fuel tank pressure?

[BPley]

just curious... is the tank supposed to build pressure?

it is something that has always always always happened on this truck, every time i fuel up, she vents when i remove the fuel cap.

however today i noticed when i pulled it to fill up that it did not vent... i checked again when i got home, and it did a little vent, then i moved the truck again, and nothing??

uhhhhhhhh...????

 

[NVW]

Does your fuel cap say "Diesel Fuel Only"?

 

[THEFERMANATOR]

Pressure is normal as the diesel fuel system doesn't vent pressure out so much, but it does let air in. As the fuel heats up, the tank heats up, and it builds pressure. I know I get a good rush of air out of mine when I fuel up if I have driven it much beforehand.

 

[BPley]

So its ok that some times it is pressured, and sometimes not?

I guess that is more or less all I wanted to know

 

[schiker]

I believe the spec is something like it can build about 4-5 psi pressure and or 2-3 psi vacuum (fuzzy memory numbers may be off a little but there is a bit of bias). It helps seal out the atmosphere and limit the exchange of air across surface of fuel to reduce oxidation & moisture exchange etc.

Gasoline tanks pressures are a bit higher and in addition to oxidation its to limit evaporation. Gas is much more temperature sensitive, lighter, and volatile. A gas cap should not be used. Our old school trucks won't code and can tolerate an open vent. Most new cars will code for an emissions issue if tank won't hold tank spec pressure.

Ground tanks are suppose to be pretty stable temp of about 69F ???? (depends on region) and pumps are calibrated for that temperature. Tank air temp will fluctuate from that or cooler mornings vs day heat plus recirculating fuel will be real warm and warm air to create a woosh outward for me most times. Woosh will vary a lot and if recent opening won't be hardly detectable.

 

[BPley]

Awesome! Thanks guys