The other day when filling up my car when I opened my fuel tank cap theres a swooshing sound like air escaping. Is this normal?

Check the fuel tank vent line, they can become clogged with dirt and cause a vaccume or pressure.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section

It only causes a vacuum in diesel cars, and will cause poor performance and stalling. The hose hangs down on the right rear axle area and can be snaked out with a coat hanger.

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'10 Chrysler T&C Stow-N-Go White. Grandpa's ride.

'13 Chrysler 200 Touring Candy Red. Grandma's ride.

Age and cunning will always over come youth and vigor.

Does anyone know where and what this vent looks like on a W124? My car has this very issue but haven't had any stalling yet (*phew*)

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Jason

00 Subaru Legacy (fun wagon)
96 Nissan Sentra (disavowed)
82 300SD (garage queen, will run again hopefully)
87 300DT (Gone: 10/15/2010)

Should be below the fill fill cap area just like the 123 car's. I try to ocassionally try to fill my car full enough once in a while to make it pee pee down that tube. That will prevent bugs from nesting in the hose.

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'10 Chrysler T&C Stow-N-Go White. Grandpa's ride.

'13 Chrysler 200 Touring Candy Red. Grandma's ride.

Age and cunning will always over come youth and vigor.

Somewhere I remember someone saying that pressure can be built if the vent is plugged. It was something about when diesel of maybe the vapors get warmer they expand somewhat, like if the car was sitting all day with a chilly night and a warm day.

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Adam Lumsden
(83) 300D
Vice-President of the MBCA International Stars Section

A handy tool to have in your arsenal is a long speedometer cable. It can be used like a plumber's snake to clean out sunroof drains, fuel tank vents, etc.

I've had spider nest webs so tough that 150 psi air pressure wouldn't budge them.

No pressure will build in a diesel fuel tank as it does not have the expanding vapors that gasoline does.

VW uses the same fuel tank for gasser's as the diesels, and we do something called venting so we can get an extra 2.5 to 3.0 gallons of fuel into expansion area of the tank normally used for gas to expand.

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'10 Chrysler T&C Stow-N-Go White. Grandpa's ride.

'13 Chrysler 200 Touring Candy Red. Grandma's ride.

Age and cunning will always over come youth and vigor.

Now, Navy, you know that I hate to disagree with you.

But, the SD does build up some pressure in the tank at times. I have not unclogged the vent as of yet, and, normally it does create vacuum in the tank. However, there are times when the fuel and air heats up in the tank and pressure is released when the cap is opened.

I think others have seen this as well.

Yes I have had it happen a couple times when I took my cap off, a bunch of air whooshed out, and I only had about 2 gallons left in there at the time it happened. It rarely happens however, maybe 1 in 7 times I fill it. Its never stalled either....

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Let me help you two guys here. The expansion ratio of diesel if you would look it up, will prove to you that your diesel fuel will not expand (liquid or vapor) in your tank. Twenty gallons of diesel will not expand enough going from 50* F to 110* F for you to even measure out of the lab. Gasoline on the other hand will, have you ever set a full gas can out of the shade into the sun light? Try it with one cool 5 gal jug of diesel & gas side by side. Let me know what you find.

What will cause you to sometimes have a pressure in your fuel tank is the unused diesel being pumped back to your fuel tank, has not yet been over come by the amount being sucked out. In other words you could have a partial clogging of your vent, allowing between driving and stops for it to slowly equalize. It will eventually cause a stall if the rest of your fuel system is air tight as it should be, but if the clogged vent tube is not completely block it will still put a strain on the whole fuel system and that's a fact.

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'10 Chrysler T&C Stow-N-Go White. Grandpa's ride.

'13 Chrysler 200 Touring Candy Red. Grandma's ride.

Age and cunning will always over come youth and vigor.

Navy, I do believe, respectfully, that you are incorrect.

If we start with zero system pressure, plug the vent, and start the engine, the loss of fuel due to consumption by the engine will result in a vacuum in the tank. Everybody agrees with this.

During this process there is a continous loop of fuel going to the injectors and returning to the tank. This loop has no effect on the pressure in the tank.

There is no possibility that the amount of fuel being returned to the tank is greater than the amount of fuel going to the engine. This could be the only case where the tank would be pressurized, under your scenario.

The cause of increased pressure in the tank is due to the air in the tank above the fuel. If you would look it up, you will find that this air is subject to heating and cooling and it expands and contracts accordingly. The space above the fuel is not strictly diesel vapor.

The large volume of air vs. small volume of fuel supports this conclusion.

I purposely stated the incorrect return scenario in a jumbled manor to see if anyone had their thinking cap on. Thank God Brian you didn't fail my little jibe completely. But if you stop and think about the air expansion in the tank idea, it still means you have a clogged vent. If you have a clogged vent where did the air come from? Diesel has an extreemly low vapor factor, unlike gasoline, and does not make a vapor pressure in the tank like gas will. What is really happening is people are mistaking the sucking sound for the sound of air excaping.

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'10 Chrysler T&C Stow-N-Go White. Grandpa's ride.

'13 Chrysler 200 Touring Candy Red. Grandma's ride.

Age and cunning will always over come youth and vigor.

Wow, Navy, if I knew there was going to be a test, I would have studied harder.


With regard to a clogged vent, this is clearly true.

With regard to "Where did the air come from", this is more complicated. If the vent was totally and completely clogged, and the cap was completely sealed, then the engine would quickly die every time you attempted to run the vehicle. However, I believe that some air is getting to and from the tank, even with a clogged vent. This air is then subject to heating and cooling.

Some people are not as dumb as you think.

Excellent reply.

I would say you are correct about some air geting in the system from the vent or fuel cap. But just how hot do you think it has to be to expand the air (not diesel fumes) in a fuel tank that much while fuel is being pumped from the tank??? No, I still would say from my experience it is strickly vacuum that some mistake for expanded air excaping.

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'10 Chrysler T&C Stow-N-Go White. Grandpa's ride.

'13 Chrysler 200 Touring Candy Red. Grandma's ride.

Age and cunning will always over come youth and vigor.