Fuel tank at a positive pressure

[Nicolas Aristodemou]

Whenever I go to refuel my 1970 280SL, upon opening the gas tank lid fuel vapours escape under moderate pressure from the tank filling neck opening. Car has the later type emissions control vent tank with fuel vapors being vented into the crankcase.

Is this normal or it there an issue with my vent tank and associated valves operation? I have no fuel smells at all inside the cabin. Should there be a built up of pressure inside the tank or excess pressure be vented to the crankcase and/or atmosphere?

[kampala]

Your thin vent lines are most likely blocked.  I use to have that gasp of air and once I put a wire through the vent lines and cleaned them out, the gasp went away.

I removed the cover that conceals vent line connections, removed the flex lines and pushed a solid core wire through the tank and through the vent lines. There was resistance into the tank and I felt the wire push through.  Then it was cleared.  Saw a little sticky residue on the wire when removed. 

The obvious: Careful causing sparks - be prepared.   

[Nicolas Aristodemou]

Thanks kampala I will have a go cleaning the vent lines, carefully...

[Jonny B]

What size wire did you use?

[mrfatboy]

Someone stated that they used a piano wire chucked in a drill to do the job once. Use slow speed.

[kampala]

What size wire did you use?

The marking is not visible on the wire but it is solid core ( not stranded) and measures 3mm on the outside with insulation.  This seems to equal 12 gauge.

The stranded is too soft and a too heavy gauge will not flex enough.   The blockage seemed to be mostly within the vent lines going into the tank. 

[dirkbalter]

I had the same problem. The braided wire and drill approach did not work for me. I used a solid wire (welding rod) with vices clamps to be able to knock the blockage "free". It also took quiet a bit of wire length to get to it. (Depending on which vent is clogged). 

[Bonnyboy]

I went to the local music store and they gave me a few used guitar strings (base and regular guitar) that I fed into the lines and when they stopped I kept a couple more inches and cut the wire and hooked up to my drill and pushed them through the blockage with very little effort. 

[Nicolas Aristodemou]

Thanks for all the responses. I will first try to spray some brake cleaner, let it soak and then will blow compressed air through. If that fails I will try the piano wire approach with a very slow speed drill.

[Cees Klumper]

I suppose it's too difficult to drop the tank and remove the lines for cleaning or, better still, to replace them with new? I would be concerned about pushing debris into the tank with the 'wire method'.

I had to drop the tank recently on two other of my cars and it wasn't too bad.

Sounds like this is something to check for all of us, periodically, since it seems to occur frequently.

[Nicolas Aristodemou]

Finally some brake cleaner sprayed into both vent lines (to/from gas tank into expansion tank) and blowing clean with compressed air has sorted out my problem. No more pressurized fuel tank!

[CJHenderson]

When I got my car the vent hose vented inside the trunk which didn't surprise me since this car was in sorry shape.

[Nicolas Aristodemou]

I had the same problem 2 years back when I bought my SL ..... fuel tank vented into the trunk and from there into the cabin while driving. Trunk was wet with fuel when filling up the tank. I installed new vent lines from tank into the vapour tank and problem was sorted out. Two years later lines got blocked (solid vent pipes attached to fuel tank so I had to clean up the lines and blow them clean with compressed air after soaking in brake cleaner for a while .....

Finally a third problem now for which I made a separate post..... rough running at low fuel levels in tank..... apparently fuel return from FIP to tank “flower pot” could also be blocked..... seems like a never ending story..... 

[Alex D]

Once we cleaned out the vent lines, I noticed a definite improvement in acceleration and performance, not sure if anyone else noticed improved performance from cleaning out vent lines? 

Also while on the fuel tank topic, does anyone hear a "bubbling or gulping" sound in the tank when you first turn the key and hear the fuel pump start? I hear it once and not again until I start the car after sitting for a few days or weeks.  If i start the car the later in the day or multiple times a day I do not hear the "bubble or gulp" sound.  It does not seem to be causing any problems, just can not figure out why it is happening.  The tank is Dansk replaced 6 years ago.  Any ideas?

[mrfatboy]

It seems like over time the return gas line is getting air in it somehow. Small gas leak in the return line/connection? When you start the car after as long period the return line pushes the air into the tank hence the bubble sound.

Any gas smell in garage?

It’s just a Sunday evening guess 🤣🤣🤣