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Stalling sometimes when braking hard


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13 replies to this topic

#1 DupermanDave

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Posted 24 August 2009 - 02:15 PM

My fiance (the subaru owner) says the subaru would idle low and almost stall when she brakes hard. I thought she was nuts, because I could never duplicate the problem.

Yesterday I figured out why. I brake slowly and come to a stop longer and she brakes more towards the end, and she brakes hard. So as the speed and RPMS go down, and we are coming to a stop, the RPMS would continue to drop. It's almost like a vacuum leak. When at a complete stop, and the brakes are still pushed, the engine would cut out suddenly.

I checked the vacuum hoses and they look good. No cracks. But I might have missed something. If the hoses are good, could this be the sign of a bad brake booster?

It doesnt stall every time, just maybe once a week or twice a week. So out of all the driving we do, it's not a lot.

#2 subaruguru

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Posted 24 August 2009 - 02:22 PM

i have the same problem in my EJ swapped coupe but it wont always dop it just once or twice every other day if you get any leads it would be helpful and i would do the same im glad to know im not hte only one.

thanks Rob.

#3 DupermanDave

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Posted 24 August 2009 - 03:14 PM

I also wondered if it was temperature related. If it got really hot outside, would it happen more than others, but that was pretty much answered yesterday when it was a good 80 degrees (pretty cool for summer here) and it still happened. And it was in the morning, so still pretty cool outside.

Also, if it was a vacuum leak, wouldnt I see a floppy or fluctuating idle?

#4 Fairtax4me

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Posted 24 August 2009 - 09:48 PM

Not necessarily.

Small leaks are often "covered up" by the idle air control valve. It can adjust enough to keep the same or near the same idle speed as without the leak, and you might never notice it.



To test the booster. Run the engine for a few seconds at normal temp, then turn it off. Press the brake pedal a few times. It should be easy the first two or three times, then should get stiff.
Restart the engine and let it build vacuum. Cut it off and wait 5 minutes. Now pres the brakes a few times again, in the same manner as before. If the pedal gets stiff sooner, or is stiff on the first press, the booster or check valve is leaking.

#5 DupermanDave

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Posted 11 October 2009 - 05:56 PM

Would it matter if the engine was warm or cold when I test the booster? The stalling mostly happens when it's cold, but has stalled once or twice when fully warmed up.

#6 OB99W

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Posted 11 October 2009 - 06:25 PM

With the engine cold, does the idle change (speed or smoothness) if you depress the brake pedal hard and hold it down?

#7 DupermanDave

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Posted 11 October 2009 - 09:26 PM

With the engine cold, does the idle change (speed or smoothness) if you depress the brake pedal hard and hold it down?


Yes, very very subtle, but we do see the idle go down.

I tested the brake booster. Turned off the engine, pumped the pedal and was able to get it to be stiff. After turning on the engine, it was able to be depressed again, and pumping it while running didn't cause it to stall. I turned off the engine again and tested it and it got stiff again after 4 or 5 pumps. It seems we can only get it to stall while the car is in motion, not at a cold idle. I checked over the vacuum lines coming from the brake booster and anything connected with the brakes. All is good and there's no split or cracked hoses. I squeezed some looking for a hairline crack and couldnt find anything. Next, to doublecheck, I think I'm going to run some seafoam through the booster to see if I can find a leak anywhere I might have missed.

I checked the fluid levels too, and they're all okay.

#8 valvestem

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Posted 12 October 2009 - 07:22 AM

Auto Trans? If so, it may be a solenoid in the trans not allowing the trans to downshift quickly enough to keep up with the braking, thereby causing a stall-speed situation. Sort of like not pushing in the clutch pedal when braking with a manual transmission.

#9 DupermanDave

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Posted 12 October 2009 - 01:54 PM

Auto Trans? If so, it may be a solenoid in the trans not allowing the trans to downshift quickly enough to keep up with the braking, thereby causing a stall-speed situation. Sort of like not pushing in the clutch pedal when braking with a manual transmission.


Yup. Auto transmission. It does feel a lot like not pushing in the clutch. That's a good comparison. Where is this transmission selenoid? Is it inside the tranny? Or is it something external that can be replaced easily?

#10 valvestem

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Posted 12 October 2009 - 03:40 PM

Yup. Auto transmission. It does feel a lot like not pushing in the clutch. That's a good comparison. Where is this transmission selenoid? Is it inside the tranny? Or is it something external that can be replaced easily?


That's out of my league, I do not know. Maybe a quick call to a good service department describing the symptoms will give an answer as to which solenoid it may be, as there are many, but only a few from what I can find out that may cause the symptioms you describe. I know auto transmissions are a mystery world unto themselves when it comes to repair, but maybe someone can be found who is honest. As far as the location of the solenoid, there are a few, some are located externally with wires attached for plugging in to a harness, I am sure other maybe located internally. I've attached a kink to a Wiki page, (I know, not the best info), that may help explain a little.
http://en.wikipedia....on_control_unit

#11 OB99W

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Posted 12 October 2009 - 04:16 PM

Yup. Auto transmission. It does feel a lot like not pushing in the clutch. [...]

There's a relatively easy way to verify or rule out such a trans problem. The next time a rapid stop causes the engine RPMs to drop, immediately put the trans in neutral. That should remove any loading that the trans might be causing, and the engine speed should recover almost instantly. If it does, then you have likely confirmation of a trans-related problem. However, if the engine remains with a low idle or stalls, the trans isn't the cause.

By the way, can we assume that there are no dash warning lights illuminating (Check Engine, AT Temp)?

#12 DupermanDave

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Posted 12 October 2009 - 04:59 PM

There's a relatively easy way to verify or rule out such a trans problem. The next time a rapid stop causes the engine RPMs to drop, immediately put the trans in neutral. That should remove any loading that the trans might be causing, and the engine speed should recover almost instantly. If it does, then you have likely confirmation of a trans-related problem. However, if the engine remains with a low idle or stalls, the trans isn't the cause.

By the way, can we assume that there are no dash warning lights illuminating (Check Engine, AT Temp)?


I was going to try the throwing it int neutral tomorrow, after it has sat overnight in freezing temperatures.

And there is a check engine light that comes and goes, and it's always the same code. It's the improper fuel/air mixture code. It hasn't come on in quite some time since I checked the sparkplugs, though.

#13 ronemus

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Posted 14 October 2009 - 07:31 PM

You may have a stuck idle air control valve; when you brake, the booster will dump some air into the intake manifold and the IAC valve should close to compensate. It's easy to pull the IAC valve and see if it's free to move.

#14 valvestem

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Posted 15 October 2009 - 09:52 AM

You may have a stuck idle air control valve; when you brake, the booster will dump some air into the intake manifold and the IAC valve should close to compensate. It's easy to pull the IAC valve and see if it's free to move.


Good answer, maybe that's it. Be a lot easier than tracking down a trans solenoid.




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