High Frequency rattle in engine compartment

AHudson · 03:46 AM

Hi Pelicans, I recently renovated my charcoal canister on a '85 3.2 Carrera (http://forums.pelicanparts.com/porsche-911-technical-forum/980170-carrera-charcoal-canister-problem-refurb.html) and in the process upset one of the components in my right rear engine compartment that now makes a very high frequency rattle/buzzing when idling.

I can feel the thing humming and have tried to reset it on its perch (barely visible in photo) but I've either knocked something loose or don't have it attached properly.

I searched the keywords I could think of to no avail. How does this thing attach and if it's now making noise, is it on the way out???

BTW, the component is the round black orb looking item in the post below with the braided hose coming out of the top center of it. Hoping you can help an novice out!

AHudson · 12-29-2017, 03:43 AM

JRSIII · 12-29-2017, 05:46 AM

That is the vacuum switching valve. See Bentley Page 100-4. This explains and illustrates its operation. At least on an 88, and I'd assume an 85 is the same, it's held in place by a clip bracket that is secured to the back upper oil tank mounting stud.

gomezoneill · gomezoneill's Garage

Additional air valve. It's like a pcv valve it lets warm air travel from the oil tank to the intake to prevent icing.That rattling is normal operation, it operates when temps drop below a certain level. I never drive my car when it's cold so I disabled it. The photograph in the vacuum limiter that's not whats rattling look further down the larger hose and you'll see the valve it has a silver bead and two hose clamps.

http://forums.pelicanparts.com/uploads24/Namnl+st1427377447.jpg

The valve is #36 in this diagram

nene · nene's Garage

AHudson,

I have an 86 and that rattle/vibration has been driving me crazy for the last past year! At one point I moved a few hoses (or something) within the engine bay by the air-filter side and was able to stop the rattling, but did not pinpoint the exact item that was causing it.

AHudson · 12-29-2017, 11:15 AM

Quote:

Originally Posted by JRSIII

That is the vacuum switching valve. See Bentley Page 100-4. This explains and illustrates its operation. At least on an 88, and I'd assume an 85 is the same, it's held in place by a clip bracket that is secured to the back upper oil tank mounting stud.

Thank you a ton. A clip bracket? My bracket has a hole in the bottom, exactly sized for the bottom of the switching valve, yet nothing to secure it. Will investigate and thank you again!

Quote:

Originally Posted by gomezoneill

Additional air valve. It's like a pcv valve it lets warm air travel from the oil tank to the intake to prevent icing.That rattling is normal operation, it operates when temps drop below a certain level. I never drive my car when it's cold so I disabled it. The photograph in the vacuum limiter that's not whats rattling look further down the larger hose and you'll see the valve it has a silver bead and two hose clamps.

http://forums.pelicanparts.com/uploads24/Namnl+st1427377447.jpg

The valve is #36 in this diagram

Wow, gotcha. Very informative and thank you. So the 'buzzing' is not from the valve but the limiter? Who'da thunk it?!? Is the securing thingy Item #33?

Quote:

Originally Posted by nene

AHudson,

I have an 86 and that rattle/vibration has been driving me crazy for the last past year! At one point I moved a few hoses (or something) within the engine bay by the air-filter side and was able to stop the rattling, but did not pinpoint the exact item that was causing it.

Well, looks like Mystery Theater is solved! That's exactly where my noise is - about to be 'was' - coming from.

darrin · 12-29-2017, 11:48 AM

Believe it's called the ambient air valve -- this thread has a lot of info -- still wonder whether it can be removed/hose plugged and not risk icing the throttle body during cold weather warmup ---

http://forums.pelicanparts.com/porsche-911-technical-forum/375188-ambient-air-valve-questions.html

AHudson · 12-30-2017, 09:36 AM

Quote:

Originally Posted by darrin

Believe it's called the ambient air valve -- this thread has a lot of info -- still wonder whether it can be removed/hose plugged and not risk icing the throttle body during cold weather warmup ---

http://forums.pelicanparts.com/porsche-911-technical-forum/375188-ambient-air-valve-questions.html

Thanks for that, read 'em all following your post. Far as your question, I just get so freaked out about rerouting, undoing, creating or losing vacuum anywhere in the FI, I just don't. Others may know the safety/wisdom therein.

I still can't figure out how this air valve is 'seated' on it's little perch. Am I missing some rubber compression grommet that holds it down? Of course what I'm hearing (and I guess all of you) is it vibrating against the metal perch. If secured I presume we'd hear nothing.

darrin · 12-30-2017, 09:58 AM

believe the membrane itself is causing the vibration and the valve body (and bracket?) is amplifying it -- think kazoo

Also, keep in mind the buzzing generally manifests itself in cold weather and I see that Alabama is experiencing the same weather that would cause mine to buzz (and then stop buzzing when temps over 50ish) --

gomezoneill · gomezoneill's Garage

The noise is the valve. It has a spring and a plate that seals the orifice. It's cavitating causing the rattle. The valve is #36 in the diagram posted above. It's like a reed with wind (in this case vacuum) passing around it.

AHudson · 12-30-2017, 01:45 PM

Alrighty then, the ambient kazoo is driving me slowly insane, making my car sound like it has caffeinated crickets in the engine compartment.

In typical OCD fashion, I'm going to try and isolate/insulate said crickets, spending $1,000 worth of time on a $50 problem, as is my norm. We'll find a solution!

NY65912 · 12-31-2017, 08:06 AM

This exact problem scared the bejus out of me a few weeks ago. It went away after a good warm up. This event has never happened before. Is this due to a failure of the part? Cheers

gomezoneill · gomezoneill's Garage

Cold weather opens the sensor/ switch allows vacuum to the plastic actuator thus allowing vacuum from the throttle body to draw air from the oil tank for heating op the intake.

AHudson · 01-01-2018, 02:53 PM

Okay, many thanks and praises for info, including those of you who've shared my head-scratching over the noise or how to alleviate.

It appears Porsche was duly aware as my perch/mount for this thing had a rubber grommet to isolate against body. I looked at the diagram kindly shared by gomezoneill and noted a securing clamp of some kind, but mine was merely a shelf with a hole in it (see pic.)

I pulled the bits out, put a grommet on the bottom of the ambient air valve to at least hold it steady to perch. Tight fit. Test drive (while 30 degrees in Alabama!) yielded NO NOISE other than than a hum which I only 'think' was the device, but thankfully no rattle. The grommet may not be durable enough, but time will tell.

Hope this helps, you guys surely helped me.

nene · nene's Garage

I was curious to hear/feel the rattling sound so I fired up my 86 and waited for "it", and interestingly enough, the sound is within the Valve. I moved it/shook it and the sound went away, so I doubt the bracket is the culprit.

One thing I noticed is that if I take the vacuum hose that connects to the top of the Valve, I feel no vacuum/suction?? Is this normal?

I expect I will be replacing the device.