190 - vacuum in petrol tank

I appear to have a vacuum in the petrol tank - every time I take the cap off, I get a hissing sound, like releasing a vacuum. There is a breather hole next to the cap which must be blocked (unless I'm mistaken) - what's the best way to unblock the breather hole?

Nice site btw!
Cheers
Andy

If it is pressure you're OK, if it is vacuum, you have a check valve or purge line clogged up. To see if it is vacuum or pressure, push down on the cap and turn it like you are going to take it off. Then let go. Be sure to stand asside because if it is pressure it could blow some fuel out. IF it is vacuum the cap will stay put until you pull it off.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM

thanks - I have checked - it is a vacuum - so what's the best way of unblocking? - and what effect will it have if I don't unblock it?
cheers
Andy

Without seeing the car I don't know where the blockage is. IE crimped line or clogged valve??? If it becomes to high of a vacuum it can starve the engine of fuel and the engine will no longer run.

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM