I have a 1988 300e when i open the fuel cap it sounds like a vacuum cleaner was just turned on my guess is a clogged vent tube if so where is it locoted and would this problem make the motor starve for fuel and run poorly

Have the same car
 

My 300E does the same thing. I think this is normal since the fuel system is under pressure. My Jeep Cherokee would also make a hissing sound when the gas cap was removed. I slowly turn the cap when opening to let the pressure release gradually.

thanks crowe but would this hane anything to do with performance thanks

Fuel supply pressure
 

The owner's manual says to "turn fuel filler cap to the left and hold on to it until possible pressure in tank has been released, then remove cap". Having pressure in the fuel system is normal, I can not see how it would do anything to impact the performance of your MBZ.

thanks again crowe for your responce