Pulling a Vacuum in the Fuel Tank ???

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I seem to be pulling a vacuum in the fuel tank lately. I only noticed this the last couple of times I filled up with fuel. I hear it slurp some air when I take the fuel cap loose. I am pretty sure it is slurping air in and not letting pressure out which is what I originally thought was happening. Is there a vent that could be pluged? It dosen't strike me a reasonable that the fuel tank should pull a vacuum. The truck seems to run fine so far.

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White '98 12V 4x4 3500 5spd 3. 54, Vac Hyd Trailer Brakes

 

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There is a small red shipping cap on the vent line that most dealers don't know about let alone remove. You can find it on top of the fuel tank module.

 

[Archive]

Yes there is a vent at the top of the tank.
In 98 and 99 some owners had problems of DC's assembly workers leaving a cap on,on the valve.
It would be odd though if you didn't notice it until now. Crawl under the truck so you can look at the valve assembly on the top of the tank and see if there is a red cap on it.

-Mike