product. His problem, however, related to a problem with the ignition shorting out. Don’t know how that would affect acceleration.

The car did not stop! It still moved on full brake until I shut it off.

Very good ideas! Thanks. By the way, the mat was actually stuck to the floor and was difficult to remove without a tusstle.

In that case, please consider this required reading: https://www.cartalk.com/content/today-what-do-if-unintended-acceleration-happens-you

The first reaction to this problem should always be to shift into neutral so you can control the car.

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