Hissing on throttle tip-in; vacuum leak? AOS? Normal? - 986 Forum - The Community for Porsche Boxster & Cayman Owners

Had the engine cover off and there is a noticeable hiss on throttle tip-in. Sounds very much like a turbo blow-off valve. Is this normal? Could it be a vacuum leak, and if so, is there a common place to look? I also tested the AOS with a manometer. At temp, readings are about 5.25 in/H2O range and with revs hitting about 3k rpm (not at load), reached about 7.25 max. Seems normal range, but could it be the AOS? Any insight will be appreciated.

If you clued us in on what you're driving... What year? Transmission? Seriously. We need to know what you're driving because these 986s have technical/parts specifications that are all over the map. Maybe include that information in your signature. A hissing shuld be easy to diagnose, and absent all that ancllary information you provided. Get one of the stethaascopes devices and start poking around with it. In the alternative... remove easy to access hoses one at a time and listen for the hissing to stop. You might find the problem. Good luck.

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Thanks. Rookie mistake. It’s a 2003 Boxster S, manual trans. What’s puzzling me is that it’s just a short burst at tip-in, then goes away. I would think a vacuum leak would be steady. Maybe it’s normal induction noise that I’ve just never heard before with the covers on. I can do some digging. Maybe a smoke test. I’ve found that usually any problem I’m having has been had before, so thought I’d start here and see if this is one of those obvious things based on collective experience.

OK. So electronic TB. Oh, I'm newbie enough I had to Google tip in. Me? I'm considering connecting with another 986 owner, local, and get their opinion on the sound. Next, I'm tightening all screw/bolt connections relevant to the TB. I'm next disconnectng the electrical harness from the TB and spraying/cleaning it with electrical cleaner. Do you hear the sound from the cockpit... or do you have a helper give it gas/pressing the pedal... with you at the sound source? With your ear ON it, that might reveal a culprit. I'm next, and carefully due to brittle hoses, disconnecting/connecting hoses listening for irregular sounds or changes in engine RPM. Are the intake boots, connecting the manifolds, properly installed and tightened? If you're hearing the hiss momentarily, methinks that once there's sufficient pressure, the leak is reduced by the laarger amount of air passing through. I know... not a scientific description... but a more technical member might chime in correcting me where I'm wrong. Good luck! Report back.

Simplest things first... Check your air intake snorkel for debris Check your air filter and make sure it is clean. If you suspect a vacuum leak the get a vacuum gauge and do a vacuum test. They cost about $20.00 -$25.00 at any auto parts store. A vacuum gauge is a basic diagnostic tool that everyone who works on cars should have in there tool kit And know how to use it correctly.

Been a long time since I’ve driven my 03 without the covers in place - probably somewhere in the order of 100k miles. My off the cuff answer is “works as designed”. I noted the sound, said “hmmm. Interesting”. And got on with life. This falls under the heading of Fix It Until It Breaks. My $0.02.

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Got one. Don't know how to use it. Yet. Think II'll spend some time this weekend testing it... learning how to use it. Learning in a pinch, car down, no bueno. Blue, Thank you.

If you can't hear it with the covers on it is likely normal. Whenever I run with the covers off I am convinced that the engine is about to grenade because of all the noises, put the covers back on and all is OK. Only my opinion.

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For a basic vacuum test: Make sure the car is fully warmed up... Best to take it for a 15 -20 drive..that way it is fully warmed up and things that use vacuum to function like the EVAP System have done there system checks. Then hook up the vacuum gauge start the car and just let it idle. Do not rev the engine just let it idle ... Hold the gauge in you hand do not set it on the engine or car body. There is a spring attached to a counter weight inside the gauge. Vibrations set up harmonic oscillations in the spring causing the needle to move or quiver giving false readings. On an engine in good condition your looking for a reading18-22 inches of mercury At sea level With a rock solid needle. If your at high elevation like say Denver Co. you have to adjust your readings. You have to add 1 inch of Mercury for ever thousand feet above sea level to the needle reading.

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Thank you, Blue! I'm 3250 above sea level. Also, I haven't used the gauge... so... newbie question here and without reading the instructions (device is in the shed, i'm preparing for a short shift/pt gig, not handy right n ow) to where am I hooking up the gauge respective to the car? I'm going too be doing some PM this afternoon or tomorrow morning (before it gets too hot). I'll be giving the gauge a run. Cheers! Thanks!!!

Modern day cars are sometimes a PITA as to finding a place to hook up the vacuum gauge. Ideally you Tee into any vacuum line that comes off the Intake manifold after the throttle body. On my 2000S I Tee into the vacuum line that connects the fuel pressure regulator to the intake manifold. So have a good look at your engine bay and find vacuum line after the throttle body that you can T tee into. Perhaps the line that goes to the resonator flap....

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Thank you. I'm giving that a run tomorrow morning. Have a good evening.

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I think this is the correct conclusion. I vacuum tested it, smoke tested and cannot find any kind of leak. Put the covers back on and am just driving it. Who knows?

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And driving it *is* why you bought it! Of course, I suspect that Cleveland is in the middle of the week of rain that NJ is seeing right now, so not so much fun driving to be done…