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240 Strange "hissing"

S

SenseiRice

New member
Joined
Aug 5, 2016
Location
Southern NH
I have a 1991 245 and it runs great, recently it started making this hissing noise however and I can't figure out what it would be or where it's coming from. I didn't notice it until it got cold outside (below 30 degrees Fahrenheit). It only happens when I'm at about 1/4 throttle and in between about 1100-2300 rpm. Then when I turn the car off it hisses loudly like it's releasing pressure and becomes a wheeze and stops. It makes the noise whether the car is moving or not but the noise seems the loudest when I change gears from 1-2, 2-3, 3-4 and when I hit the throttle to downshift. From the inside of the car it sounds like it's coming from the passenger side but under the hood I could've sworn it sounded like it was coming from the power steering pump. I'm at a complete loss, thanks in advance!
 
Try recording a video(s) and post it(them). There is no way to determine the cause from your description, only wild guesses.
 
It could be a dying animal. joke.

Make sure the check valve at the intake manifold is good. Use your mouth. no joke.

And after that put your head down there while somebody else drives. kinda joke.
 
Black and white one right off the inelt manifold. Go look. Pull it off and suck and blow on it and it shoudl only go one way.


In the end, it could be anything in the guts of the dash. Maybe a servo acting up, climate control stuff. I dont really know the common points of failure under a volvo 240 dash, but it's nothing more than hoses and valves and stuff. You gotta spend some time learning it or just listen around with a stethoscope or length of hose stuck in your ear while a buddy drives and see what you can find out. It could take hours, it could take minutes, up to YOU to dig around and find it.
 
I believe Z is referring to the hose from the intake manifold to the Brake Power Booster.
 
lifeofgold said:
I believe Z is referring to the hose from the intake manifold to the Brake Power Booster.
Click to expand...

Nah, think it's the one for the HVAC system. 700's have a different setup than 240's do. The brake booster check valve isn't attached to the manifold.

It's the valve shown on the short hose coming off the head side of the manifold in this shot here:


The 700/900 series cars just have a hose coming off of it, going to somewhere between the engine and the HVAC system where there's a check valve.

To the OP, does it make the noises if the "REC" button is depressed? If not, then it's probably the fresh air flapper door actuator that has an intermittent leak going on. My parents' original '84 245GL started making a rapid flupping sound along with the hiss whenever the REC button was not depressed. My 242 makes the hiss, but no flupping.

-J
 
I haven't used the REC since I bought the car, sorry for the lack of videos and replies but the car has stopped making the noise altogether after I took off that valve and blew through it.
 
Really sounds like the brake booster check valve.

I had a similar hissing issue. Turned out to be a blown diaphragm on my brake vacuum booster.
 
First suspect for an audible hissing noise (especially when the car is running fine) is something in the dash for the heat/ac. All the doors that direct the air flow to wherever it needs to be are all operated by vacuum. Any of those vacuum actuators could be leaking. If you've messed with anything inside the dash around the hvac controls you could have pulled a vacuum line loose from the control panel. Its easy to do..
 
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