I have two 90 245 with the same issue. When driving and I let up on the gas the vacuum sound happens. It also happens when turning off the car. I figure this is a common problem but the archive search yields nothing. Seems to be related to the vacuum reservoir.

Is there a fix that anyone has found for this particular problem?

https://www.brickboard.com/FIND/index.htm?forum_menu=&cx=003544480620222770647%3Aki7j2gik3xi&cof=FORID%3A11&q=Vacumm+leaks+in+heater+control+240&x=0&y=0

You can remove the side panels in the centre console and get a look at the reservoir and its fittings. If a second person can then start and stop the car you may be able to hear where the leak is originating.

A lot of years since I had that area all apart (blower motor R&R) but I don't think you have to disassemble a lot of stuff to remove the reservoir.

Another possible source of your hissing noise may be the selector buttons and their little manifold which send vacuum to open/close valves to direct the heater output to defrost, floor, etc. Try operating those while listening for the noise.
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Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), and XC60. Also '77 MGB and four old motorcycles. Long gone: 1981 244, 1994 940 and 1998 S90.

Also look for disconnected or broken hoses to all of the vent, defroster and recirculate doors are controlled with vacuum.

You can sometimes tell which system has the leak if you switch it either on or off and listen if the sound changes.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502

Check the one way valve coming off the intake manifold which is where it gets its pressure from.

The one-way valve was my thought as well. I think they can be cleaned with carb cleaner; not sure, I bought a new one. Kick panel removal will make examining the various connectors easier - and you could buy a Mityvac or equivalent and pull a vacuum via this connector and check them out (probably with an assistant.)

I ran into an interesting phenomenon on my current 240, when I'd drive it I'd hear a hissing/slight whine that was RPM-dependent when driving sometimes. It sounded exactly like a vacuum leak. I always have an ear on my cars when I drive them and it eventually drove me somewhat nuts: one day it would be fine, the next I'd hear the noise at high revs/vacuum.

It turned out that I was using a cheap Monoprice iphone charging cable while listening to Spotify via aux-in, and when the phone was plugged in to charge via an equally-cheap Monoprice USB cigar lighter adaptor I would get interference from the ignition (I guess) HV stuff through the stereo. For what it's worth, the ignition cables are relatively-new Bougiecords with (I think) "Ignition Suppression" on the jacket.

Probably not related, just an anecdote- but the plural of anecdote is data, right??

Had to chime in here after hearing "but the plural of anecdote is data, right??" So true.

You'd think this would be, as the OP hints, a common symptom with a single solution. It might be -- I've heard of it often, but not experienced it myself exactly as reported.

What I've had years ago is the leak a tennis ball fixes. The recirc flap actuator dry rots and that's so difficult to reach to fix properly, wedging something to keep the flap in place against its spring is more likely a daily driver solution. To stop the hissing, just keep the recirculate button pressed in, which cuts off the flow.

Those check valves in the engine compartment seem to almost fall apart in my experience, and soapy water is my choice of cleaning solution, which has fixed the one way valve in every case I've tried, but I think the symptom of failure is not a hissing inside, but a change in function (airflow) when the throttle gets opened.

Best advice has, in my opinion, already been given. OP should maybe abandon the hope of skipping to the "common symptom with single solution" guesswork and sniff around a bit more to see how the climate system's vacuum switches alter the symptom.


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Art Benstein near Baltimore

"the worst data are better than the best theory," said Antonio Ereditato

Regardless of the buttons (all three) are in or out the sound of leaking air happens when turning off the car or letting up on the gas. My guess is that the sound is coming from near the reservoir. I'll attempt to open up the console while marking electrical contacts I remove in hopes of getting an eye on a vacuum hose that has split or something.

It sounds like the fresh air intake is moving when I depress the recirculating button.

I already swapped out the vacuum valve in the engine compartment with a car that doesn't make this noise.





My current cars: 90 245 white (CA), 90 245 white (rebuilt front end), 91 245 maroon, 83 245 blue (daily driver)

Borrow a vacuum tester.


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Art Benstein near Baltimore

Two peanuts walk into a bar, and one was a salted.