thumbs

Ok can someone help me understand this thing. I am to understand that some of these tanks are vented and some have a vent in the cap. Mine seems to have the vent in the cap. I can see a little spring in the center of the inside of the cap and I can push it in a bit. I am also getting a hissing when I take the cap off to fill the tank.

The reason I ask is I notice my fuel pressure drops about .5 psi after running for a while, maybe 5 or 10 miles. I was just goin to get another cap to replace the original figuring it may be clogged and creating a vaccuum in the tank causing the fuel pressure to drop. When I looked at the new cap it said open until the hissing stops then take off the rest of the way.

Now the question. Are these caps set to a particular vaccuum before the valve in the cap opens causing the hissing I hear when I take the cap off? In other words is there a particular vaccuum that is supposed to be in the tank after you run a while. As the vaccuum increases to a certain point then the valve in the cap opens to allow air to enter the tank and evel off the vaccuum. If there is supposed to be a certain vaccuum is this the reason for my drop in fuel pressure? Is this right or do I have the theory wrong.

Do you guys notice a slight drop in fuel pressure after you run a bit??

thanks for the help

BearKiller

I really can't answer for the vent/cap situation, as I haven't really studied one yet.


But, I believe there should be a SIZABLE vent-hose on the tanks top, something at least 7/16-ID and maybe 1/2-ID.

Without a larger vent than the fuel draw-line, the tank's sides can start to collapse under hard acceleration.


Also, without a rather large vent, AWAY from the filler-neck area, adding fuel will be a slow frustrating process.


Were it me, if a tank did not have a big vent-hose, I would soon see that I remedied that.

TruckU

Actually, most new vehicle tanks operate on a small amount of positive pressure now for emissions capture purposes.

if you open the cap REALLY quickly it will (or should) push it out.

On my '93 if I fill it past the point of self shutoff it will eventually start dripping oout from the top of the tank.

Power Ram 250 C

Mine has a slight hiss when I unscrew the cap to refuel too. I don't have a fuel press. gage to check it thought.

BearKiller

You need to check the snug-ness of the big plastic collar that holds the sending-unit.

Just recently, the wife's truck came home dripping fuel after a fill-up; I was able to get almost two full turns on the collar.

After I hand-tightened the collar = no more leak.

thumbs

Thanks guys
No there are no leakes that I can find anywhere. I just put the gauge in along with a lift pump. I think I am ok but thought I would ask anyway.

One other thing. Could someone take a look at their cap and see exactly where the rubber seal is placed. The new one I looked at is different than mine. I think mine had the grove in the rubber around the plastic lip in the top of the cap. The new one I looked at just had the rubber with the groove faceing out away from the white plastic cap. I would thing the groove goes around the plastic at the top of the cap but ya never know.