Car runs rough/stalls when brakes pressed

Tiny
QM1K
  • MEMBER
  • 2007 CHEVROLET IMPALA
  • 3.9L
  • V6
  • 2WD
  • AUTOMATIC
  • 100,000 MILES
Hi, I'm hoping someone here can help me. I'm having a heck of a problem and my car has been broken down on a random street in a neighborhood close by for 3 days.

I'm thankful it hasn't been towed but I need to do something to fix it any help is appreciated.

So the engine runs fair but as soon as I press the brake it stalls or runs extremely rough. I've checked the brake booster I don't hear any leaks. I've checked the intake clean filters in case it was just getting gunked up. Please please I hope someone can help because I'm kind of lost as to what's wrong with it. The brake booster was recently replaced the check valve for the brake booster is good I've checked the hoses from the intake.

The key here is it idles fine until I press the brake literally even if I just barely press the brake with my finger barely not even enough to stop the car if it was in motion the it starts to run super rough as if it's going to stall. In fact half the time it does still. As soon as I let off the brake it goes back to normal. The brake boosters been replaced though and I've also had someone else hold the breakdown it started a run rough while my head was under the hood and I did not hear any leaks. Like I said I replaced a brake booster before so I know what that leak would sound like.
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Sunday, August 8th, 2021 AT 8:23 PM

28 Replies

Tiny
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I added a video to show how it runs rough with the brake pushed in. As I mentioned when I let go of the brake pedal the idle returns to normal.

If I pull the brake booster check valve out of the brake booster I hear the psshhhtt of a good tight vacuum releasing it's vacuum. So I don't think it's the booster or any of the respective hoses. But then I don't understand why pressing the brake would cause the engine to sputter.
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Sunday, August 8th, 2021 AT 10:08 PM
Tiny
ASEMASTER6371
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Good morning,

I would take a pair of vice grips and clamp off the line that goes to the booster. With it clamped, check and see if it still stalls.

If it does not, it is the booster.

Just be careful when driving this as you will have manual brakes so do it in a safe location. However, it will prove out a leak internal to the booster. Basically when there is a leak it bleeds off engine vacuum and causes it to be unstable.

If it does then we need to look elsewhere.

Below is the process from the manual on the booster. Let me know what you find. Thanks

Roy
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Monday, August 9th, 2021 AT 10:52 AM
Tiny
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The brake booster was replaced in the past year so it's possible but I don't think it's that. My last brake booster was bad, I could hear the vacuum leak when I pressed the brakes and the brakes aren't soft like they were then.

I made a discovery and I think we're getting closer to solving this mystery. I noticed without pressing the brakes if I turned on the headlights or even just the running lights it would cause that extremely rough idle. When I mentioned pressing the brakes, just barely pressing the brake pedal with a finger caused it "with my pinky finger" I didn't push the pedal hard enough to actually create pressure in the system. But I would say it was hard enough for the brake lights to come on indicating I'm braking. So that tells us it's electrical. It's not the brake system causing anything. It was the fact that the brake pedal being pressed turns on the brake lights thus using the electrical.

Once I realized that I started disconnecting the battery, unplugging the PCM and re-plugging etc. And I managed to get the car home but not easily. It took about three hours of stalling pulling over and messing with the battery and PCM.

Now going from here which of the silver computer boxes in the air filter box control the headlights, taillights, brake lights and running lights portion of the electrical system? Because I think it's a wire shorting out on one of these PCM boxes. Just like how the blower motor wire shorts out from being too short on these cars.
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Wednesday, August 11th, 2021 AT 4:19 AM
Tiny
ASEMASTER6371
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Okay, did you do the test I suggested? Please do that test first.

Disconnecting the battery and the ECM does nothing at all. Not a good idea to do that.

Roy
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Wednesday, August 11th, 2021 AT 4:53 AM
Tiny
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I did the test you recommended. The car stalled after clamping the hose so the brake booster is good. It didn't stall immediately but it stalled after about 15 seconds.

The car still runs normal idle until I turn on the running lights, headlights or push the brake pedal enough to engage the brake lights. If I start the car without my foot on the brake and without any lights on and then turn on any of the lights then it idles rough to the point of stalling. If I quickly turn on and off the lights the idle goes rough and then immediately returns to normal upon turning the lights back off.

So I'm still not sure which box it is that runs the electrical system for the lights but I know it's one of the two that are in the air filter.
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Thursday, August 12th, 2021 AT 1:39 PM
Tiny
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I think I've isolated the issue to being an electrical connection shorting out at the PCM and that's the only reason disconnecting and reconnecting did something. Because I'm hearing you out where you said it will do nothing but on the contrary it did manage to get the car to drive 6-7 miles back to where I live so I don't have to worry about the car getting towed from the random neighborhood it broke down in. Thank for all your help so far, we're getting closer to figuring this out.
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Thursday, August 12th, 2021 AT 1:42 PM
Tiny
ASEMASTER6371
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You are welcome.

