Hello, folks!

My car's making a strange sound, and it's coming from the turbo.

It spools up like normal, making the regular sounds, but if I enter the boost area of the meter and then let my foot off the gas suddenly, dropping from boost to vacuum, a strange growling sound comes from what I suspect is the turbo itself, but could be anything else in the exhaust side of the car. There's a corresponding slowing down of the needle when it makes the sound.

What the heck is that sound? It's gotten worse; it used to only make it in the first quarter of the vacuum part of the meter, but now it does it almost all the way down--from the moment it enters vacuum to the moment it finishes slowing.

Also, the car runs fine; again, the turbo spools up perfectly, without making any abnormal noises, and there's plenty of boost to go around.

I'm wondering if there's perhaps a lack of lubrication; there's been an oil leak that I haven't been able to get to yet that keeps me putting in a quart every two hundred miles or so, so the oil is pretty regularly cycled.

But I don't know at all what the sound could be, and it worries me. Please help a worried Volvo owner.

Thanks,

Volfro

'85 740 Turbo, 160000 mi on the engine and 260000 on everything else

Compressor bypass valve?
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Jim McDonald

Well...I'm not exactly sure what that is or does. I'm not familiar with the turbocharger on this thing. Or anything. I know the basics, but that's it.

Where is it? How would I diagnose it? And how would I fix it?

Thanks

Vincent

The bypass valve, should be somewhere up by the front of the engine, up top bolted to the head, it should be fairly visible, I mean right up at the top of everything. Trace the hose from the turbo outlet one should keep going to the intercooler, and there should be a branch that goes off to this little saucer looking thing.


That bypass valve lets the boost go back through to the intake, so that the turbo can remain spooled up when the engine throttle closes, like for a shift on a manual, or if on an auto trans, just for any reason you'd lift off the gas. So if that were making the noise, it'd be making it at the time you described.

-Matt

Thanks! I know the part you're talking about.

Is there a way I can diagnose it? I sure hope my turbo's not going...

Thanks again,

Vincent

The two allen bolts on valve often come loose and need tightening. There is a gasket underneath it which may need replacing.
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Paul NW Indiana '89 740 Turbo 103,000

Hello,

Thanks for that tip! I'll try it tomorrow.

However, how tight is too tight? Do they normally need much tightening? I've never encountered this before, and I want to be very careful with it...a turbo's an expensive thing to waste.

So what would happen if I tightened them too much? Do they just keep the valve tight to anything, or do they open or close it, or what? How would tightening them affect the sound, other than by stopping it, if that's the problem?

Just trying to learn more about my car...

Thanks,

Vincent

Mine was very loose. If it's not loose your problem may lie else where. Just tighten the bolts with a allen wrench and see if it diminishes the sound. If it does, replacing the gaskets (there is one on each side of the valve) may eliminate the sound. I wouldn't worry about overtorquing them unless you go crazy.
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Paul NW Indiana '89 740 Turbo 103,000

well mine has done it for about 6 years know and being the volvo mechanic I am I have checked everything. it is a garret on a 87 760 turbo and when I let off hard it goes warbwarbwarb and with help from a turbo manufacturer he says that it is called a sonic inbalance. Mine even become more noticable when I went from 12 psi to 17 psi however never no problems. But this does not mean dont check all the great ideas everyone has suggested becouse the bypass valve can make a noise but normally when that gasket is leaking bad it has a rough idle. good luck and happy boosting!