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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.
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???
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.
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?
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.
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.
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.
No, it's the diaphram failing and it's inability to handle oil flow leading to (1) smoking and oil consumpltion, (2) hydralock, as well as it's (3) brittle parts 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.
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.
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.
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
10-24-2023 | 12:37 AM
vid inside - big vacuum leak / hissing at idle - runs fine otherwise
