Tank venting / periodic hissing noise

[munition]

I hope that someone here is maybe able to help.

After 20-30 minutes of driving, I hear a regular hiss of about 2 seconds duration from the rear / from the trunk. The hissing goes along with the opening of the fuel tank vent valve, so whenever the fuel vapor is fed into the intake. I suspect there is too much vacuum in the tank.

I thought at first of a leak in the lines or at some connectors and therefore I used my smoker / leak detector to inspect the vent lines the tank. Everything is tight, not a little sign of leakage.

My questions: Is there always negative pressure in the tank while driving ? Is there a breather valve somewhere to let air into the tank or is there a kind of safety valve if the negative pressure gets too strong? Or do I have overpressure and it pushes the fuel mist out somewhere (I don’t recognize unusual fuel smell)?

Any help is very appreciated.
Andreas

[Alfan]

Is this happening when you turn off the car? Is it causing problems?

I don't recall this happening for any of my cars, but when opening the fuel tank cover, it can be very different, if there is audible pressure regulation or not...

[munition]

No, it's happing only during driving. There is a also a hissing noise when opening the tank cover, but I think this is a normal noise.

The car runs fine so far. The only thing that I could observe with the Marelli tester is it enriches the mixture at idle quite a lot. The values are still in the range, but close to the limit. At higher revs no issue at all. I recently fixed some false air sources that I was able to detect with the smoker (oil cap seal, oil stick seal, AOS return hose). Fixing the false air sources has not helping to fix the rhythmic hizzing...

[Alfan]

I don't know about this, so to understand the system better, you most likely have to look into the details of the system in the workshop manual.

The rich mixture could be related to the coolant temp sensor, if it is maybe slowly going bad and of course the lambda probe, but I guess you have monitored the values of that with the tester.

[munition]

Yes, values seem to be ok. The lambda probe sends voltage between 180mV and 800mV at idle; water temperature was >95°C when I had the Marelli tester connected with the car.

I need to do an emission check next Monday, hope the car will pass the test.