everytime when i fill up, i take off my fuel cap and i have a vacuum in my tank. i bought a new cap, and still the same thing, to temperarily aleviate the problem i took off the rubber O-ring on the fuel cap.

does my tank have some sort of blockage, is there a fuel line that is plugged?

 

IIRC the rollover valve is also the vent. Perhaps it has gone out.

 

The tank is vented by two sources, the cap and a vent on the tank module.
Both are designed to hold 3 psi of vacuum or pressure.
You should hear a hiss if everything is working correctly.

Tank vent is the one labeled remove the red cap below

 

I still get a vacuum when I remove my fuel cap...The bleeder is working ok...I assume now that a little vacuum is normal..

 

i bought a new cap, two of them in fact and both did it... so how do i check the tank vent cap/bleeder thingy ma jig?

 

I checked mine by pulling it out and blowing through it. The rollover valve/vent as Infidel showed a picture of is just a grommnet fit, tight but it will come out. I cant remember if it is a one way valve or if it has to be in a certain orientation (like upside down for the rollover valve to work (block the flow). I just held it in the same postion as it would be mounted on the tank, right side up and blew through it. Air went through so I assumed it worked fine. Thats how I was told to verify its operation.

 

did you have to drop the tank to do that?

 

I had my tank down for another reason so it was dropped when I took it out, I also had the tank module out of the tank as well so it was not hard at all at that point. You might be able to get it out without dropping the tank but it is tight up there. I believe there is clearance for it to come out it is probably more a question of whether you can get it out with the room available.

 

on 24 valve cummins there should be NO pressure or vacuum in the fuel tank at any time

 

So what can happen because of this fuel tank pressure? I too have pressure in my tank when removing the cap to refuel. Been doing this for years. I just thought it was supposed to do this or it had something to do with the fass pumping so much fuel. My Chilton book shows a hose on the tank vent. I guess I can check this for blockage. What else can cause this?

 

i have a NEGATIVE pressure in my tank...

 

By negative pressure, do you mean when the fuel cap is removed there is a suction of air into the tank? That's what mine is doing. I'm still not sure why this is a problem.

 

that would make the lift pump work THAT much harder to pull the fuel outta the tank and pump it to the engine...

 

i agree, i would say pressure is better than vacuum because of the transfer pump

 

when i was pullin my 10,000 K 5th wheel, i was getting 8 MPG... after i took out that O-ring (on the gas cap) it jumped up to 12-13