The other day my wife's 300SD had fuel spilled on the quarter panel and I chided my wife about not being careful enough during fueling (note to self: bad idea to chide wife about, well anything).
After cleaning it off it reappeared the next day, depsite the fact that the tank is only half full. I swapped fuel tank caps with one from my parts car and the next day same story. Today when I took off the fuel cap, there was quite a bit of pressure. The car hadn't been used in a day.

What is causing this over-pressure problem and how can I fix it?

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1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

Fuel tank pressure = Your vent valve is plugged.

Look at the picture to find it, remove it, clean or replace it.

Have a great day.

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1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

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Very strange indeed. Sounds like the car is throwing up. But I agree the vent may be plugged. Blow some compressed air through it and see what comes out.

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1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair

whunter - I have the same problem. Can you explain the general location of the vent in your picture?

Driver side sub frame mount area on the W126.

make sure you clean out that vent, I had a 300D in the past that had so much pressure it actully ruined the fuel tank it looked like a vacuum sucked the tank in.

Whether the problem is pressure or vacuum, the cause is likely the same-the venting system is compromised. The same valve vents both positive and negative pressure. However, it takes more positive pressure to open the valve than negative. So up to 50 mbar positive pressure is normal.

What should I use to clean that?

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I'm not a doctor, but I'll have a look.

'85 300SD 245k
'87 300SDL 251k
'90 300SEL 326k

Six others from BMW, GM, and Ford.

Liberty will not descend to a people; a people must raise themselves to liberty.[/IMG]

If you have been married for more than a year and 'chided' the wife...you do not have the skills to fix the problem!

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82 300D....went to MB heaven
90 350 SDL....excercising con rods

You must be married...

I crawled under the car and found that fitting, but it appears to be just an inverted cone on the end of a steel line. I can push a wire a few inches inside, then it quits, due to the curve I think.

Is there something I'm missing? I can't find any valve, just the cone.

This car (300SD) has pressure in the tank - everytime you open the filler cap, it releases pressure and spits out some fuel.

Following that line back, there is a larger connection near the tank - could that be a valve?

Thanks for everyone's help.

__________________
1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

The larger unit in that line near your tank is the valve.
You must ROD the sludge out of the tubes also, a used speedometer cable works very well.