My car is a 1981 242 B21F, 5-speed manual (m46).

I've been having the strangest problem...when I drive, I can hear a hissing sound behind the dash. It seems to emminate from behind the center console. It sounds like steam coming out of a kettle, not whistling, just hissing.

It seems to be tied to my RPM, that is, the higher my RPM, the louder the noise. For example, when I shift from 4th gear at about 4000RPM to 5th gear (dropping down to 2500 RPM or so), the sound goes from wooshing to almost non existent.

I suspect it may be some sort of vacuum leak, since most of the temp controls and vents are vacuum controlled, but how can I test this? Any ideas???

Thanks,

Adam C.

Interesting stuff...I found the vacuum line connection inside the car, where the tube comes from the intake manifold and connects to the first white plastic vacuum bubble. I disconnected this hose at the bubble and drove around. The hose still pulls plenty of air in, and having it disconnected or completely plugged has no effect on the noise.

More on the noise: The best way to describe it is like that sound that you make when you take the first drink a cold can of Coke - Ahh! - but it stays, so it's like Ahhhhhhhhhhhh. You get the idea.

Also, I noticed that the noise is dependent on what GEAR I'm in as well as the RPM. For example, at 2500 RPM in 3rd gear, the noise is much quieter than at 2500 RPM in 4th gear. Basically it gets noticably louder from 1 to 4, and then 5th gear is pretty similar to 4th gear.

Thanks,

Adam C

It's still the vaccuum check valve under the hood, that is black and white, and resides atop your valve cover off of your intake...or Klaus's snake.

Chris-

I'm not sure I have such a thing. The only check valve that looks like that and comes off of my intake is connected straight to the tube that I disconnected. So how could the dash system still make the noise if it wasn't even hooked up to the check valve?

see pics of my engine compartment at:

http://oregonstate.edu/~calamara/volvo/

Thanks,

Adam C.

Last I knew, as long as the engine is running, you are creating vacuum, as long as you are creating vacuum, that blown vaccuum valve will make that noise.

Chris-

I gotcha. I don't have much of a brain for these air pressure physics things. Next thing to do will be to buy a new valve. Thanks again.

Adam C.

You've got a brain! and perservance...pat yourself on the back! Some people wouldn't have even heard the noise, much less looked for it or tried to solve it.

My guess is as said previous, it's the check valve. It's leaking air, and making it sound like the airs coming from the dash? Or possibly the hose going from the firewall to the resevoir has a leak?
Just some thoughts.
--
John '82 242GLT / '83 244DL / '66 122S (crunched front end, looking for a home)

Vaccuum check valve atop your valve cover, from your intake...black and white...bet your defrost doesn't work.

Or a snake that doesn't like high rpm's? Open the hood, grab the throttle and ease it open. You will find it.
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95 850, 88 245

check vacuum balls, behind center console