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Discussion Starter #1 (Edited)
Although this info is in my big Misfire thread, it is way at the end of many, may posts on that thread so I thought I would put this information front and center for others to find easily.

If you are chasing an illusive rough idle and/or Lean codes at idle that you have not been able to find, beware the power brake booster is one more problem area. We have been seeing power brake boosters leaking vacuum but still functional and no obvious lose of braking effort. Most of these booster failures are happening after year 10, usually around year 11-12 on most of the cars that have had booster problems.

Often power brake boosters leaking vacuum will cause the STFT to by very high, +20 to +27% and often LTFT may or may not be over +10%. One problem that makes booster failures hard to find during a smoke test is the input check valve for the booster blocks smoke from entering the booster and even if there was not an input check, many might fail to think to look under the dash for smoke from booster leaks.

Symptoms of booster leaks are higher than expected Fuel Trims, cold start and rough idle issues. Additionally if you know what to listen for, you can actually hear the booster leaking vacuum from under the dash. It may sound like a fan running under the dash and may change in volume as you apply the brakes while stopped or the hissing noise may diminish when accelerating normally from a stop. Keep in mind that some DME boxes may have a fan in it so do not confuse a DME box fan as a booster leak.

In order to hear a leaking booster, you will have to turn off the main HVAC fan by running the fan speed all the way down manually, have the windows up and radio off. While stopped and idling listen for a hiss or fan sound from under the driver side dash. The lower hush panel will muffle the sound and may need to be removed to easily hear booster leaks. When the boosters first start to leak, you can apply the brakes and the hissing will drop in intensity and may even stop while the brake is applied. If you have an OBDII scan tool connected to watch Fuel Trims, you may find that the Fuel Trims drop back toward 0% when the brake pedal is applied and when the brake pedal is released you will probably see the STFT rise first.

To confirm if the brake booster is a problem the booster hose can be removed and plugged or the booster hose might be able to be pinched off for a test. The booster vacuum hose is molded and pretty stiff so it may be hard to pinch off.

As for a solution, you could consider a used booster, but in my opinion you do not know how long the used booster will last and the amount of work to replace the booster is probably not worth risking on a used booster? But as they say, you either have time or money, if you had both you would not be here on this forum working on a 10+ year old car.

A new ATE power brake booster with a 2 year warranty is currently available from www.OEMbimmerparts.com for around $177 shipped. These boosters appear identical to what was installed at the factory, so I assume ATE may have been the OE supplier? You could also consider a re-manufactured booster as well. I did not spend much time looking in re-manufactured booster pricing and availability because I thought the $177 figure was pretty reasonable for a new booster. The dealer booster pricing was too high and the OEMbimmerparts.com booster came with the same warranty as the dealer booster.

Replacing the booster is not too hard, it is just time consuming. Expect to spend about 4 hours replacing the booster. On cars with the ABS controller in the passenger side drug bit it will be a bit easier. On later cars with the ABS controller under the booster and master cylinder it will be a bit more difficult.

Under the dash is not too bad, not sure all cars have the safety loop to keep the push rod from falling off the brake pedal if the clip falls off, but I had to remove the brake pedal from the fire wall and twist and turn the pedal to get the booster push rod off the brake pedal.

Once the master cylinder and booster are loose, getting the booster out of the cavity and the new booster in is a bit tricky. It will require a bit of patience and you will have to twist, turn and pull on the booster but it will come out of the cavity where the booster is tightly tucked into.

Once the new booster is in and the master cylinder is reconnected, you will have to bleed the master cylinder and brakes. I connect everything up, made sure not to cross thread the master cylinder hydraulic lines, then leave them partially loose and fill the master cylinder with brake fluid and then let the system gravity bleed for a few minutes, this is usually just a fast drip of fluid from the connections. Once I have let the master cylinder gravity bleed for a while, I snug up the brake line connections on the master cylinder than then bleed the master cylinder using the 2 man method of pumping the pedal and then cracking the master cylinder connections just like a brake bleeder screw. Do half of the master cylinder, then move to the other half. I usually bleed the master cylinder with between 5-10 pumps for each half.

Some may worry about damage to the master cylinder due to over stroking it. A good master cylinder should not have problems with over stroking, but a 10+ year old master cylinder might. If you are worried, put some phone books under the brake pedal to limit the travel when manually bleeding the brakes. I have found that usually bleeding just at the master cylinder is sufficient, but you may want to fully bleed the braking system after the master cylinder has been bleed. If for some reason the master cylinder fails during bleeding to over stroking, it needed to be replaced anyway. Keep in mind the ABS controller will not be an issue as you will not introduce any air into the ABS controller when bleeding the brakes. So not get worried, just bleed the brakes as the car did not have ABS. You do not need special software or anything to activate the ABS controller.

