vid inside - big vacuum leak / hissing at idle - runs fine otherwise

01/98 C2. Video attached of hissing at idle. Runs great otherwise and doesn't 'miss' anywhere in the powerband. stumbling/rough idle and sometimes climbs to 1-1.5K rpms randomly. Started a new thread here (posted earlier about this after getting car washed thinking the wash was related - still hoping it's not). Have replaced: -ignition coils x6 -spark plugs x6 -maf Misc: -thoroughly cleaned both intake plenums and manifolds (small amount of oil found in both) -cleaned throttle body & idle control valve (small amount of oil found behind throttle body) Is this what a bad AOS sounds like? Can easily remove oil fill tube cap. No smoke ever from the exhaust. No oil leakage except for a bit showing from bank 1, cyl 3 spark plug tube. Idle is a bit iffy and sometimes climbs to around 1-1.5K randomly before I tap the gas and it comes back down (not always). Something to note: I snapped the serpentine belt not too long ago and it took that vacuum line that goes underneath the second intake plenum with it (the one that goes to the same plenum holding the evap valve). I replaced that vacuum line. When I reinstalled everything, I think I accidentally covered that vacuum nipple in the plenum holding the throttle body (the vacuum line coming up from the right). I just discovered that the other day when I pulled both plenums. So that vacuum line opening may have been covered for ~1K miles. I didn't realize that the first plenums boot should actually favor the left side, and the opposite is true for the rear (front) plenum. Anyways something to note. Read that again if confused! The vac line that was covered is the one that goes underneath bank 2's intake manifold to something (fpr??? I don't know). The hissing noise in the video is constant. Sounds like a vacuum leak to me, but a part of me is thinking something fuel related? doesn't sound like it's coming from the AOS either. I have a longer video but couldn't email it to myself. Hoping to get some input with the video. I have an AOS so might just install it tomorrow and tackle that job with the minimal tools I do have. Thanks in advance

Codes? I�d try to isolate the sound with a length of tubing. If that failed, my next step would be a smoke test. Id suspect that vacuum line you describe going to the rear plenum is for the intake resonance flap.

How about a recording lasting longer than 4 seconds, with maybe some RPM changes, steering changes, in gear vs out of gear, A/c on vs A/c off , steady RPM at 1500 RPM vs idle RPM ..ect.. It kills me when someone asks for help to diagnose an unusual never heard of noise strange noise , but then has no video/audio, or a 3-4 second clip..

Video wouldn't give you a better understanding whether it was 4 or 20 seconds. Shortened it to email it to myself. The location of the hissing is unidentifiable and revs drown it out. Only appears when car warms up. Small oil drip found this morning. Looks like it would come from the AOS? Pic attached. This is looking at bank 1's side, to the driver side / front left of oil sump (if looking from behind car). Above that blue circle is the AOS and that spring-looking tube connected to the AOS. The tube itself is still connected but obviously may be leaking... The characteristics: - Sometimes the idle stays around 1-1,500 RPMs, or climbs there by itself. Tapping the gas will sometimes bring it back down. - The hiss doesn't get louder with revs and the noise of the engine drowns it out. - The hiss is only present when car has warmed up. - No loss in power or misses through powerband. If you don't think it's AOS (still no smoke from exhaust and maybe a little bit of vacuum when removing oil cap (but not a lot)), what else???

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Thanks on the smoke test and tube-test. So basically just put my ear up to a piece of hose? I'll do that. It is, but I think I messed something up with the vac line in the first intake plenum on the right (see OP) when I reinstalled the plenum. I think I drove the car with the vac line covered by the rightmost plenum boot. Learned the hard way that the first plenum should favor bank 1's side, and the 2nd plenum should favor bank 2's side for the sake of those vac lines not getting covered. Basically I am trying to avoid doing the AOS if it doesn't need to be done right now. Don't have all the necessary tools and would probably take me ~15 hours of work. Edit - will grab codes when I get to autozone / use their reader.

Only when it warms up? Just a wild guess, is it a very noisy SAI? Cant miss it if you get close to it. Mine made a heck of alot of different noises. Some extremely loud. Now it's silent. For now...

