fuel tank overpressured? is this normal?
 

on my 1987 560SL, took it for a ride with the top down today and noticed for the first time a humming noise coming from rear of car. Once I got home I turned off the engine and could still hear this loud hummnig sound coming from gas tank area.

After looking around for a while, on a hunch, I took the gas cap off and as usual I could hear air under pressure come out of the gas tank...the humming noise immediately cut off at this point! There followed a very very faint buzzing sound that eventually went away as well.

my question is this....is the gas tank supposed to be in an "over pressured" condition? I think I read somewhere in this forum that this is to keep the fuel lines under pressure and not boiling over when the engine is hot.

the car starts fine with the engine either cold or hot. Getting 15.5mpg. I can live with the humming noise if this is normal behavior but would like confirmation of this from y'all.

tia

dom

It sounds like one of the vent valves may be getting clogged.

ok, sounds reasonable, where exactly are these vent valves and how do I get to them?

or do you mean the fuel pump check valve?

thanks

after some more searching of this forum and others, was wondering if this humming noise is telling me that the fuel pump is going south on me....what are the symptoms of a failing fuel pump?

why does the noise stop when I take the gas cap off?

one more symptom
 

after a long time with the noise not recurring, it started up again this evening during a drive.

on a hunch, after I'd noticed that I was below half a tank of gas, I went to the local gas station (humming sound quite evident) and filled up the gas tank.

driving home, no humming noise at all!

I wonder what condition, when the tank is below half full, that causes that humming noise....it appears to be quite a piece of work to get to the check valve on the gas tank too....

so maybe I'll top her off when she's at half tank from now on.....

so much for previous posting, the humming noise appeared again today after a really hot day's drive home...and the fuel tank was at the 3/4 mark.

again, the noise went away soon as I opened up the gas cap after stopping the car. weird.