Always glad to help.

Roy
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Thursday, August 12th, 2021 AT 2:16 PM
Tiny
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So I've learned that the two computers in the air intake are the PCM and the transmission ECM. I'm assuming the lights would run off the PCM right? Another question, do both have to grounded to the chassis of the vehicle?
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Friday, August 13th, 2021 AT 2:20 AM
Tiny
ASEMASTER6371
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The lights are controlled by the body control module.

This is a separate control module for the control of all body functions.

Roy
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Friday, August 13th, 2021 AT 3:11 AM
Tiny
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Is it possible i'm experiencing symptoms of a bad crankshaft position sensor?
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Friday, August 13th, 2021 AT 4:48 AM
Tiny
ASEMASTER6371
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No, not at all.

You would have a complete set of issues not related to the ones that you have currently.

Roy
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Friday, August 13th, 2021 AT 5:02 AM
Tiny
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Hey, I just wanted to take the chance the to show what I'm talking about. I've been busy and haven't had a chance to mess with the car at all. So I'm going to upload a video so you can see what's going on. I understand where you're coming from with suspicion of it being a brake booster. And you probably deal with a lot of people who have absolutely no idea what they're doing or how to do the tests you recommend.
Unfortunately, I disconnected negative of battery while trying to get the car home. So I lost the DTCs that had popped up at the time and I didn't have a scan tool with me that day to check it before disconnecting battery.
But I'm telling you I worked on this car since the day I bought it three years ago, and based off what's going on there's something amiss once the vehicle is drawing just any electrical power. I have a brand new battery and I recently replaced the alternator. I'm thinking it could be possible that I got a faulty alternator?
After I installed the new alternator I used a voltmeter and it was holding 14v constant while running but I haven't checked it since all this started happening. It was running great and then I stopped to get gas and that was all she wrote didn't stay running or handle electrical current after that.
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Saturday, August 21st, 2021 AT 6:50 PM
Tiny
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In this video I have a voltmeter hooked up to car while I turn the headlights on and cause the car to stall out from the electrical draw. As you can see the battery hold above 12 volts after it stalls, and it stays above 14v while running so the car isn't losing its power at all but yet it stalls out whenever the car draws power.
Picture 1 is while vehicle is off. Shows at 12.20 volts. Picture 2 is while running, shows at 14 volts and then the video of voltmeter as I turn the headlights on and car stalls from electrical draw.
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Saturday, August 21st, 2021 AT 7:07 PM
Tiny
ASEMASTER6371
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The alternator stopped working when it stalled and the voltage went back to battery voltage.

This is normal. There is nothing there for that.

Roy
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Sunday, August 22nd, 2021 AT 2:19 AM
Tiny
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Okay, so all the issues I'm having is solely because the alternator is no good?
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Sunday, August 22nd, 2021 AT 10:26 AM
Tiny
ASEMASTER6371
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No, I did not say that at all. What makes you think it is the alternator?

Roy
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Sunday, August 22nd, 2021 AT 1:10 PM
Tiny
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My friend, I sincerely appreciate all the help you've been offering but did you read all my posts or watch the videos I attached? (Specifically the one I posted minutes before the voltmeter)

My car keeps stalling out, any time and every time anything electrical draws power from the system. That's what led to the video with the volt meter to show my voltage is good until car stalls. And battery holds good. When it stalled in that video it was because I turned the headlights on. In the other video right before that post I demonstrate how turning on headlights, turn signal, and brake lights all cause the car to stall out or struggle. If I turn the lights off fast enough the car won't stall out and will return to a rough idle.

Plus I mention that I recently replaced the alternator, and asked if it could have been a faulty alternator. So when you said the alternator stopped working in your reply I figured okay so it likely was a bad alternator then. Please if you can, please watch the video I uploaded in the post right before the voltmeter post.

https://www.2carpros.com/images/question_images/410623/video.mp4

Thank you again.
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Monday, August 23rd, 2021 AT 12:25 PM
Tiny
ASEMASTER6371
  • EXPERT
I did watch the video. The voltage dropping when the car stalled is normal. There is nothing abnormal about that at all.

When the engine shuts off, the only voltage will be battery voltage.

Roy
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Monday, August 23rd, 2021 AT 2:27 PM
Tiny
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Not that video.
this one:
https://www.2carpros.com/images/question_images/410623/video.mp4
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Friday, August 27th, 2021 AT 8:35 AM
Tiny
ASEMASTER6371
  • EXPERT
Can you put a voltmeter on the under hood fuse block and measure the voltage at the block?

Want to make sure you are not losing voltage.

Roy
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Friday, August 27th, 2021 AT 2:13 PM

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