Gas tank has alot of vaccum????

[dyeager535]

Honestly I'm not sure that it causes any REAL problem with fuel delivery.
Come on physicists...does a pump have to work harder to move fluid in a vacuum? I'd guess not, since the entire system is within that vacuum, the only "leak" would be the injectors as they opened.

The real threat is the tank "imploding" if vacuum gets too high IMO. From what some of these guys are saying though, the tanks must be pretty sound structurally if they are seeing that much vacuum without causing physical damage.

My truck for some reason stopped venting correctly (again, no EVAP, I was an idiot), so for now I just don't put the cap on tight. When I re-do the truck, it will run EVAP. As far as I can tell, it's the best possible way to vent a tank.

 

[British Slave]

My 87 with TBI always has a slight pressure on the tank when I remove the cap. I have seen fuel tanks colasped due to no vent. The tank should be vented thru the canister for emmission control.

 

[Burbdood 74]

Keeping an eye on this thread since I've had the same exact problem for about a year now.

 

[Fordum]

Well, you have to remember, you don't "suck" fuel, you produce a vacuum on one side of the pump, and the atmospheric pressure in the tank pushes the fuel to fill that vacuum. If the tank has less pressure than the vacuum the pump can produce, the fuel will go nowhere.
I have had outboard motors quit when I forgot to open the vent, and lawn mowers stop when the holes in the tank cap got stopped.

Its the same reason that you cannot draw water out of a well pipe if the water level is below about 28 feet.
32 feet is the theoretical maximum, but few water well pumps can produce enough vacuum for deeper than 28.