Unplug the MAF Sensor and remove the rubber boot from the throttle body... Make sure the throttle plate shaft is resting on the mechanical stop screw .. Start engine and let idle then take your thumb and place it over the hole to the left of the throttle plate ( where the air goes into the idle speed controller) .. This should cause the idle to go down to below 600RPM, IF the idle speed does not go down below 600RPM, then YOU HAVE AIR LEAKING IN SOMEWHERE IT IS NOT SUPPOSE TO..

Just used brake cleaner. Sprayed it back towards the AOS and the idle changed significantly. Tight everywhere else. Can also smell burning oil, assuming from the oil that dripped down as shown in the pic. My guess is it's not bad enough to smoke but the AOS is bad. Don't know how there'd be a loose connection. Nothing looked disconnected last I had the plenums off / was underneath the car. How does a bad AOS cause a vacuum leak? Really don't feel like doing the AOS myself - anyone know of good indy's in the Hermosa Beach, CA / surrounding areas?

Might just be the rubber accordian boot in the back side of the aos?

codes: P1124 P1126 P0130 P0150 P0102 P1319 P1313 P1316

Aron @De Jeeper u da man. Got my finger up behind that lower tube (the ribbed one) and sure enough, there a hole in it on the backside that you can't see. That's the leak right there. I'd bet the AOS is still good. Can I replace that tube with the AOS still on the car? Looks like I could loosen those two 10mils but I don't want to un-seat that lower part of the AOS from the block accidentally. I wonder if this is most people's AOS's issue.... just that ribbed tube and they end up replacing the entire AOS thinking it's bad. Given the location of the AOS, I don't think Porsche planned on it ever going bad until 200k+ .....just my random opinion.

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The real issue is the diaphram failing and it's inability to handle oil flow leading to (1) smoking and oil consumption, (2) hydralock, as well as (3) brittle parts that fail which can lead to intermix. Porsche may never have planned on it failing initially, but they sure did find two stop gap measures quickly. (1) the development of the motorsports aos, and (2) TSB that was issued to run less oil in your engine. You would do yourself a favor by eventually replacing yours, especially if its as old as the car. Your hand crafted early build 996 won't prevent intermix and hydralock.

Quotes source post: 19076326

gotcha. So really the only dead giveaway for failed AOS is that big plume of smoke. in my case I experienced this loud hiss only when warm, but no smoke, indicating a vac leak coming from that ribbed tube. I also had oil drip as shown in the pic. Hoping this will solve it - gonna try replacing just the tube tonight. I have the tube from the vaico AOS I bought. If still bad will replace entire AOS.

Quotes source post: 19076339

A plume of smoke is a good giveaway. If you are enjoying the twistes on the PCH, the plume can get worse and when the flames start coming out if the back, you may be real close to hydralock. You know my recomendation, if you are going to replace it, you might as well go the extra step and get the UAOS track version so you don't have to worry about twisties, overfilling, hydralock, smoke, etc again. No brainer.

The test is simple. Buy a $40 manometer and put a barb into an old oil cap. If it reads more then -4-6 in-hg rhen its bad. That rubber boot connector in back is held on by a spring clip that comes with a preinstalled "lock" that allows u to slide it on and remove without tools. If u have a manual trans u should be able to get up there and replace it without removing anything

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Sweet - my AOS's boot didn't come with that colored finger lock on the bottom (I know what you're talking about) so am going to have to remove the boot and the clamps together / then reinstall using the OE clamps I think.

99951263000 mk1 part# of clamp with lock. Its the top clamp. The lower one is just a hose clamp.

Quotes source post: 19077037

on the AOS currently in the car they both have those hose clamps on it. This would prob make it easier but think I can just finagle it with a pair of hose clamp jaws that lock in place (mine do and makes it easy)...unless I'm misunderstanding you and you really think this is the way to go.

