Why do I have vacuum in my diesel tank.....................?

[Lampbum2]

Why do I have vacuum in my diesel tank? When I pull the cap off it really sucks in the air, I feel it should be free flowing?
Thank you ahead for your help!!!
Jim

[04 Diesel]

Mine does the same thing.

[Mustang_man298]

I wouldn't call it normal, but he needs to mention what the car is....some car's had pressurized tanks and some had free to air vents pending year and model. In any case, I don't believe a vacuum in the tank is correct.

[vstech]

assuming you are discussing your 90 SDL, there should be a vent in the rear driver's wheel well look in there and see if it's plugged up. should be vented to atmo to avoid implosion.

[Hatterasguy]

Its a closed system, a little bit of vacuum or pressure is normal.

Thats why when you change the fuel filters after the car has been sitting in the sun for a bit...you crack the fuel tank cap...so you don't take a bath in diesel.

[barry123400]

Usually the vent line is blocked creating the accumulated vaccum in the tank. As the fuel level drops no air is introduced from the vent line to compensate. Bad enough obstructed vent and your car will not be able to continue running.
The vacuum is pulling against the lift pump. If enough vacuum is created you starve out for fuel.
There is a one way valve in the vent line . . It's function is to retain pressure or fumes that would be vented to the atmosphere. When a vaccum forms in the tank the vent line valve is supposed to open and relieve any vacuum being created in the tank. The fumes do not escape as air is flowing into the tank at that point. Common failing for the little plastic item on the end of the line to obstruct. Should be cleaned with that symptom. In theory a quick check is your should be able to blow on that line with the fuel cap off and air should flow into the tank.
This particular problem a couple of years ago got me a nice 167k (not tweeked odometer) 1984 300d for 500.00 american. It would quit after about every 140 miles on the highway.
Usually by the time it was towed into a garage the tank pressure equilized enough to start again. So the vent line was only almost totally obstructed. Cost me a little under 2.00 to fix the burnt out wire on the starter from excess cranking and cleaning the vent line.
In theory it could have also imploded or collapsed the fuel tank. The vacuum should never build greater than what it takes to open that small valve. Thats not very much. So pay attention if the vacuum is strong when you take your cap off.

[Lampbum2]

Sorry, I forgot to mention which car, It is the 1984 300SD that has the vacuum in the tank when I go to fill it, My 350SDL does not have the vacuum when I go to fill it.
Thanks,
Jim

[Alberta Luthier]

Originally Posted by Lampbum2

Sorry, I forgot to mention which car, It is the 1984 300SD that has the vacuum in the tank when I go to fill it, My 350SDL does not have the vacuum when I go to fill it.
Thanks,
Jim

generally has pressure. I asked here and after a bunch of discussion and a kind fellow member who sent me a file from the manual, the consensuses was that there is a vent line with a restrictive valve that will hold either pressure or vacuum but not much of either. check your vent it may be plugged up.
link:
fuel tank pressure??

Pressure or vacuum is not normal. At most there should be a few millibar press/vac.

If you have pressure or vacuum you have a plugged or restricted breather.

[Alberta Luthier]

The MB file is called:
'47-720 operation fuel tank positive and negative venting system'
I can't Cut & paste out of a .pdf file so I'll just note a couple of highlights as I am a crappy keyboarder and won't do it all.

"ALL models the tank cap allows pressure out at 100-300 mbars"

-Looks like starting in '81, I think on W126 and W116:

"The vent valve opens at a pressure of 30-50 Mbars
And a vacuum of 1-16 mbars"

As I say I don't have my cd yet to read further. And I really don't think in Mbars but that can't be much pressure or vacuum... so the vent must be plugged eh?

Originally Posted by Alberta Luthier

"ALL models the tank cap allows pressure out at 100-300 mbars"

"The vent valve opens at a pressure of 30-50 Mbars
And a vacuum of 1-16 mbars"

1.4-4.3psi
0.4-.7psi
2.9"-4.7"Hg vacuum
You might hear a little hiss if you listen for it, but if you get a woosh there is a problem.

The valve is usually just plugged with dirt. Its in front of the left trailing arm and looks like a rubber horn. Also remove the tank cap and blow back through the vent tube to make sure its not blocked with sludge.

[300SD81]

I drilled a tiny (0.63mm) hole in the tank cap to fix a problem where pumping from my aux tank in the trunk into the main tank would cause it to build up significant pressure. It should help for vacuum as well, but the proper way to fix it would be to clear the tank vent, I think it has a check valve in it, because I never had a problem with vacuum, only pressure...

[mccauleybil]

Busted Benz, unfortunately I was unable to get the air out of the fuel delivery system and thus not able to restart the car. Sent it to a knowledgable garage to sort out.

Prior to that I was removing the fuel cap after each 10 minutes of driving.

[mccauleybil]

300SDI, I inspected the bell shaped valve on the third fuel line (close to the drivers rear wheel) and could not detect any issue. However, I did notice that there is a crack in the final 90 degree bend, most likely admitting air into the system. I believe this to be a non issue as my fuel tank had a vacuum (not over pressurized).

Thanks for the help...

[spark3542]

I had significant vacuum in the fuel tank of my 97 E300D a few years ago. I didn't address it and it ended up partially collapsing the fuel tank (gave me about 4 gallons less capacity). I found the vent tube underneath the fuel tank and gave it a blast of compressed air (with the fill cap removed). Solved the problem...it was as simple as that.