Is the diesel fuel tank cap supposed to breathe both ways?

[Eurasiaoverland]

Fuel system on a diesel 60 Series.

Putting together a very long rebuild project, I recently tested the one way valve from the tank separator/vent assembly (which lives inside the rear right quater panel, next to the filler neck). The valve was open one way (expelling positive pressure via the breather pipe which runs into a cross member on the body), but also did not seal the other way (i.e. when there is a vacuum pulling air into the breather). So that will be replaced.

That got me thinking on the fuel tank cap - it has a sprung metal valve inside which I assume should only allow air into the tank when there is a vacuum, but it is open both ways. I assume this is not correct, otherwise the one way valve on the breather would be redundant. Time also to replace my filler cap?

 

[TheBussman]

I just checked a 1990 80 series, I'm pretty sure it's the same metal cap as a 60. This one lets air in to relive vacuum but don't let at least low pressure out. i think if you build up enough pressure it will release though, that's what the spring is for.

 

[Eurasiaoverland]

I just checked a 1990 80 series, I'm pretty sure it's the same metal cap as a 60. This one lets air in to relive vacuum but don't let at least low pressure out. i think if you build up enough pressure it will release though, that's what the spring is for.

Thanks very much for checking! I had a look at the valve on mine and it's set up to let air in when there is a vacuum. I gave it a bit of a poke and it seems to be working properly again. It doesn't seal perfectly for positive pressure, but the venilation valve attached to the separator in the rear quarter panel should relieve positive tank pressure before the filler cap I think. Once I get the car built I'll look for any signs of diesel creeping out past the filler cap. They are still available new so I'll probably pick one up just in case. No knowing how long these parts will stil lbe around.

Cheers!