any way that u can get it to work is the correct way. I was just adding info to the thread

update- *fixed* Changed out that ribbed tube on the bottom of the AOS and used hose clamps instead. prob wouldve been fine with a new clamp (see jeeper's post). No need to unloosen the AOS or disconnect. Definitely undo that ground wire with bank 1's variocam connector though for more space. First installed it with hose clamps, then decided to try to do it the OE way reusing those snap clamps and even with the clamp jaws I have, it's almost impossible (I'm sure if I kept trying I would've gotten it) to get those clamps back on and get that tube to seat fully/correctly. That, and you have to watch that you don't pinch the tube anywhere. I gave it a solid hour. Anyways tried my best to get at least one of the snap clamps on and it just wasn't working, so went back to the hose clamps. I loosened the clamps all the way and put the tube on first. Tube goes on very easily without clamps. Then slide the hose clamps into their slots and twist to get them to seat correctly. Tighten (but don't overtighten) both clamps. I actually put the bottom clamp on by undoing it fully and then reconnecting it around the tube. I only had one clamp wrapped around the tube when I initially installed it. Found it to be easier to get it to seat correctly that way. clamps use the 8mm socket. Anyways she idles perfectly and is running great. Thanks all for the help. edit- the hiss from the OP video is also gone.

Now please get a manometer and check the vacuum at the oil cap. That way u can verify the aos is operating correctly and u can check it when u do oil changes.

Or how about instead of a manometer, just go to a local autozone to pickup some window washer fluid and have her turn into a smoke machine? Dude..... unreal amount of smoke. Turned it off immediately and towed it home. Looks like that AOS is going in this weekend. Going to bite the bullet and do it myself. Only thing am worried about is that I had to crank it a few times to get it to turn over in order to get it in my driveway (no other possible way with towtruck and hills out here). Then I pushed it into my garage. Hoping I didn't do any lasting damage and it will finally be fixed after changing the AOS. I wonder if fixing that hose / vacuum leak somehow caused the AOS to finally show symptoms. For the record it was running great for about ~50 miles there before a very loud squeal started to come about this evening, then the smoke followed.

No, what happened is that the AOS started failing first and causing high vacuum on the crankcase, this put extra stress on the old tired rubber accordion hose on the bottom of the AOS and caused it to fail which let vacuum/air from outside to the inside and caused your idle problems.and missfires... Fixing the rubber accordion hose ONLY caused all the vacuum to go back on the crankcase/oil sump....sucking up a massive amount of oil... You will know if you have killed the motor by (1). as soon as you replace the AOS and start driving if you start loosing coolant through a cracked cylinder or (2 ) if it bent a rod you may not know for quite a while if it starts burning oil or if you have a noise....

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So is everyone letting me fix that accordion hose without mentioning that if the AOS is bad, it will only make things worse? Lol, did I miss something here? No one has even mentioned deleting the AOS altogether. Or the fact that you've been driving with an AOS delete / catch can setup for 25,000 miles (that was in 2019 at least) and have been fine with it. Leaning towards an AOS delete / catch can setup now, at least to start driving again. Maybe will pull plugs and crank first to get any possible oil out but I don't think there's much. Also I found a quote from you on a thread I went deep into saying something about the rubber piece inside the AOS failing due to extreme vacuum fluctuations, or that original AOS's rarely fail (even though they obviously will eventually). I thought it'd be interesting to note that the guy driving my car at the car wash had no idea how to drive a manual, so he'd rev my car up, then let off the gas completely / use the clutch to move - rinse and repeat. Not sure if that had anything to do with mine failing, but I guarantee you my car ran perfectly before that day at the car wash. Originally I thought it was water being sprayed on the coils via pressure washer but now am thinking it was the dude's driving that did it in, even if it was for a short amount of time. Anyways, I like 808Bill's AOS delete setup a lot and will more than likely go with that. If I'm understanding correctly, his AOS completely failed, like mine, and his catch can setup works just fine.

Expensive car wash. Between that and a different post about a �friend� roasting someone clutch, these cars can be expensive enough for a single owner, let alone, �renting� it to someone else, even for just a few mins� The car wash could have just been a coincidence, or a straw that broke the camel�s back. The AOS on these cars is routinely at the top of the list of failure items, along with water pump, bore scoring, IMSB, Y cable, solenoid/tensioner pads on 3.4s, etc. Not much you or anyone can do about it, aside from preventatively replacing it, which is not super easy to do ($1000, much of which is labor). I�d consider the UAOS as well. Nothing against the catch can, but the UAOS was designed by a pro and installed by other pro engine builders, which means data points of hundreds. I�m just saying it is recommended more than any other AOS option. I guess this thread is also a reminder to anyone else who has a loud hissing/vacuum sound that suddenly emanates from the engine bay. The 996 experts who have been around the block would know, but a newbie would not. Many of the folks on this thread were mentioning the vacuum test. Again, it�s not really your fault you didn�t drop everything, quit your job, and get on this asap. It�s just unlucky.

