I have a 89 240 with a an unknown number of miles. I think it might have a vacuum leak under the dashboard somewhere. I hear a hissing sound every time I get on the throttle and when I turn the car off. I unhooked the vacuum line that goes to the cabin from its check valve and the noise went away. Does that mean my check valve is bad? Are there other possibilities? I blew and sucked on this valve and got air both ways-- Bad valve I know. I can this be the source of a vacuum leak? I'm a little confused about how this works.

 

Most likely it is the check valve. Take it out and try to blow through it. Air should flow only one way. Replace it if air flows both ways.

When mine went bad, you could hear hissing every time you turned off the car and when you accelerated hard.

 

I get the same exact hissing sound.

r I cant seem to get the clamp off, I'm going to try some plyers pretty soon. Last night I just took the hose off and tried sucking/blowing. Air flows both ways. How serious of a vacuum leak could this cause? I'm chasing some surging idle issues and I wonder if this is the cause.

 

It really shouldn't cause any vacuum leaks if all of the vacuum plumbing inside the car is intact. Basically the vacuum runs from the intake manifold to a reservoir in the center console. From there it goes to the heat control push buttons which routes it to four different flappers that control the various heat vents. You can use a mightyvac to pull a vacuum on the hose running through the firewall and see if it holds a vacuum.

To check if this is the cause of surging idle, just pull the check valve and cap off the nipple on the intake manifold. If the surging idle continues, then this isn't the source of the idle problem. For surging idle, make sure that the throttle body is clean and adjusted properly. Also make sure that the idle switch is adjusted properly and the idle air controller is functioning.

 

I put a cheap universal check valve right in front of the old one and the hissing sound when I step on the gas or turn the engine off is gone. The surging idle is intermittent and may be the result of several issues. It seems like the car idles better, though.