When I remove the filler cap there is a vacuum. Is this the way it should be or do I have a plugged vent line somewhere? If there is a vent line where is it?
Thanks for any info.

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1983 300SD
200000miles

There are vent lines that run off the top of the tank. It is normal to have a little vacuum in the tank. However something in my 300TD got plugged and the fuel pump pulled enough vacuum to partially collapse the tank. If you open the cap and there is a loud rush of air rather than a little hiss you may have a problem. After having 2 fuel senders crushed I ended up drilling a couple of very small vent holes in the cap rather than try and sort out the venting system.

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Bill Reimels
Now down to one:
1972 300SE 3.5 W109 (Euro delivery)

I don't know if it would be the same on my car but where do the vent lines run to?

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1983 300SD
200000miles

The lines run to a charcoal filter canister somewhere in the trunk. I believe that the canister can become clogged. I took the easy way out with the holes in the cap since I did not want to deal with sorting out yet another German over-engineered system! I'm not familiar with the SD model so can't give you specifics. Do a search on vent and tank vacuum as this topic has come up numerous times.

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Bill Reimels
Now down to one:
1972 300SE 3.5 W109 (Euro delivery)

I doubt that the small expansion tank in the trunk (or in the wagon it's located behind the spare tire) gets plugged, more than likely the air vent line gets stopped up. One vent line I saw had mud daubers nested in it.
This line runs down to the under panel, in my 115 sedan it was under RR fender. In the wagons it on the LR fender.
Stick a coathanger wire up the line (hose actually) to see if its plugged. I can't believe a plugged vent could cause the fuel tank to collapse, but it wouldn't surprise me if that was true!
It seems like fuel would not be pulled to the IP and the engine will be starved and simply die from lack of fuel if this line was plugged up.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

Air pressure is around 14.5 psi.
If you draw a vacuum of 2 inches, that translates to approx 1 psi. How many square inches of the fuel tank are there? say 4 ft * 15inches on one face that applies a staggering 720 pounds force trying to crush the tank. AND the same force is on the other side of the tank too.

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Tony from West Oz.
Fatmobile 3 84 300D 295kkm Silver grey/Blue int. 2 tank WVO - Recipient of TurboDesel engine.
Josephine '82 300D 390kkm White/Palamino int.
Elizabeth '81 280E, sporting a '79 300D engine.
Lucille '87 W124 300D non-turbo 6 cylinder OM603, Pearl Grey with light grey interior


Various parts cars including 280E, 230C & 300D in various states of disassembly.