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it was designed by pt3 above if I’m not mistaken. id like to drive the car today though and blow the oil out / smoke. Planning on covering the TB (I have the mechanical cable TB) and just undoing the hose to the left that comes from (goes to??) the AOS. More or less to see if she’s still healthy and so I don’t have to tow it to a shop / can drive it there. anything I’m missing? Obviously can’t leave that hose hanging there with no end as oil will come out (because my AOS is bad), but was thinking I could zip tie a hose and just route it to an empty can / bottle for the time being (however long it take for the smoke to disappear). If that works I’ll prob just go the catch can route and route the vac/vaper line to bank 1’s header. Saw 808bill route it to a nipple on the oil cap but am unsure of how that works correctly…. edit - getting some quotes at nearby shops. Think I can find someone to do it for ~$500 cash and they'll use my vaico aos. Have two shops I've reached out to / have been recommended. 8 hours is an absolute rip off. Can be done with bank 1 manifold on car and exp tank still there. Lowering of engine needed and alternator removed helps but that's it. Dude told me this over the phone and then admitted to charging 8 hours when it takes him 1. Can't blame him, prob makes a ton of money.

808Bill is running the UAOS on his car now, and has been for quite a while. In fact he just ordered a UAOS for his wifes ( new to her) car that I will shipping out tomorrow.. The UAOS is not just a catch can/AOS delete, It is a Patented redesign of the complete AOS System....It has a diaphragm that is twice as thick as the factory one and moves it to close to the throttle body for easy service, and is 4x more effficient at removing oil from the blow-by gasses, and incorporates an in-line accumulator that ( on the Track Version) has another oil drain back to the engine..and makes the "AOS Body" a "Llifetime Guaranteed part".....so no need to ever have to spend the time/money to get way back in there.. You can temporarily disconnect the AOS breather pipe from the intake and plug the intake, and leave the breather hose open ( it will have oil vapor/smoke coming out) or even run it to a catch can ( it will still have oil vapor/smoke coming out of the catch can) just to run the engine to see if there is any damage and to clear the smoke/oil from the exhaust/cylinders..... After you have assessed the engine for damage, then you can decide on what to do next....

Thanks pt3. Yep, aware that it's a fully functioning replacement and easier to access. I was saying 808's catch can DIY seemed to last him a little bit before replacing the aos entirely with your system. I did just get a confirm on a local shop to do my aos for $500 cash, so may just go that route. I have big trust issues though, even though it's a reputable shop and was recommended to me. I have some time this weekend, just doing some soul-searching on why I'm being such a p*ssy about doing it myself lol. Prob because I've already spent a good chunk of money between plugs, coils and a new maf (all of that unnecessary) and my hands are cut up / my neck hurts (ah, to be 29...). I do have a new vaico aos on-hand from FCP euro. If I do it myself it'll be fixed this weekend and I know it'll be done right (truly right would be with uaos, but I need my car). Maybe uaos next time around. Shipping is really tough to CA and takes forever unless you spend ~$30. Think my plan for today/this weekend is: Pull TB plenum off to inspect manifolds for oil. Clean out best I can with paper towels. Then pull all 6 spark plugs and inspect for oil. If coated, will pull them and crank (using starter) with rags sort of "stuffed" into the tubes to catch spewing oil. If not coated in oil, I'll just reinstall the plugs and take a breath. I also want to get my iphone camera up over bank 1 intake manifold and take a video for possible pooled oil. If less oil than suspected, I may just go ahead and give the AOS a shot. Have to pickup a piece of wood from home depot to support motor/sump with jack but I think I have everything else. Those two snap clamps are going to be an absolute b*tch with the accordian hose so may stick with the worm clamps like it is now, but we'll see... I think I can pre-install hose with top clamp and just worm clamp the bottom with ease. ^ what do u think of that? Cheers - will keep thread posted. Using it as my journal at this point... obviously fingers crossed the engine is fine and I haven't bent anything... I only started it 3 times and it only ran for ~5 seconds each because I had to move it (hand rolling impossible where I was at). The last two times you'd think I had a bonfire going. Car never died itself and each time I was the one that turned it off. It has sat overnight though so will not start it again until I've pulled plugs and installed new aos.

