When I open the tank cap on my 1996 X300 4 litre it sounds like there has been a very strong vacuum in the tank. I can really hear how the air rushes into the tank. Can this really be correct, or is there something wrong with the ventilation? The car runs perfectly.
That is very wrong and can lead to an imploding tank 
There is a vacuum controlled valve, which I think is called a Rochester valve that is properly faulty. It can just be a disconnected pipe causing the problem.
Hi Peter! Thanks for the information. I suspected that something was wrong and I’m glad You confirmed this. I’ll investigate further before I use the car again.
You don’t happen to know where the valve is situated?
#16 in diagram.
My garage found the vacuum hose that goes from the manifold had become disconnected, causing excessive tank pressure. You might be as lucky.
Thank You so much!!
Now I know where to start looking.
All the best
Arne