As far as I knew, my model Volvo doesn't have a blow off valve. I have a 1998 S70 GLT.

Well today leaving a parking lot, I noticed a sound I've never heard in the two years owning my car. I pulled out, got up to about 3500 RPMs, let off the gas, and I heard a "PSSSSHHHH!" sound, fairly loud too. So I tried to recreate it, sure enough, it does it consistently now.

So I'm a bit confused. I haven't noticed any different performance since hearing it.

But yeah, anyone have an idea what's going on?

 

1. Stock you do not have a BOV. You have a CBV which is kind of like a BOV but isn't.

"A compressor bypass valve (CBV) also known as a compressor relief valve is a vacuum-actuated valve designed to release pressure in the intake system of a turbocharged or centrifugally supercharged car when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor."

"A blowoff valve, (BOV, sometimes hooter valve, dump valve) does basically the same thing, but releases the air to the atmosphere. This creates a very distinctive sound desired by many who own turbocharged sports cars. Some blowoff valves are sold with trumpet shaped exits that amplify the "psssshhh" sound, as not all bov's are the same, some make different noises and these designs are normally marketed towards the tuner crowd. For some owners this is the only reason to get a BOV. Motor sports governed by the FIA have made it illegal to vent unmuffled blowoff valves to the atmosphere. In the United States, Australia and parts of Europe cars featuring unmuffled blowoff valves are illegal for street use."

With that said have you installed a cone filter on your intake? If so you would hear this when letting off the gas and you may think it is a BOV. I hear this every day but I have an aftermarket intake installed.

 

I haven't made any modifications under the hood.

I rang up my local dealership today, and told them what I was experiencing. They assured me it's completely normal to hear it. Though couldn't explain why I've never heard it before.

From the description you gave me there on the difference, the CBV recycles the air, therefore I shouldn't hear any being released, right?

I just want to make sure I'm not being put off so I'll have a larger problem to deal with down the road.

 

You will always hear a little bit of noise.

 

That's exactly what they told me. I'm still worried, I hate not having explanations for things I suppose. Two years driving the same car, leaving the same parking lot, and all of the sudden I start hearing all this air being released when I let off the throttle. Compared to my last two years of driving this car, I'm 100% sure this is not a normal sound. Thanks for putting my mind at a little ease though, I'll take yall's word for it.

 

It may be starting to go bad.
I wouldn't worry to much about it.
If it starts stalling when you let off the gas then it is bad.

 

Check the CBV it might be tearing.

They are basically the same but different. You can recirc a BOV as well.