Cylinder #4 and #6 are the ones to most likely get the most oil, and possible damage ... I would turn the engine by hand to make sure no cylinder now has so much oil it is hydrolocked. Then take an inspection camera and run through the throttle body over to the intake on each side to see if oil is pooled in the low spots...If the engine turns by hand and no oil is pooled, it is safe to start the engine. IF the engine won't turn by hand or if oil is pooled in the intake, it must be removed or it can cause damage ( if it hasn't already) ...If there is just oil wetness or just a couple onces of pooled oil it's ok, there is always oil residue in the intake from the breather system oil vapor..It will never be completely dry.. I would never install a factory style AOS for any reason, only the UAOS System makes any since.....If I had to drive to car temporality I would disconnect the breather pipe and plug the intake like mentioned above, then wrap 2-3 shop rags around the end of the breather pipe as a filter then place a zip-lock bag over the whole end and cut 2-3 1/4" holes in the top of the zip-lock bag to let the filtered blow-by gasses/ oil vapor out...I would drive modestly like this until it could be done right with the UAOS..I had to do this on my car when the Factory AOS failed at Talladega and I had to drive home, and I drove it for a while with this temporary cheap set-up as I designed the robust UAOS System .. On the 996.1 manual cars the UAOS System can be installed in about 1.5 hours . No need to remove the intake ect. Can be done as you already know mostly from the bottom just lowering the engine down on the cross member , and a little work near the throttle body. If installing the Track Version, you need to remove the alternator and oil fill pipe, but not necessary on the Street only version..

Got under the car and the accordion hose was off at the top... Ripped again too. I didn't use the snap clamps, I used worm-style hose clamps that were tightened with an 8mm. Maybe the clamps ripped it? They do have sharp edges and I could see how it would dig into the hose. But pt3 you're telling me the suction from the bad AOS ripped the hose.... it was a brand new hose this time around. Just crazy it creates that much vacuum to actually suck the hose out of its place. I swear it was on there good. Maybe those snap clamps are on there for a reason..... lol

You may want to go to Skips threads on the UAOS to understand what happens when the oem AOS fails. You don't want this to happen. People don't take the repercussions seriously enough. Best case you get a little smoke. Worst case you get intermix and/or hydralock. The oem aos is a brittle piece of plastic and rubber that will fail in many places. Especially if its as old as the car. Do yourself a favor, get the car into the best porsche specialist you can find to make sure you haven't damaged the car, and if not, get yourself the UAOS track version. You won't loose any sleep again. While you are in there, replace the water pump with another oem water pump. That was you will mitigate two catastrophic failure points.

Yes, the factory system is designed to have 4-6 "inches of water" vacuum on the crankcase., when it fails you can have as much as 400 "inches of water" vacuum ( 29 inches of mercury) ... that's 100x the vacuum the system is designed to handle...or anything in-between.. Glenn has been writing a paper to inform on all the advantages/benefits of the UAOS System. I never mention them all in a single post because there are so many, like I didn't mention the warning system or the fact that all coolant lines are no longer used due to the relocation of the low pressure/high velocity zone...I spent a year designing and developing the UAOS System..

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Yeah, i am by no means an expert nor plan to be. Just a happy customer that paid full price for the UAOS track version that wants to help Skip to collect all of his thoughts and information on the UAOS and condense into a simple summary. Similar to what Pete did with his explanation on UAOS instalation. UAOS failure is a topic with complex implications that most gloss over. After experiencing 2 factory aos failures and a motorsports aos failure I appreciate what Skip has designed.

Quotes source post: 19080196

Yea, that's another Myth that is hard to bust, many believe the Motorsport AOS is leaps and bounds better than the Factory AOS when the only thing it does different is it holds 1/4 quart more oil and lowers your oil level by 1/2 quart when it's full and can't handle any more...

