Rush of air when opening fuel filler cap

[cheesebox]

The last few times I've filled up with fuel, when I've unscrewed the filler cap I've noticed a massive rush of air into the tank.

Based on what I've read in other forums, I'm guessing there's a problem with the system for equalising pressure in the tank as fuel is pumped out. Maybe due a blocked pipe. I've not found any e30 specific info though.

Does this sound the likely cause, or could there be other possible issues?

Note the engine runs fine and the fuel economy is probably the best it's been.

I'm not clear on how the system should work, or which pipes are which on top of the tank, so any guidance would be appreciated.

[DesktopDave]

I'd start with the easiest thing...check the check valve under the intake boot & associated lines up front.  I'm not sure if UK models carried the NA carbon canister under the P/S reservoir.  LMK if you'd like pics.

I'd guess it's usually a plugged line; never had that trouble myself (yet).

[cheesebox]

Not sure my car has a check valve. Are you talking about the part mentioned here?

You say it's under the intake boot. You mean the rubber boot feeding into the TB? That's not what the pic shows. Confused now...

[DesktopDave]

That's the fuel pump check valve...holds fuel pressure for warm starts (and boy did my old e34 need one!).

I'm talking about the vacuum purge valve.  It connects the fuel tank to the intake and the vac canister.  Here it is on RealOEM, part 19.  Might be for catalyzed models only?  I'm not 100% sure on that.

[cheesebox]

Mine's a non-cat model. I've noticed a smell of fuel too when the engine's cold. The tank was replaced recently, but that was a few months ago. The problem only start recently. It might coincide with having the inlet manifolds off to replace the rubber pipes under the TB. Maybe I've disturbed something around the fuel pressure regulator?