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Old 04-16-2017, 03:11 PM
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Join Date: Jun 2011
Posts: 14
Answering my own question: Turn the dial counterclockwise to "O"

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Old 04-17-2017, 10:45 AM
Benz Mondi
Join Date: Nov 2006
Location: Sacramento area
Posts: 308
air swoosh?

Originally Posted by Diseasel300 View Post
Again, no. The only reason the cap is sealed in the first place is for emissions (preventing vapor from escaping) and to minimize moisture ingress due to temperature fluctuations.

The tank has a vent to prevent building pressure and has a valve to prevent vacuum. You can remove the cap and drive around all day and have no impact on how the car runs or drives. On a newer car, you can set a check engine light if the purge valve checks for vacuum at the cap (most cars run very slight vacuum to prevent fumes escaping), but this has absolutely no effect on how the engine runs.

All fuel pressure is created by the fuel pump. It doesn't push air in the tank to do this. Not sure where you're getting your information, but I'd check the reliability of your source.

What is the cause of all the air that comes out when you undo the cap?
03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio
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Old 04-17-2017, 12:39 PM
Diseasel300's Avatar
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Join Date: May 2016
Posts: 6,023
Air temperature changes will cause the air and fuel vapor level to change in the tank. The vent system is designed to maintain a slight positive pressure (~1PSI) in the tank for emissions purposes. When the fuel system runs, the fuel is also heated both by the pump and by the engine. The amount returned to the fuel tank helps to heat up the rest of the fuel which then expands and causes more vapor. Excessive pressure is vented directly to atmosphere in the older diesels, or through the charcoal canister on gasoline vehicles.

If you want a great visual, take a plastic jerry can and fill it half full of gasoline then seal it tightly. Keep it in a cool shady place overnight, then go place it in the sun for ~30 mins an come back. It'll be puffed up like a balloon.

You can also have a very slight vacuum on the tank depending on conditions and fuel consumption. When you pull the cap off, you hear that whoosh - it can be positive pressure OR vacuum. Keep in mind that 1PSI can feel like a LOT of pressure when it really isn't.

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