clogged sedan tank vent - PeachParts Mercedes-Benz Forum

funola · #1 04-27-2011, 05:54 PM

I cut the metal vent line where it enters the top of the tank and I am still getting vacuum as fuel is consumed. I stuck a wire in there and it stops in about 1". I assume it makes a 90 degree turn 1" inside the tank? Does anyone have a pic of how that line is routed inside the tank? I tried blowing into it with compressed air and it still won't clear. I assumit it makes a few serpentine turns in the tank and ends up at the top inside of the filler neck. There are 2 openings in there and I stuck a wire in both of them as far as it will go and still can't clear the clog. I don't think this will be easy to unclog since it is clogged with gunked up VO (I am using the main tank as a VO tank). I will drill a hole in the cap to vent the tankl for now. I have a spare tank which I may install at some point and not fill the tank to the brim, which I think is the cause of the vent clog. Anyone else with vent line clogging inside the tank?

Orv · #2 04-27-2011, 06:37 PM

If it's really congealed VO, maybe gently warm the area with a heat lamp for a while, then try blowing it out again?

funola · #3 04-28-2011, 12:58 PM

Took the easy way. 5 minutes later, tank is vented and no more vacuum. Plus side is if it clogs, it is super easy to unclog!.

.

Stretch · #4 04-28-2011, 01:39 PM

The FSM is crap at these sort of things isn't it... even the description of the fuel venting system fails to show what happens in the tank. I guess it isn't too complicated - do you know anyone with one of those Gucci endoscope cameras?

All I could find was from a link for a different car in this thread:-

Cleaning/Sealing a fuel tank

Real link is here:-

http://www.sl113.org/wiki/Fuel/Tank

It shows a cut away tank - but I get the feeling the W123 tank is different.

funola · #5 04-28-2011, 03:57 PM

Quote:

Originally Posted by Army

The FSM is crap at these sort of things isn't it... even the description of the fuel venting system fails to show what happens in the tank. I guess it isn't too complicated - do you know anyone with one of those Gucci endoscope cameras?

All I could find was from a link for a different car in this thread:-

Cleaning/Sealing a fuel tank

Real link is here:-

http://www.sl113.org/wiki/Fuel/Tank

It shows a cut away tank - but I get the feeling the W123 tank is different.

Yeah, the FSM sucks in the tank vent. I'll take a look at the links later. This is where I think the vent terminates in the filler neck (with the restrictor removed)
.

Stretch · #6 04-29-2011, 05:32 AM

Quote:

Originally Posted by funola

Yeah, the FSM sucks in the tank vent. I'll take a look at the links later. This is where I think the vent terminates in the filler neck (with the restrictor removed)
.

Oh no - not on mine it doesn't - I'm not sure exactly what that extra pipe in the filler neck does but I guess it helps stop the glugging when you fill the tank - and so your nicely ironed action slacks don't get splattered with diesel.

The vent on mine comes out under the car just in front of the left hand rear wheel. There's actually a good picture in the FSM of the rubber bung bit on the end of the fuel line that runs up and over the subframe and off into the top of the tank. I don't know if this is exactly the same on the American fuel tanks that have that strange looking expansion blob... my Euro 300D doesn't have one you see.

layback40 · #7 04-29-2011, 11:01 AM

You have probably got some bits of french fries stuck in it. Maybe a bit of biodiesel may soften it & allow you to blow the c##p out.

Stretch · #8 04-29-2011, 01:17 PM

Quote:

Originally Posted by layback40

You have probably got some bits of french fries stuck in it. Maybe a bit of biodiesel may soften it & allow you to blow the c##p out.

Mon Dieu!

I think he's blowing in the wrong bit anyway - I guess even the most stubborn French fry won't last long against some (hot) air!

layback40 · #9 04-29-2011, 08:25 PM

Quote:

Originally Posted by Army

Mon Dieu!

I think he's blowing in the wrong bit anyway - I guess even the most stubborn French fry won't last long against some (hot) air!

