Hi everyone,

I am the new owner of a well loved 1985 245DL and I am not every familiar with these types of cars. When I shut the car off or when I step on the accelerator (when i'm driving) I hear a hissing sound behind my dash.

Is this sound normal in these cars or do I have an issue that if I don't fix now will become a big headache later?

David

 

could be a vacuum leak in the heating/airconditioning/ventilation controls. try turning OFF the A/C, Fan, and leave all three mode buttons 'out', then see if you hear that hiss.

the vacuum line comes from the engine, and goes to the back of the pushbutton controls, each button allows vacuum to a different air flap actuator, which is a sort of bellows that opens/closes the various flaps that control floor air for heat, dashboard air for defrosting, and fresh vs recirculated air.

 

Yes, the hiss happens when everything is off and the buttons are out. Could the hiss be the A/C? I was told it hasn't been charged for years.

 

no, its probably a vacuum leak.

on the engine, there's a bunch of vacuum lines that come off the intake manifold right on top/middle. one of them has a black+white 1-way vacuum check valve something like this ...


that then goes to a thin vacuum line that goes through the firewall into the back of the dashboard. note there might be T's in the vacuum before or after the one way valve (I don't think any 240's I've seen have had the vacuum stuff hooked up quite the same way, it all works).

temporarily disconnect that vacuum line through the firewall (and nothing else), and plug up the line that comes from the manifold, and take a drive, see if your hissing has gone away. if it has, then its the ventilation air control stuff. its not that hard to fix, but you'll probably have to take apart a fair amount of stuff to get to all of it.

 

It's the check valve as Pierce suggested. Easy check, easy fix. It's located on top of the intake manifold and looks like the picture he posted. Remove it and try blowing through both ends. If you can blow in both directions then it is bad. Should only be able to blow one way.

 

it also could be a leak in the vacuum lines behind the dash. I'm not looking at the docs right now, but IIRC, the line goes through the firewall, connects to a 'vacuum reservoir' (sorta like an egg crate of plastic spheres), and to the back of the button control unit, the control unit in turn connects to each of the flap actuators I previously mentioned.

 

If it was a leak it would hiss constantly. Hissing when accelerating or when you shut the car off is typically due to evacuation of the vacuum reservoir under the dash due to a failed check valve.

 

ah, yeah, ok, I'll buy that.

 

Good to know. My car has hissed in exactly the same way the poster described for as long as I've owned it. It's just been the very bottom on my list of priority fixes on the car. The only inconvenience is that for long uphills, the vents will change their configuration due to lack of vacuum. I just keep forgetting to get the valve when I get things.