Normally the pressure in the tank is somewhat lower than the atmospheric pressure. If the compensation valve is faulty, the pressure equalization between the atmospheric pressure and the tank will be incomplete or doesn't performed at all.
If pressue builds up in the tank during the engine is running, it means the vapour evacuation is not working,
and the vapours are not reaching the intake manifold.
Aha, I understand...
I'm driving the car without the valve since 3 days and I'm pretty sure it runs far better since I took away the compensation valve, could be an impression but definitely her way to run is changed a bit. Feels more ready and agressive on acceleration, no noise from the tank, no air pressure from the fuel cap anymore.
But of course this facts makes me think that the valve itself is faulty, maybe I can blow easily with my mouth so it looks fine, but as soon as I put the valve back, the tank gets pressure, so this makes me think it is faulty and as the atmospheric difference is small, it is not enough to open the valve and equalize the pressures between the tank and the outside pressure.
The system needs to suck air from outside through the valve, but this pressure is small and and also the valve (after 20 years) it's not that sensitive to the small pressure it was build for and doesn't open enough or at all...
Any other ideas or suggestions?
Where can I find a valve that does the job...?