I noticed a hissing noise, that sounded like a vacuum leak, coming from the dash area. It would happen immediately when the car was shut off, and sometimes when you stepped on the gas to accelerate from a steady cruise. I checked all the vacuum capsules that operate the vent flaps, and made sure all the hoses were conncted. Everything seemed to be working properly. It turns out it was the check valve at the manifold, which was doing nothing, and would air to go in the reverse direction under conditions of low or no manifold vacuum, which occur exactly at the times I mentioned. I put on a good used one, and now all is quiet.

Interesting! I haven't had a failed one in the 240. Were your vents acting normally?

-Ryan
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Athens, Ohio
1987 245 DL 314k, Dog-mobile
1990 245 DL 134k M47, E-codes, GT Sway Bars
1991 745 GL 300k, Regina, 23/21mm Turbo Sway Bars
Buckeye Volvo Club

The vents were acting semi-normally. During steady cruise, or at idle, they would receive sufficient vacuum to do their thing, but when the throttle was opened, they would slowly reverse position (if engaged) accompanied by a hissing noise. This noise was readily apparent, and coming from somewhere in the vicinity of the vacuum tank, NOT from the failed check valve under the hood. Apart from the normal sounds of actuation when a button is pushed, all is quiet now.

I just got this car a week ago, and as I haven't owned a 240 in many years, I am not all that familiar with their peculiarities any more, so I didn't immediately notice that there was anything unusual going on with the air vents.

Good info. My car has been 'airing down' but I swear the hissing was under the dash. Defrost and Heater works normal otherwise. I guess I will blow through the Check valve at the Intake.

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'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me http://home.no.net/ebrox/Tony's%20cars.htm

FWIW - I've been troubleshooting this same problem here recently - and found that the defroster air switch has somehow failed and I've had a constant air leak there for some time. I haven't pulled the switch off yet to figure out how it failed, but I get a hissing from the switch itself when I manually actuate the defroster actuator behind the glovebox. All the lines hold vacuum when scientifically tested (disconnect and apply suction on them to see if they hold vacuum), so it's not one of the actuator bellows. I'll post more info in a separate thread when I receive a working switch for comparison later this week.

Zach