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  1. #1
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    Hiss behind dash

    Hi guys hope you're all well?
    Just wondered if anyone could shed some light on a strange hissing noise I have Even under gentle acceleration that sounds to be coming from deep within the right hand side of my dashboard before I strip it all out looking blind. My car has cruise control and I'm aware that there are vacuum connectors behind there however the boost gauge and cruise control both function fine so I was wondering if it could be anything else? It is quite noticeable and has done it for months! Can the turbo cbv leak or would that cause running problems as it runs great.

    In the engine bay all of my hoses are new and tightened and vacuum hoses are fine Other than the large one to the brake master which looks to be ok and so Iv Never fiddled with it

    Thanks!

  2. #2
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    Try clamping off the hoses that run to those two items you mentioned, to see if the noise stops. Not the the brake servo though
    ...or maybe....

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    Could be a boost pipe leaking.
    19t, greens, 3" inlet, 3" downpipe with race cat, V70R catback, autotech map...

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    Boost pipe I'm not so sure about as they are new and I checked them pretty thoroughly but worth another look I guess! Jamesy though yes that's a great and very simple idea! Don't know why that didn't occur to me. Certainly will Rule bits out! Thanks guys

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    Hmm so tried that and hasn't solved it. Does sound like either in the dash or just behind the fire wall. Could my exhaust be split/leaking?? Runs ok and no smells

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    if you thinks it's the exhaust fella get it up on a ramp with the engine running & hold a piece of tissue paper near all the joints/holes to see if any movement. or a BBQ lighter if you are feeling brave

    if no free access to ramp Kwik Fit or similar would love to find a leak for you (even if you don't have one...)

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    Aww and I just had it on the stands... lol

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    Brake servo?

    Apparently they can leak at the bellows (grey bellows in pic below) under the dash behind the brake pedal. If you remove the drivers under dash cover you should be able to see it poking through the firewall. Maybe coming from there.

    Try disconnecting and plugging the brake servo vacuum pipe (obviously don't drive it but you said it hisses at idle), that should rule it out.

    1996 Olive Green 850 AWD - Forged rods, VXR injectors, 19T, Ostrich 2.0, cold air con. Follow the project here
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    Thanks that's a good plan. If that was at fault would it also affect the brakes etc?

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    I'd have thought a vacuum leak in the servo would have some impact on the braking 'assistance', depends on how big the leak is. It would still operate, there would just be less of a vacuum to help.

    Also check the connector on the front of the servo (white thing the pipe from the manifold connects to). This is also a non return valve which stops the boost pressure getting into the servo, could be that the non return valve is broken and pressurising the servo (but I would have thought braking would be affected a lot more if that was the case).
    1996 Olive Green 850 AWD - Forged rods, VXR injectors, 19T, Ostrich 2.0, cold air con. Follow the project here
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    Perhaps its a long shot, but:
    Is this noise definitely coming from behind the dash?
    There is a rubber gromett plug thingy on the steering column where it connects to the boot under the pedals and if its not sitting properly on its place then it can cause weird noises in the cabin.



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    Thanks guys Il investigate this too. Htomi that's a good point you make there too, I haven't checked that yet but I was thinking along similar lines initially too. Might just be the turbo noise I guess but it doesn't sound like it should be there if that makes sense?!

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    Right so had a bit more of a dig today and had some fun with a vacuum cleaner, fairy liquid, and duck tape

    It seems if I pressurise the exhaust from the rear I get quite a lot of bubbling where the turbo meets the manifold, where the exhaust meets the turbo and a little bit in the Flexi joint. All of the bolts are tight so not sure why it's leaking but it can't really be heard from the outside even with the Bonnet open and the engine running. I was starting to think my engine was sounding bad but I think the rattly tapping noise I can hear is probably this leak. I'm also getting a sweetish smell briefly when accelerating when thermostat is about halfway between cold and running temperature which I was starting to worry was coolant. however it's not dropping at all so perhaps this is a symptom of a leaky manifold as well.

    Next question then, should there be a gasket between the turbo and the manifold? I spoke to Volvo and they seem to think not which to me is odd and may explain why i have a leak especially with the engine being moved about a bit when the clutch was done recently.

    Also does anyone know a good stainless exhaust fabricator in the Bristol area?

    Thanks!


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    So think I may have found part of the problem, had my new exhaust fitted today 2.5 inch turbo back going to a 3 inch rear silencer (cost limitations mainly) have to say it drives like a totally different car, turbo spool is much better and seems faster. However closer inspection of the old downpipe has shown quite a considerable crack most of the way round which probably explains some of the noise !

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  22. #15
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    Annoyingly though some of the hiss still prevails even though it is a bit better. Mystery continues LOL

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    Maybe less mystery... seems to go away when pressing the brake reasonably hard when driving, any ideas? Master cylinder or just vaccum pipe hole somewhere?

    Thanks

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    SOLVED
    It was indeed the boot that goes around the steering column. I thought I had been seated properly but on closer inspection obviously hadn't and now it's very silent!

    Thanks for everybody's help

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  26. #18
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    Nice one!

    Something so simple....
    1996 Olive Green 850 AWD - Forged rods, VXR injectors, 19T, Ostrich 2.0, cold air con. Follow the project here
    1996 Nautic Blue 850 AWD - The daily, resisting the urge to modify

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  28. #19
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    And cheap lol. What's annoying though is that I thought I'd checked it. Turned out that bearing at the base was meant to be inside the boot whereas I thought it was there to allow the column to turn AGAINST the boot. Live and learn lol

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    Yeah, I've had mine pop out a couple of times, mainly when I've undone the universal joint on the steering rack to drop the subframe
    1996 Olive Green 850 AWD - Forged rods, VXR injectors, 19T, Ostrich 2.0, cold air con. Follow the project here
    1996 Nautic Blue 850 AWD - The daily, resisting the urge to modify


 

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