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

Mine does the same thing.

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.

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Chris
64 190D R.I.P.
80 240D W/617 engine -for sale
82 240D -for sale

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.

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

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.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200

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.

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??

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Thanx,
Alberta Luthier
1983 300CD ('Stinky')155k miles, 2.47 diff, EGR removed, AAZ injectors with 265 nozzles from Sean,and vogtland lowering springs.
1984 300SD ('Old Blue')150k Klicks from Japan originally, came with rear head rests, no sunroof and never had an EGR

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.

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?

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Thanx,
Alberta Luthier
1983 300CD ('Stinky')155k miles, 2.47 diff, EGR removed, AAZ injectors with 265 nozzles from Sean,and vogtland lowering springs.
1984 300SD ('Old Blue')150k Klicks from Japan originally, came with rear head rests, no sunroof and never had an EGR

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.

1984 300SD. I have the same vacuum problem building up in my fuel tank. I checked the fuel vent filter and it is not blocked, however I tried blowing air back through the vent line (with fuel tank cap off) and there is deffitently some sort of restriction. I tried poking a wire up the line but was not sucessful. Is there a method of clearing this line?

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My infants:
1984 W126 300SD - 118000K WVO conversion
1979 W116 300SD - 160000K Registerd Classic Vehicle

If you couple another hose on that vent line that extends higher than the car. Fill it with a solvent. It might work its way up to the tank pretty quickly. You might have to add a small amount of air pressure to overcome any valve resistance in the line though. Others may have beter ideals.

I am a great believer in laquer thinner as a solvent for petroleum products. So much so that even when not painting cars I buy it by five gallon cans. Cheapest is called gun wash at body supply places. Sold by gallon and sub units at hardware stores as well.

Do not spill it on the cars paint. Probably simpler to remove the vent line and soak it. The thinner works on many other types of chemicals as well. I think of it as a kind of universal solvent.

Have not had to buy any for awhile but paid about five dollars a gallon the last time. So it is pretty economical. Actually that is cheaper than diesel fuel at present here if the price has not elevated. Use the stuff in a safe fashion as the flash point is pretty low and you can get high on it with the concentrated fumes. I found out this additional benifit by accident years ago.

So do your cleaning outside. Another real beauty is there is no other chemical that I can think of that dries faster leaving no residue behind. So if you clean an oily gasket surface for application of a sealant it is dry in far less than a minute after the solvent has removed any oil or other substance. .

I would feel better about soaking the vent line from the top of the tank to the drain, rather than introducing any solvent into the tank. Where do you acess the top vent line fitting. Do I need to remove the rear seat back or the rear trunk wall panel?

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My infants:
1984 W126 300SD - 118000K WVO conversion
1979 W116 300SD - 160000K Registerd Classic Vehicle

My 1982 300 SD w 126, will crank, but not start. The fuel system draws a vacuum that is sufficient to hold the fuel cap in place. I've released the vacuum and pumped more fuel via the small (leaky) hand pump near the IP. My car stranded my wife and I last night (in formal wear) at a fundraiser. I am unsure of my next steps? Any help / idea? Hunter what do you think? I've checked the bell shaped device on the third fuel line, and while I cannot confirm if the bivalve is working, I noticed that the line itself has a small tear at the final90 degree beng.