Basic question: should the cap on the fuel tank of an '85 300TD
 

be the vented type?

I have the identical car and year and mine doesn't.

Don

Typically the tanks are vented at the top - through a tube that runs back down beyond the bottom of the tank. The cap makes a relatively tight seal.

If you find yourself upside down, the tank won't leak through either the cap or the vent (because the opening of the tube will be higher than the tank).

:)

Ken300D

P.S. Make sure you get the cap back on with both tabs (of the cap) hooked inside the filler. If you miss and hook only one with the other outside the filler - you've got a mess. Fuel will slosh out the filler and down the side of the car. Some will leak back into the trunk. Not a disaster, but not fun either.

I am question challenged by the wording of this post query. One
solution is to purchase either a gas cap for any W126 model
or a key lockable MB gas locking gas cap ($25-40.) from the
dealer. As for gases (meaning vapors) escaped from the fuel
column, that's entirely possible since diesel fuel is a hydrocarbon
that can be exchangeable from a liquid to a vapor. Dont worry too
much about which gas cap will fit the W123. Simply tell the MB
parts personnel that you want either of the above for a 123
fitment and they'll recognize the stock part needed. BTW, the new, new, newest generation MB gas caps probably wont fit but
the writer doesnt know this for fully certain.

The safety concern's a good idea, but I have to ask. What about road dirt and such? Is the vent tube protected somehow from that?

Thanks!