It farts! (on brake pedal release)

[tomwitte]

Recently, my 2000 996, 6spd, 73k miles, has developed an interesting sound. Upon brake pedal release, either hard or light, I get a quick farting sound from the rear. Only a quick 3 or 4 braps, you know, the baked beans and beer kind. Have not noticed any change in the brakes. One or two times, I believe it happened on a clutch pedal movement as well.

I assume that I have a vacuum hose leak someplace?

Probably unrelated, but just in case, I had the a/c compressor replaced late last summer and soon thereafter it apparently lost it's charge.

Any thoughts or if it is obvious to someone with more experience than I, can you point me in the direction of the offending culprit? I like getting greasy and can handle most jobs in the engine bay if I don't need a rack or engine removal.

Otherwise I'll have the shop track it down when the fix the a/c again!?

Thanks in advance

[number9ine]

Recently, my 2000 996, 6spd, 73k miles, has developed an interesting sound. Upon brake pedal release, either hard or light, I get a quick farting sound from the rear. Only a quick 3 or 4 braps, you know, the baked beans and beer kind. Have not noticed any change in the brakes. One or two times, I believe it happened on a clutch pedal movement as well.

I assume that I have a vacuum hose leak someplace?

Probably unrelated, but just in case, I had the a/c compressor replaced late last summer and soon thereafter it apparently lost it's charge.

Any thoughts or if it is obvious to someone with more experience than I, can you point me in the direction of the offending culprit? I like getting greasy and can handle most jobs in the engine bay if I don't need a rack or engine removal.

Otherwise I'll have the shop track it down when the fix the a/c again!?

Thanks in advance

Does it sound like the motor is making the noise? Is it accompanied by a change of any kind in engine speed? I'd agree with your assessment of a vacuum leak. Is your MIL lamp (check engine light) on? Have you checked your brake fluid reservoir to make sure it's above the minimum line?

I'd get the car started in a driveway and have a helper pump the brake while you listen for the noise at the rear. Might help narrow it down.

Mark

[tomwitte]

Does it sound like the motor is making the noise? Is it accompanied by a change of any kind in engine speed? I'd agree with your assessment of a vacuum leak. Is your MIL lamp (check engine light) on? Have you checked your brake fluid reservoir to make sure it's above the minimum line?

I'd get the car started in a driveway and have a helper pump the brake while you listen for the noise at the rear. Might help narrow it down.

Mark

No change in anything (speed, rpm, etc.) that I can tell. It is not a mechanical sound. It is the sound of air escaping between two surfaces, one of which is somewhat flexible. The only expansion on my description is this. It is rather loud. It is very consistent. More like a bellows or flapper valve than a cracked hose. That last description of a flapper is what is making me scratch my head.

No trouble lights on. I'll double check the reservoir but pretty sure I topped all those off when I pulled her from her winter's slumber. I am starting to place the master cylinder at the top of my list.

No, I don't have hydraulic shocks.... No turbo therefore no bypass valve.

Thanks for the suggestions.

Tom

[number9ine]

Your master cylinder is at the front, I'm not sure any part of the braking system would be causing that noise. I suppose it's possible since the brake booster could be causing something to "fart" on the motor from undue strain placed under braking. I would assume you'd feel something in the pedal, like increased effort or uneven braking.

If something is loud enough for you to hear in the cabin, I assume it'd be easy to localize outside the car. If the sound doesn't reproduce at idle, you could place it on jackstands and have a helper bring the car up to a very slow speed and brake. This assumes a flat surface, jackstands that can withstand the weight, proper jacking at the jack points under the rocker and liberal application of common sense. Do this at your own risk.

Mark

Edited June 1, 2011 by number9ine

[tomwitte]

Your master cylinder is at the front, I'm not sure any part of the braking system would be causing that noise. I suppose it's possible since the brake booster could be causing something to "fart" on the motor from undue strain placed under braking. I would assume you'd feel something in the pedal, like increased effort or uneven braking.

If something is loud enough for you to hear in the cabin, I assume it'd be easy to localize outside the car. If the sound doesn't reproduce at idle, you could place it on jackstands and have a helper bring the car up to a very slow speed and brake. This assumes a flat surface, jackstands that can withstand the weight, proper jacking at the jack points under the rocker and liberal application of common sense. Do this at your own risk.

Mark

It's going to the shop next week. Since it is tied to the brake system, I don't need unnecessary risks. I'm stumped. To your point regarding the strain placed under braking, etc., that is not an issue either. I get a quiet fart when just parked and idling at a stop sign. It's a louder fart if i have been up to speed (but not crazy speed, just city driving). Again, this points me back to something related to vacuum. Engine speed at idle, low vac. Higher revs from driving = slightly higher vac ( I assume).

I have tested carefully and the sound comes when I take pressure off of the pedal. This in turn, I assume, releases a backflow or check valve or some other similar device in the vacuum or interface between the vacuum and hydraulic system. I am guessing here, not a mechanic and certainly have never delved into the intricacies of brake and vacuum systems enough to accomplish anything other than barely educated guesses. Pedal pressure feels fine and normal. Braking performance feels unaffected.

Thanks again for your suggestions. I'll post the fix to the board after the shop lightens my wallet some.

Tom