How does that expression go? "Blow in her/his ear & she/he will follow you any where !"

100 psi back blown through the vent line normally clears it out, even years of dust from dirt roads.

Stretch · #10 04-30-2011, 05:34 AM

Quote:

Originally Posted by layback40

How does that expression go? "Blow in her/his ear & she/he will follow you any where !"

100 psi back blown through the vent line normally clears it out, even years of dust from dirt roads.

That should work - shouldn't it? But he might also burst the tank and pop out his back window!

Yeah that blowing thing was also adapted to cigarettes wasn't it? I wonder who sent me that old advert?

layback40 · #11 04-30-2011, 06:51 AM

Quote:

Originally Posted by Army

That should work - shouldn't it? But he might also burst the tank and pop out his back window!

Yeah that blowing thing was also adapted to cigarettes wasn't it? I wonder who sent me that old advert?

I thought I posted my lazy way to bleed air out of a fuel system.
You disconnect the return line from the filter & put the air hose on the return line. You will hear the air venting out the tank vent but you still get pressure in the tank. Reasonably quickly all the air will be out of the filter etc. Be careful when you take the air hose off as the fuel will run back up the return line. Quickly put it back on the filter. There you have done bleeding without any pumping.
If you get too much air pressure in the tank, it blows the fuel cap off across the shed.

Guess how I know!

Yak · #12 04-30-2011, 11:24 AM

Quote:

Originally Posted by Army

Oh no - not on mine it doesn't - I'm not sure exactly what that extra pipe in the filler neck does but I guess it helps stop the glugging when you fill the tank - and so your nicely ironed action slacks don't get splattered with diesel.

The vent on mine comes out under the car just in front of the left hand rear wheel. There's actually a good picture in the FSM of the rubber bung bit on the end of the fuel line that runs up and over the subframe and off into the top of the tank. I don't know if this is exactly the same on the American fuel tanks that have that strange looking expansion blob... my Euro 300D doesn't have one you see.

I think Army is right. The vent line comes out the top, goes through some plumbing, then ends at the little bell housing thing in front of the left rear wheel subframe area.

I presume there'd be some sort of baffle plate or cage-/strainer-like thing inside the tank to mitigate fuel splashing out the vent line, so that may be preventing the wire from moving too far.

Pressure on the vent line "might" clear a clog, but may just press whatever's there in deeper. Maybe try suction on the cut line at the top with the Mity-Vac?

I thought, per the FSM description, that the valve was on the far end under the car, so depending on what specific steps you've done, you may be trying to clear a clog that doesn't exist at the tank in order to fix a broken valve at the bottom of the car. This COULD be the case if you snaked the line, then put it all back together, then got vacuum. The "clog in the tank after snaking" proof would be if you drove with the cut line open to the trunk and an intact fuel cap. If it still generated vacuum, then it's clogged at the top.

There's nothing wrong with your fuel supply and return lines is there? Not swapped or clogged? I would think that returning fuel would reduce the vacuum.

Link to a cut-open W123 tank here, post #3: fuel tank internals

Doesn't show the internal vent line though.

funola · #13 04-30-2011, 12:11 PM

All vent lines below the tank is gone, i.e. they're rusted out and has been removed. The metal vent line at the top of the tank was cut with a hack saw 3/4" before it enters the tank. Compressed air was blown in at the cut nipple at the top of the tank which did not clear it. I think the clog is a plug of partially dried up VO that has the consistency of chewing gum and cannot be blown out easily with compressed air, it just form a tighter wad as pressure is increased. I also think that the vent line makes four 90 degree turns inside the tank before going to the highest point at the filler neck. I have not tried blowing in the vent at the neck since I need to make a special fitting to make it fit the odd shape. There are actually 2 openings in the vent line at the neck. My vented cap works just fine and I do not have an immediate need to clear the clog. I am still curious of the internal routing and will look inside a spare tank that I have with a Borisscope.