Took me an hour to get it out. Left bank 1 manifold on, expansion tank in, alternator in, and didn�t touch motor mounts. Would've taken less had I not unbolted the AOS prior to getting the bottom hose off (not the accordion, the one to the right). So much crap going around about this thing. About to make a thread about how to change this in less than an hour. Installing it is going to be easier than taking it out. Couple of things: 1 - I still stand true that my AOS was fine and that the initial vacuum leak was caused by a failing / ripped / old bellows tube. It was not a faulty aos that caused the rip in the tube. The fact that the tube was still �there� and seated (albeit with a small tear) is the reason the aos never smoked / failed completely (it would've eventually obviously). The reason the aos started to fail in the first place is because the bellows tube was ripped on the backside. The reason it became dislodged on my go-around is because I used hose clamps and probably over-tightened it. When it dislodged, the aos failed completely and at a more aggressive rate, showing the smoke and loud squeal. 2 - the "rubber inside" talked about was 100% fine prior to me ripping it up to take it out and take a picture of it. No "rips" or whatever people say happen to it were present. The spring was functioning just fine. It�s a rubber piece covering a circular piece of plastic suspended by a spring. 3 - my aos and car would probably still be fine had I not re-installed a bellows tube with worm-style hose clamps. Should've just unbolted the aos from the block to give me more wiggle room and bought the finger-safety style snap clamps that were on the car originally. 4 - the reason i found a small amount of oil in my intake manifolds originally (when I made that other thread about the car wash a few weeks ago) is because the bellows tube was ripped and preventing the aos from working at its full capacity. I still don't believe there was anything inherently wrong with the aos as-is (before I stabbed with a flathead, obviously). My take on this is that: The bellows tube malfunctioning / ripping will cause the AOS to malfunction. It is not the other way around. I don't see why an original AOS wouldn't last well past 200,000 miles. There have been many people state that they see original aos's last for a very long time. My proof for this is having an original ripped bellows tube cause a vacuum leak, but it was still seated properly and had a small tear so the aos never showed smoking/aggressive symptoms. My aos never "failed" during this, it was just on its way. It was only when I changed the bellows tube with hose clamps that the bellows tube slipped completely off the north port and that�s when sh*t hit the fan. The car was running great for about 20-ish miles there and then the squeal came on very aggressively out of nowhere. My guess is the hose slipped due to me using regular worm-style hose clamps. So replace the bellows tube, or put your finger up behind it to feel for rips before you change your AOS, and don't use hose clamps like I did. Use the snap clamps with the finger safety so you can get it on without removing the aos. And unbolt the aos so you can get the tube on easier, don't try to do it with the aos bolted to the block. F*ck this thing and the internet. Just break the coolant tabs off the aos and remove the lines that way. Saves 30 minutes. You're replacing them with a new aos anyway........ and use hose clamps on the coolant lines. Position them when reinstalling so you can get a socket on them. Reach under the manifold with your right arm and over the bank 1 side with your left arm like shown. ^ this pic is supposed to show how you get your right hand behind the manifold. Look under the manifold to get eyes on the two hoses while you finagle them out. this rubber piece was fine before I ripped it up to take it out. Looked brand new in there. ^ murdered it with a flathead. spring that was suspending the circular plastic piece.

No way that bellows being torn caused the AOS diaphragm to fail, period....believe what you want...

Reinstalled everything. Smoked out the entire city of hermosa beach for about 20 minutes before it went away. I�m getting a faint whistle now. There�s another thread I found where a guy replaced his AOS and had the same thing - even took it to a dealership. I have re-checked everything a thousand times. Everything is tight. is it possible that a brand new AOS gives off a faint whistle for a bit? I�m driving it and it seems fine. ICV had to re-learn the idle but overall idles great and pulls hard now. Edit: stopped whistling just now for the first time. Only thing I did was Jack it up one more time and have a look. Wiped away some excess oil that had dripped from before, but no leaks present. I pushed in on the hose one more time above the variocam connection but it didn�t move. Dunno. Seems to be running fine. No knocks or smoke anymore. Fingers crossed.