At times the negative pressure in my fuel tank is so great that i can't get the fuel cap off, which is real bugger when I need to fill it up!   It's not always, but regular. I can hear something "buzzing" which is connected to the pressure issue because it disappears when i can manage to get the fuel cap off.
 
I assume the Charcoal Canister has something to do with venting the fuel tank but I have no experience in this area.
 
Any ideas why my fuel tank is creating such a large negative pressure and how I can fix it?
 
I have a 1990 turbo.
 
Cheers
Colin

There should be a vacuum line from the charcoal canister to the throttle body. Make sure that's there.

Also, the buzzing comes from the charcoal canister. If you tap the canister while you hear the buzzing, you may hear an interruption in the buzzing.

Colin, the vacuum is caused by the fuel being pumped out of the tank, and air needs to take it's place.
The filler cap contains a vacuum check valve that I imagine would be to release excessive vacuum. Perhaps that may be faulty.
The function of the carbon canister is to absorb and vent fuel vapor, and if it was clogged it may cause excessive vacuum.
 
Do you have a PDF of the service manual?
If not you can download a copy free at: http://manual.teq.org/
Look at section after EC5 that tells you how to test the canister.

Thanks Robk.  I checked and there is a line coming out of the top of the charcoal canister going directly to the throttle body.  Thanks for the advice.
 
Red, I didn't realise the filler cap was meant to vent, which it clearly doesn't.  I'll grab a new one and that should solve my problem.
 
Cheers
Col

It could be worth trying a new cap.
 
If that doesn't fix the problem my guess is the excessive fuel tank vacuum would probably be caused by the canister or it's hoses blocked.
 
BTW my '90 model NA has exactly the same setup as the turbo models, and occasionally I have noticed vacuum in the tank when filling up but have never had trouble removing the cap.
I had tested my canister according to the service manual procedure and it was OK, so I am wondering if some fuel tank vacuum is normal.

FYI, the last time my engine was removed and re-installed, I struggled to get my fuel cap off the first time I tried filling up. I never encountered the problem prior to that. I noticed that the vacuum line from the charcoal canister to the throttle body hadn't been connected when the engine was re-installed. I reconnected the vacuum line and I haven't had a problem for years since then. 
Apart from the fuel cap itself, there could be a problem with your charcoal canister, or there could be a blockage in the vacuum line or something like that. 
I should also ask, are you sure that the pressure in the tank was negative (vacuum), not positive? I'm just asking in case it could be related to boost pressure. I haven't given this much thought so it may be a silly idea. 

Thanks Red + Rob,
 
Rob, when I do get the filler cap off I get a loud "swoosh" noise.  Sounds like it's sucking in and I don't get a sudden petrol smell so I assume it's negative, but you're right, I'm not 100% sure.
 
I've also suffered from a strange problem for many years that I've never been able to workout.  Every now and then when my foot lifts off the accelerator pedal ever so slightly (like going over a bump in the road) my car suffers some form of retardation (only for 0.5 sec, so very quick).  However if I'm up in the revs and under strong boost in 2nd gear the retardation can be quite severe and the car kangaroos (at 50+ km/h).  It's always felt like some sort of fuel cut?   It's not the AFM, BPV or the TPS. All tested.  It's only just occurred to me recently that if the negative pressure is big enough, maybe it's sucking back some fuel into the tank when the foot comes off the accelerator ever so slightly?  Not sure if that can actually happen but it's seems almost logical.  I'll be interested to see if a simple fuel filler cap can fix such an annoying problem that I've lived with for so many years.

I'm confused. I don't see the charcoal canister having a role in venting the petrol tank if the cap is a breather type. Nor if the canister clogged, why would the petrol tank become a vacuum.
To me it seems a) the petrol cap must be a breather type and b) should the charcoal canister clogging it  would just be like blocking off the line to the inlet manifold. No problem.
Or am I missing something ?
Also on my 90 Turbo the canister purges directly to the inlet manifold, yet the manual describes a vtv controlled by the ECU to determine when the purge should occur. Since Col, Robk and I don't seem to have that vtv "switch", maybe it was introduced in later years ?? 
Does anyone know, as I am reinstalling the engine currently and unsure of what is necessary and what isn't .
 
cheers 

In theory, people remove the charcoal canister and block the lines up. So why would the canister be an issue if you can do that and block all lines too and from cause the charcoal canister to be an issue?

Most likely a issue with the cap.