Remember that brake fluid can damage paint and it is water soluble, so have a garden hose or lots of water available for rinsing any brake fluid that leaks into the engine compartment and also protect the exterior paint on the car from brake fluid.

I would like people to report in when they are finding bad power brake boosters on their cars and how old the cars are. We do not need mileage as the brake booster life is not significantly affected by mileage. But the build month and year along with how old the car is and where in the country the car is located would be helpful. Stating how old the car is based on the build date will save people from trying to do the math when the actual reply was made in the thread.

I can think of over a dozen cars on the forum here that I know have had failed brake boosters recently and I assume there are more out there that are leaking vacuum that have not been identified and there are more boosters that will be failing in the near future. There may be a lot more failed boosters that have not been documented here as well.

Again the initial data seems to support that most of the failed boosters are 10 years old and it seems the bulk if the failing boosters are 11+ years old, many are concurring in year 11 & 12, some boosters may last longer depending on where in the country the car has spent the main part if its life.

So please report in booster failures as this will be able to give owners an idea when they are typically failing and how common the failures are. Please post the build month and year, age of car and where in the country your car currently resides.

I had to replace the booster last year on a 2001 325xiT that had a 9/2001 build date and the booster failure was noticed around year 12, however, I think the booster had failed sometime in year 11.
 

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My brake booster is whistling when letting off the brake pedal, only when the car is running. No loss in braking power, still functioning, but I assume a slight loss in vacuum. Build date of 03/2003, so this seems about right in the time frame you suggested.
 

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The other day when I ran my Engine with the MAF unplugged by accident, the RPM would increase when I pushed on the brake peddle. Has this anything to do with a failing, or leaking Bake Booster, or hose?
 

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Discussion Starter #4
It can and may be an issue.

Best to turn off the blower in the car, windows up, you may need to remove the lower "hush panel" under the driver dash.

Listen carefully to what may sound like a fan running, press the brakes and see if the noise changes/goes away.

If you hear a noise, it will likely diminish when you accelerate away from a stop sign.

The other option it to get a vacuum pump and pull a vacuum on the booster and see if it holds it or after about 5 minutes after the engine is shut off see if the booster still has any vacuum/boost left for a few brake pedal applications.
 

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My brake booster is going. 2002 330ci. It sometimes will squeak when the pedal is released, and on long trips will sometimes go into a steady squeal. The car runs dead smooth other than the occasional pee-shiver at idle at a random time, sort of like you get when the AC is kicking on.

I've been dreading the job.
 

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Discussion Starter #6
Today would have been a great day to the the job, not sure you will have too many good days moving forward.

Not a hard job, a bit timing consuming.

Plan on about 4 hours just to be sure you have time to get it done.

GL
 

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So I DO have the sound of moving air under the dash. However it does not go away after I release the bake, and seems to be at a constant, quiet level. tomorrow I'm having a smoke test performed to pinpoint vacuum leaks I discovered with a DIY smoke test I did last Thursday. I'm leaning toward CCV, and Brake Booster Hose Leaks. Place bets now.
 

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Hey Fanatics!

Thanks again for another useful thread jofj

*RESPONSE migrated from Brake Booster Thread

My Wife's 2003 325i (M54) 120-ish k miles is starting to display similar symptoms, and I think this will be valuable left here for others troubleshooting efforts.

Car has no SES.

This morning my Wife tells me she hears the sound of air following the "braking" action of the brake pedal.

Upon inspection, I found that there is a hissing sound, and after pumping the brakes a bit, there is the sound of what first i would have described as a: "modulating bubble rustle" listening to it a few more times, I have found a better way of describing the sound, which is very similar to a plastic diaphragm...sort of "buckling" plastic on plastic flexing and buckling also hissing as the brake pedal is cycled.

The pedal seems to be only slightly firmer at this point.

I will have the part in by this afternoon, as this makes me uneasy happening on my wife's car; fortunately, I work a second full time job with a euro parts house, "because 2 e46's"...
.

This just occurred to me as I was typing, and reflecting on this thread:

The Car has stalled a couple of times as it would come to a stop, or gently accelerating from a stop very much like a vacuum leak would cause...but never an SES light on this matter.
 

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jfoj, you're the man!

I'm not sure if you remember my thread (#1045691) few months ago when I had an issue with fuel trims, high rich fuel trims that just would not go away. Well, in the mean time, I did another smoke test, vacuum propane leak test, sprayed my engine with carb cleaner....nothing brought me to a solution of my problem...

Well today I got an "eureka effect" when I was stuck in traffic looking at my stft... - I got a wifi obd2 adapter few days ago.
To my surprise, repeatedly pressing on brake pedal in quick succession resulted in stft of +15% in addition to my already large ltft numbers (~10% each bank), that's roughly +25% fuel trims.

