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Car started to intermittently accelerating by itself

Tiny
CHRISTA DICKSON
  • MEMBER
  • 2000 TOYOTA AVALON
  • 250 MILES
Cleaned throttle body already, no check engine light on. I literally have to hold the brake pedal hard to keep it from surging forward into the car in front of me and as soon as I let off the brake pedal it surges forward. A guy O'Reilly's mentioned that he had the same problem wrecked his car and it was a brake booster issue. I would have never thought to look there but at times not consistently but at times it does seem like the brake pedal is very hard to press and I guess since it does pool vacuum as possible. Just wanted a second opinion on what this could be or if you think it is brake booster.
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Monday, April 5th, 2021 AT 3:33 PM

16 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
More then likely it is a vacuum leak and not necessarily the booster. That uses engine vacuum so if it is leaking then it can cause this as it is low on assist and the engine is getting more air that can cause high idle or rough idle.

https://www.2carpros.com/articles/engine-idles-too-high

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let's check these guides and go from there. Let me know if you have questions. Thanks
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Wednesday, April 7th, 2021 AT 11:33 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,536 POSTS
To check if the booster is bad with the engine off, remove the check valve on the booster to see if vacuum escapes. But I think this video will show you how to fix the problem. (IAC motor service):

https://youtu.be/XMB16Gg-qNg

Let us know what happens and please upload pictures or videos of the problem.
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Wednesday, April 7th, 2021 AT 11:36 AM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 141 POSTS
Cleaned IAC no fix. Here is a video. Sorry my editing skills are not good. If I let off brake the car will just keep accelerating so it's really not safe to drive. I took off check valve it is good and hoses look good.
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Sunday, April 11th, 2021 AT 4:00 AM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 141 POSTS
Oh, and when I took check valve off I did get sound like pressure. It was a small burst. So if understanding correctly that would indicate a bad booster, correct?
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Sunday, April 11th, 2021 AT 4:07 AM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Yes. You are correct. However, when you showed the RPMs it looked like it was surging around 1,000 RPMs up to about 2,500. Was this you accelerating the vehicle or was it doing this by itself?

Have you checked for a vacuum leak?

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge
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Sunday, April 11th, 2021 AT 7:44 PM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 141 POSTS
It was completely doing it by itself. That's the scary thing is if I just let off the brake the car will accelerate and continue accelerating until I hit brake and even then I have to hold the brake pedal firmly to keep my car from surging forward into someone else's. I guess a vacuum leak would be the next thing to check for but think it's kind of unlikely it just started one all hoses look to be in good shape no dry rot or damage. But I will check that next I guess.
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Sunday, April 11th, 2021 AT 11:06 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Thanks for the update. Yeah, I would use the brake clean method (just make sure it is combustible) to see if you have a leak at the manifold or something else that is not visible.

Next thing we need to monitor is the throttle position sensor with a scan tool to see if the throttle is moving. If it is not then I would revisit the vacuum leak again because if the throttle plate is not moving then the only other way for the RPM to raise and drop is to get more air from somewhere else.
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Monday, April 12th, 2021 AT 6:52 PM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 141 POSTS
I did notice that it was not accelerating roughly but I did spray carburetor cleaner all around the upper intake manifold, brake booster and throttle body and noticed no difference whatsoever in the idle. Thank God I got a backup vehicle and I'm driving that now. But Avalon was daily driver and I'd like to get it running correctly so not sure where to go from here any advice would be appreciated.
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Sunday, April 18th, 2021 AT 9:00 AM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 141 POSTS
It's honestly tripped out and kind of scary I have to stay with foot firmly on that gas pedal to keep it from surging into a car in front of me! I noticed the gas pedal is intermittently stiff and hard to press. It's mostly normal and not stiff but at times it happens. I did replace brake booster though and no difference:(
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Sunday, April 18th, 2021 AT 10:15 AM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
I just want to clear something up from that last post. When you say the gas pedal seems stiff, did you mean the brake pedal because you said the booster was replaced.

However, if you meant the gas pedal then that is interesting and something I did not think of. You have a throttle cable on this vehicle and they were known to stick and the throttle body would stick open and that would cause this issue.

I would at least remove the throttle body and clean it and then make sure the cable moves freely.

https://www.2carpros.com/articles/throttle-actuator-service

Below is the info from the manual on this.
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Sunday, April 18th, 2021 AT 6:49 PM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 141 POSTS
I'm sorry I meant the brake pedal.
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Wednesday, April 21st, 2021 AT 11:26 AM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Okay. If the brake pedal seems stiff then that is due to no vacuum in the booster which basically means you have a leak and it is not assisting in pressing the brake.

If that is the case then that could be the source of the vacuum leak as well. Was the pedal doing this same thing with the last brake booster as well?
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Thursday, April 22nd, 2021 AT 7:12 PM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 141 POSTS
Yes it was and what I'm trying to explain to you guys is that the car is actually accelerating on its own which I'm thinking is a different issue besides just the brake booster problem. I've already replaced the brake booster but I'm saying you had to hold on the brake pedal. I'll take another video of that I'll help you guys understand what's actually going on. I have no problem with that and I can do that when I get home. Let me know if that'll help with the situation. Thanks
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Thursday, April 22nd, 2021 AT 8:17 PM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
I do understand. If you have no vacuum leak then we need to run through the rest of this guide:

https://www.2carpros.com/articles/engine-surges

I would suggest starting with monitoring the fuel pressure while this is happening.

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

This is the next step so let us know what you find with this. Thanks
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Friday, April 23rd, 2021 AT 7:49 PM
Tiny
CHRISTA DICKSON
  • MEMBER
  • 141 POSTS
Just updating you guys. I honestly think it could have been something in software or computer related I was reading Toyota's have had unintended acceleration issues. But I got a quote at Carmax and they are offering $1,800.00. I only paid $1,400.00. So I will make a little profit and just let it be there problem. I told them what it's doing and the test drove and still want it. So going to go that route. Thanks for replies, though. :)
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Monday, May 10th, 2021 AT 4:01 AM
Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Well, you have to love it when things just work out perfectly. That sounds like a great deal even if it weren't having this issue. Then add in the fact that you were honest with them and you still make out.

Thanks for the update. Please come back to 2CarPros in the future. Thanks
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Monday, May 10th, 2021 AT 7:13 PM

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