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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > Does your brake booster hold vacuum with the engine off?



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      11-22-2012, 11:40 AM   #1
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Does your brake booster hold vacuum with the engine off?

I've recently noticed a trend between cold starts where the brake booster has lost its vacuum and the brake pedal is stiff on the next cold start, where the car has rested for a few hours.

I'm thinking it used to hold vacuum overnight, even for 12 hours when I make the regular commute into work the next morning. My car is a 2010 E92 (Steptronic) with Cobb AP tune, ETS IC, iCarbon DPs and AFE S2 intake.

The car doesn't lose vacuum immediately after the engine is shut off but there must be some small leak in the system that drains the canisters over a few hours. This leak is not significant enough to trigger boost codes and the system holds vacuum well enough while the engine is running. I don't recall this difference after any of the mods were installed, though it is possible some hoses had to be removed and plugged back in.

This might also be normal behavior with the N54 as it ages but I figured I should ask around, see what others experience and get some pointers on what to look for.
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      11-22-2012, 11:44 AM   #2
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My brake pedal is stiff in the morning too
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      11-22-2012, 11:52 AM   #3
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sounds normal to me... mine holds vacuum for a couple of hours only
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      11-23-2012, 09:22 PM   #4
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This never happened with my N55 335, but i have since noticed this with my N54 335. I notice it most every morning, but the brakes seem perfectly normal and responsive when the engine is running. Can anyone else chime in on this please?
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      11-23-2012, 10:11 PM   #5
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Mines stiff after a while too. Dont see a reason to keep that vaccum overnight.
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      11-23-2012, 10:20 PM   #6
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Quote:
Originally Posted by bimmer35 View Post
Can anyone else chime in on this please?
Almost all vehicles with "power brakes" use a brake line pressure "booster" that is supplied with manifold vacuum. When supplied with vacuum, the brake pedal is easy to depress and assists with force to the brake hydraulic system. When that vacuum is lost, the brake pedal becomes stiff to depress as your leg is doing all the work--the power assist is lost through the brake booster.

The N54 maintains several other vehicle controls that run off of engine vacuum, especially the turbocharger wastegates. Turbocharged engines don't supply a steady amount of manifold vacuum and often employ vacuum canisters which hold a reserve amount of vacuum for operation while under boost.

Somewhere along this system, the vacuum is lost. It might take several minutes or several hours, depending on how well the system is sealed up. This is what you experience when pressing the brake pedal in the morning on a cold start. As soon as the engine starts up, it begins supplying vacuum and immediately restores the brake booster function.

I posted this up just to see if my experience aligns with others and if several known sources of vacuum leaks exist that might be worth checking into. It seems this behavior is actually normal with the N54.
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      11-23-2012, 10:30 PM   #7
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Mine is the same way. get in the car, put the key in, push the pedal (rock solid) hit the start button, brake pedal instantly works like normal. It would seem its normal with our cars
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      11-23-2012, 10:39 PM   #8
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Yeah....I'm stiff in the morning too.

But seriously.

I have always suspected the o-ring seal on the vacuum line when you disconnect the large vacuum hose that routes around the stock airbox.

I have disconnected that line so many times to work on the car that I am almost certain it has become worn.
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      11-23-2012, 10:44 PM   #9
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Old news. Mr.5 did a DIY fix on this. There is a rubber grommet in that attaches the brake booster to one of it's vacuum lines. It is in a VERY tricky position. When the grommet stops sealing correctly around the vacuum line, you get a slow vacuum leak. Overnight, your brake pedal becomes hard. Ultimately, if this is left unchecked for a very long period of time, it will cause vacuum issues at idle and during deceleration.

The solution is very cheap: $5.00 for a new gromet and a case of beer for the guy with a set of midget hands to have it replaced. I sure as hell can't fit my hands in that spot.


EDIT: It was not Mr.5 after all:

http://www.e90post.com/forums/showthread.php?t=589753
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      11-24-2012, 10:09 AM   #10
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Mine is also stiff in the morning... Just started happening a week or two ago when it got cold... So what's the conclusion, is it normal or not?
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      11-24-2012, 10:57 AM   #11
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Quote:
Originally Posted by psmitty95 View Post
Mine is also stiff in the morning... Just started happening a week or two ago when it got cold... So what's the conclusion, is it normal or not?


lol...Thats what she said!



On the side note yes when it gets colder out, my does the same thing in the morning during a cold start. Once its warm on the next start the brake pedal is no longer rigid..
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      11-24-2012, 01:58 PM   #12
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Quote:
Originally Posted by psmitty95 View Post
Mine is also stiff in the morning... Just started happening a week or two ago when it got cold... So what's the conclusion, is it normal or not?
Quote:
Originally Posted by cn555ic View Post
lol...Thats what she said!
..
Well if "she" said that then I hate to break it to ya.....she's a he.
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      11-24-2012, 02:53 PM   #13
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Quote:
Originally Posted by Ilma View Post
Well if "she" said that then I hate to break it to ya.....she's a he.


I was referring what she said about mine!
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      11-24-2012, 10:06 PM   #14
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Quote:
Originally Posted by cn555ic View Post
lol...Thats what she said!



On the side note yes when it gets colder out, my does the same thing in the morning during a cold start. Once its warm on the next start the brake pedal is no longer rigid..
Hahahaha
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      11-25-2012, 01:30 AM   #15
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Mine looses vacuum
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