380SL 1981 erratic idleing on 1/2 tank
 

Good Day all,
My 380sl 1981 has a nasty problem:
-When the gas tank is more than half full than the engine runs fine.
-When the tank is less than half full than the engine runs erratic, from 500 RPM to 2000.
Finally shuts down, and than a gurgling sound comes from the tank like it is sucking air back in. After that the engine is very hard to start. The problem goes away when I add 40 liters (the tank is that more than half full). The previous owner already knew this problem.

I figure it must be a dirty filter inside the gas tank, if there is one. Things are fine when the filter is completely submerged, after that the trouble starts.

Any help would be appreciated.

or vacuum lock. there is a line that exits under the car, check to assure that it is not blocked, or yours may go to a charchol canister , make sure that is it venting correctly. Also the gas cap must be proper to vent as well
Try running it without gas cap when the issue arrises and see if that cures the issue

Meltedpanda,

Thank you for your answer.
I will have to trace the mentioned venting lines. My SL has a charcoal filter.
The filler cap is hard to remove, and have to check that one somehow.
I siphoned 3 liters of gas into a tall clear flower vase, and no sign of water.
Reading further the many answers on-line, one was interesting. The strainer in the tank is fully submerged when the tank is between full and ¼. That may explain the trouble when the tank is less than ½ and parked on a slight slope and the Internal Tank Filter (strainer) is clogged (which I suspect). This does not explain that gurgling sound.

try removing the fill cap next time the issue surfaces

No filter in the tank. However there's a "swirl pot" which you can see in pics at this link.

Help: Anyone familiar with fuel tank internals? - Benzworld.org - Mercedes-Benz Discussion Forum

Though it should allow gas to enter even at low tank levels, it could be partially blocked. It could also be the tank screen if you haven't cleaned or replaced it, or a half dozen other issues. ;)