How to tell tank vent tube is blocked?
 

It seems that every time I open the 'diesel' cap to the tank I hear an inrush of air. What is causing that, a blocked vent tube? If I do nothing, would the tank collapse due to vacuum when I run the tank dry/very low?

Your advice is appreciated.

Yes your vent tube is blocked.

I've actually seen a picture on here of a guy who had his tank collapse in on itself with the vacuum. I'm not sure who it was, but it looked crazy.

Definitely get it unblocked.

The vent valve on the 124 is designed to hold a certain amount of Vacuum or pressure, i have looked it up before for a similar question,dont have the cd at hand right now.At least for that model the vent is not open to atmosphere,yours may be the same or not. Found it,normal operation pressure up to 50mb or vacuum of 1 to 16mb.the pressure relief control is in the fuel cap,the vacuum control is accomplished by the vent valve.Don

It is not possible to draw that conclusion on the basis on the information presented.

I have the same symptoms, even though I recently cleaned the tank very thoroughly. I believe I blew into the vent tube and I verified that the air was passing thorugh although slowly - I thought that was because of the small tube diameter. Anyway, even though the air could pass through it I still get some rush as I open the fuel cap... hmmm - I'll be watching this thread as well...

Are you sure it is vacuum pulling air in, or is it pressure pushing air out? The only reason I ask is because my '95 does the same thing, for a long time I thought it was sucking air in, that is, until it got worse and I could feel it blowing the fuel cap off when I twisted it to remove it. Also, the pressure began pushing fuel past the cap. Anyway, it could be either and sometimes it is difficult to tell.

I really do not know whether it is sucking air in or pushing warm air out but it seems to be happening in the SDLs and 300D. If the vent tube is OK then the tank should be at atmospheric pressure, there should be no hissing sound. Am I correct?

There are 2 ways I can try:

1) Connect compressed air into the tank from the fuel line and see it stays pressurized - It would not pressurize if the vent tube is OK.

2) Get spare cap, drill a hole, hook it up a MityVac and drive the car around. If it shows a vacuum then the tube is blocked.

Would any of these 2 methods work?

On the '83 anyway, run a stiff wire up through the vent cap, see if there is any obstruction.

Your right. I did a quick search too and now I feel guilty. However I still feel that it is clogged, it's just easy to troubleshot it by running a wire through it and checking for obstruction.

What other suggestions do you feel can help?

As oldiesel correctly stated above, the tank vent system incorporates a relief valve which actuates at a slight pressure differential, both positive and negative. That means that some degree of pressure differential is perfectly normal.

It would be much easier to remove the relief valve from the vent line and test it. With the valve removed, you could also verify that the line itself is clear.

Back when I owned my little gasser accord I had a huge vacuum problem in the tank. One of the things I noticed rather quickly in my Benz was a hiss when I did my fill up.

I waited until the next time and let the car sit an hour. Still a hiss so I did the wire method and I guess I pushed the sludge back into the tank since I don't have any suction when I remove the fuel cap.