Check out my video below showing what is happening:
https://www.youtube.com/watch?v=-L-LBQL4wco&feature=youtu.be

Do you think this is a bad membrane inside of the booster? My brakes seem fine, I checked that one way valve on the booster and it is in an operational condition. Quick vacuum test on the booster showed that there is no drop in vacuum over a 60 seconds... its just like there is a vacuum leak only when I press the brake pedal.

Any suggestions?

PS: Car is a 330Ci, manufactured in October 2000, sold as MY01.
 

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Discussion Starter #12
I have not tried the Fuel Trim brake pump test, some of this may be normal because while you are pumping the brakes, vacuum stored in the booster will be depleted and the engine will have to recover the lost vacuum so in a word/way you are actually creating an additional short term vacuum leak.

Pumping the brakes at idle close to 10 times is somewhat "not" a normal condition so we could need a few others to try and see if most cars behave in this manner.

I have not timed or counted the "normal" amount of brake applications after the engine is shut off before the vacuum reservoir is depleted, but I would assume you should expect about 4-5 brake pedal applications after the car has sat with the engine off for 10-30 minutes.

The car that tipped me off to the bad brake booster did not trigger the SES/CEL/MIL as I recall but did have a rough/unstable idle as I recall. I was test driving the car for a totally different reason when I heard what sounded like the HVAC fan running when I was at a stop. It took me less than a minute to figure out what was going on by just listening to the noise in the drivers seat and having the noise drop when the car was accelerating away from a stop.

Might need to circle back around and gather more data on the smart phone and try to sort out what may be causing the fuel trims to be high. More often than not the problems tend to be soft fuel pump, vacuum/crankcase air leak or MAF related, but could be something else. FYI, there have been some documented "bad" OEM CCV valves out of the box that may cause Fuel Trim or idle/start issues and these may be very hard to identify and may be overlooked because of the "I replaced the CCV already" smoke screen.
 

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Great tip on the brake booster problems. Your detail to vacuum related problems,,mirror mine on the E39 Forums. Had to sit in the car and think for a minuet,while at idle I can hear something like a door opening or closing once every 30 seconds or so and it's tide into the brake peddle someway. If my foot is off the peddle, you can hear it plainly, if my foot is on the peddle it's a little harder to hear. I'm a stickler for vac leaks and have listed everywhere imaginable about where to find them,,except on this one. Let me know your thoughts,,have already looked into doing this job and figure it will take at the most 2 hours on my E39.
 

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Could rpm rise when turning the wheel be a symptom also? I have an elusive misfire, rough idle and intermittent ses light once a month for about a day so far no one thread has narrowed my approach
 

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I finally replaced my brake booster yesterday. It was constantly whistling, car was a little sluggish, and I finally threw a lean code on banks 1 and 2. Ever since I've owned the car, I always thought the brakes were a little soft and not very effective (this was with a fluid change and new brakes).

After the replacement, no more lean code, no more whistling, and my brake feel has improved 100%. Very happy with the results, and the car pulls very strong now. I used a brand new ATE booster from ECS. Whole process took just under 4 hours; it wasn't too bad of a job except for moving the DSC unit out of the way to get the old booster out and new booster in. I did not remove the DCS or the master cylinder, just pushed them towards the front of the car.
 

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Discussion Starter #17 (Edited)
Thanks for the update, I see you have a 2004 model year, so it is starting to look like 10-11 years these boosters are failing.

Should not be long before EVERY E46 will need a booster.

I think as of now, RMEuropean has the best price on the ATE booster for around $156? But check sources before you pull the trigger.
 

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I believe I have started the process, but the hissing noise happens intermittently on hotter days in Southern California. I went to my Indy mechanic today and told him about the hissing; however, we tried for a long time, but could not replicate it. He suggested we wait till it get to be a consistent hissing noise. I am not sure they fail and leave you with limited braking capabilities?

I am the original owner and I have one of the first E46 with the following: Built date in Germany 12/1998 E46 328i Sedan M52 USA Left hand drive. Let me know if you think I should do anything else while waiting for it to fail consistently.

Thanks :)
 

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Also want to add, check the booster rubber hose line. I was out there tonight changing my upper and lower intake to my new Mishimoto upper and lower hose and mine had a small crack in the line. I had no SES codes or anything. The hose was like totally dried out, hard as a rock. I just ordered a new one, $19 from BMW. The last time a checked it the hose was fine. I pulled the old hose off, so unfortunately, the car will be sitting several days until I get the replacement. Glad I found it but sucks the car spends most of these days sitting in the garage waiting on stuff.
 

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Here is what the brake booster check valve, hard vacuum line across the engine and soft vacuum line to the sucking jet pump look like on a RHD E46 with the latest parts.

check_valve_small.jpg hard_line_small.jpg soft_line_small.jpg
 
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