Fuel Vent Reservoir in Trunk

[Richard Madison]

New unvented locking gas cap: after driving on a warm day, when the cap is taken off a blast of gas fumes comes out...if the cap is not taken off, some gas condensate forms on the bumper below the cap.

A lot of pressure is built up in the tank while driving. I was told several times that "unvented" is the correct cap.

The car is a 1969 Euro model and has a Fuel Vent Reservoir Tank mounted on the right trunk wall (shown in Parts Group 47 as "Behälter"). It has one line to, and one line from, the gas tank and one line that vents to the outside through the trunk floor.

Maybe the vent line is clogged causing pressure to build up?

How does the fume reservoir cover come off? What's under the cover?

Any other fixes or suggestions for the pressure build up?

Thanks,
Richard M, NYC
 

   

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[ja17]

Hello Richard,

All the W113 gas tanks are non-vented. You have a blockage in this system somewhere. If you look at the "gas tank tour" you will see a tiny vent line snaking along the inside top of the tank to the small opening at the fuel filler. The second line at the fuel filler goes to the charcoal canister in the tank and finally is vented out of the car near the right rear bumper. **Latest models have a plastic line channeling the fumes back to the engine block. You have a blockage. Check the exterior vent near the rear bumper first. Next disconnect the lines going to the filler cap and see which is blocked.

Joe Alexander
Blacklick, Ohio

[mdsalemi]

Richard,

I experienced your symptoms a couple of years ago, and lasted for 2 years.  I had all kinds of running issues when the tank went below 1/2 full in concert with this pressure build up.

I changed all the vent lines. in the trunk.

I changed the fuel expansion tank.

I snaked out the vent lines at the tank.

All fuel lines were blown out.

All filters and screens were checked, cleaned and or changed.

Problem never really went away until the tank was changed.  No obvious problem was found in any of the parts swapping or cleaning/blowing out until the tank was changed.

The mechanic who finally told me the tank needed replacement said that it was beginning to rust from the inside out and very fine particles were causing all kinds of issues.  When the tank was changed all the problems went away.  For what its worth the mechanic worked on the Sindelfingen line years ago as an apprentice and is quite familiar with the cars.

May your problem be as simple as a clogged vent line.

But ask yourself the question--what clogged the line and why?  Will cleaning the line eliminate the symptom, or solve the problem?

Michael Salemi
1969 280SL
Signal Red 568G w/Black Leather (Restored)
President, International Stars Section
Mercedes-Benz Club of America

[waqas]

Richard,
I noticed a similar problem just this weekend ("whooshing" sound when cap is removed). Today, I disconnected the two little hoses from the filler neck and used my little vacuum gun ("mity-vac") to test the tank side (with the gas cap removed). One line was clear, but the other seemed to be holding vacuum (albeit with a slow leak). I then used a steel guitar string (low-E if you want to know) and pushed it through a few times. Subsequent test showed the line was no longer holding a vacuum. I'll report back after my next drive whether the tank still builds pressure after a drive.

Waqas in Austin, Texas