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Turbo overrun boost noise?

Help, Advice and DIY Tutorials on Volvo's extremely popular car line -- Volvo's 1990s "bread and butter" cars -- powered by the ubiquitous and durable Volvo inline 5-cylinder engine.

1992 - 1997 850, 850 R, 850 T5-R, 850 T5, 850 GLT
1997 - 2000 S70, S70 AWD
1997 - 2000 V70, V70 AWD
1997 - 2000 V70-XC
1997 - 2004 C70

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Turbo overrun boost noise?

Post by BlackBart » Mon Apr 29, 2019 5:11 pm

Should an 850T (1994) on overrun make an audible blow-off valve noise? ....FFT-FF-FF-FF-FF-FF ....or CH CH CH CH CH? You've heard it on Subaru rally cars.

I don't remember it ever before. Our system is a bypass valve, not a blow-off valve, correct? At WOT or near it, accelerate from low speed, let off the throttle - I get the FT FT FT FT noise from under the hood.

I'm wondering if there's a intercooler hose leak, or a turbo controller issue, or a vacuum line leak? I'm also trying to diagnose a recurring but inconsistent acceleration stumble and wondering if it's related.

The only code it has is EGR Flow Too Low, which it's had for a long time. I've checked the manifold opening and it's clear, I've cleaned the EGR, I've sucked on the diaphragm and it clicks. So I don't know what that's about.

I searched and found a thread on 850 vac lines, so I should check those. I just replaced these on the P2, and bought extra for this car.
viewtopic.php?f=1&t=63615

Thanks for any advice -
Last edited by BlackBart on Tue Apr 30, 2019 4:36 pm, edited 1 time in total.


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Re: Turbo overrun boost noise?

Post by hausmeister » Tue Apr 30, 2019 4:11 pm

Well what to answer...
Stock the t5 is pretty silent imo. You can hear the turbo spool a bit (at least on mine) and during boost mabe a refined wooshing as in a lot of airflow. I don't really hear the bypass valve when it recirulates, maybe a little bit, I don't recall...
Some time ago my plastic intake hose failed near the turbo, you could hear it nicely then. But you would really have to listen for it witn the windows down I would say.
If you get funny noises chances are something is wrong. The turbo farting noise (wu wu wu wuu...) could be a faulty bypass valve. Or maybe a loose hose on it. Or a split hose anywhere...
Whatever it is, you should check all hoses and maybe do a pressure/leak test...


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Re: Turbo overrun boost noise?

Post by Klausc » Tue Apr 30, 2019 7:51 pm

A leak in the intercooler hose will not allow pressure to build and therefore, no boost. I also would check for leaks in the 3 vacuum hoses to the turbo, as well as the airfilter to turbo feed.


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Re: Turbo overrun boost noise?

Post by abscate » Tue Apr 30, 2019 11:23 pm

I heard a lot of monkey chattering from my turbo when I got the hoses mixed up

I was dumping excess boost into the manifold, and had the snorkel vacuum connection onto the Blow off valve.

Chattered whenever I lifted.

Wizechatmanager helped me get them right

When troubleshooting remember boost is controlled by ECU, not revs. You can race your engine all you want in neutral, it won’t boost


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Re: Turbo overrun boost noise?

Post by Sommerfeldt » Wed May 01, 2019 2:24 am

Strictly speaking, we have a recirculating bypass valve. I've only ever heard them make whoosing noise, like they should. If you're getting Subaru noises or turbo chatter, that's boost surge. Is your CBV blocked or stuck somehow?

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Re: Turbo overrun boost noise?

Post by BlackBart » Wed May 01, 2019 9:49 am

Thanks everyone -
I'll check on that when I change the small vac lines. Interesting to note there's no boost in neutral / no load. So you're not going to hear or see anything from underneath to find a leak.

Mr Abscate -

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Re: Turbo overrun boost noise?

Post by wizechatmgr » Wed May 01, 2019 10:40 am

BlackBart wrote:
Wed May 01, 2019 9:49 am
Thanks everyone -
I'll check on that when I change the small vac lines. Interesting to note there's no boost in neutral / no load. So you're not going to hear or see anything from underneath to find a leak.

Mr Abscate -

WCM? RWCM? Royal Wellesley College of Music?
^^This guy =)
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Re: Turbo overrun boost noise?

Post by BlackBart » Wed May 01, 2019 11:23 am

I actually don't know what RWCM stands for.....

I looked up Compressor Bypass Valve!


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Re: Turbo overrun boost noise?

Post by abscate » Wed May 01, 2019 1:16 pm

BlackBart wrote:
Wed May 01, 2019 11:23 am
I actually don't know what RWCM stands for.....

I looked up Compressor Bypass Valve!
Sorry about that - I had the worst sleepless night last night after a redeye flight - every muscle ached, and the Aleve I took sent me into stomach cramps. I slept downstairs with the dogs rather than P&M and keep the better half up all night.


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Re: Turbo overrun boost noise?

Post by BlackBart » Wed May 01, 2019 2:01 pm

Obviously from your stressful recital at the Royal Wellesly College of Music!

Take care of yourself!



PS - Ohh, that's a person, got it.
Nice to meet you Mr Wizechatmgr.


ex-1984 245T wagon
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Want: '67 122 wagon

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