89 420 sel lost it's fuel tank pressure
 

I posted a few weeks ago about an idleing problem with out success.

I have noticed that since the 420 started missing or idleing rough, the fuel tank no longer hisses when I remove the fuel cap. The car runs great from 5 mph to 80 mph, no smoke no leaks.

Any know if the two are related, and what keeps the tank under pressure, what keeps the tank with pressure?

Thanks

The tank is not pressurized.
Pressure may build up and should be released when driving through a charcoal evaporation system.

Rob