I did the large fuel tank vent a few weeks ago.

When I open my fuel cap I get a large amount of pressure release on the front tank. I would assume this means that a tank is pulling a vacuum which isn't good.

I've been looking around to locate vented gas caps which would solve the problem. It appears that my filler necks are from previous to 87 as I have a small metal cap with two tangs on it.

Do I have any option for a vented gas cap with this design? Or is pulling a vacuum normal?

Thanks guys

 

Its good you bring this up.

What did you use on the top grommet for a vent? 3/4" base with a 5/8" barb elbow?

I did the 38 gallon rear tank with the vent mod, but I couldnt find anything that would fit the grommet besides a 3/4" pipe thread. And I couldnt find a 3/4" male pipe thread to 5/8" barb elbow ANYWHERE.

I ended up using a 3/4" pipe to 1/2" barb elbow made of nylon. The venting isnt as good as I expected... And I also experience the vacuum you speak of when opening the cap to fill up. I can just picture the tank walls getting 'sucked' inward as the tank gets lower on fuel and imagine the same thing, that its not good.

Hopefully someone will have some info.

Edit: And i had the same issue finding the 'roll over' vented tops for these tanks. I searched the PN Jim had listed on his website... and found a few threads in here people said they were discontinued.

 

Just the opposite. Are you sure it's a pressure release you're getting and not a vacuum release? Is the air rushing in or rushing out.

Sounds like you need the right filler necks.

 

I utilized the factory roll over valve and drilled it out to accept a 5/8 or 3/4 barb and epoxied it in.

I only did the saddle tank as the rear will likely not have enough slope. I wanted to leave the rear so I could at least fuel one up in case the mod didn't turn out.


You are correct those parts are obsolete and unavailable anywhere except if you get lucky and find a set used.

 

I honestly don't know I hear a pretty good sound of air rushing when I pop the cap. My caps do say "vacuum and pressure" on opposite sides of them.

I am assuming that this sound isn't good and can lead to issues with fuel flow?

I will start looking around for the right necks.

 

I have the same thing on my 94 7.3

I get a small hiss like a pressure release from the front tank, not sure about the rear, I've never tested to see if it works.

I figured it was normal, just air being released from the tank. I usually only get this when I get close to about 5gal left in the tank before I fill up again.

 

Interesting bud much appreciated! The tank was about the emptiest I ever ran it to E and below. I usually fill up at a quarter of a tank.

 

Do you have the stock mechanical fuel pump? I'm wondering if it has something to do with that, just back pressure being released from the lines. I do notice it cranks for a couple seconds more after a fill up, most likely positive pressure being put back on the lines?

 

I run an electric fuel system, so it seems like it is the same all the time.

 

That is one mod I plan on doing, hopefully get some better MPG's.
I still have the mechanical lift, so it must have something to do with near empty tanks, pulling in air vapor.

 

My front tank does this as well. A slight hiss when you pop the cap. The back tank not so much.

I do notice that the back tank is very hard to get full. The front one slightly but no where near as bad as the back. I don't know if it's related in any way or not.

 

I just did this mod and get the hiss as well. I'm betting is pressure release I'm (and you) hearing. In my case, I just did the mod as I was I replacing the midship tank. I have only filled it once halfway to check for leaks and haven't drawn fuel from it yet. I was curious if pressure would build so cracked the cap after a day and sure enough there was pressure. Though I suppose running a tank from full to dry would pull vacuum. And is probably not good for a fuel pump...

...though neither is building pressure - I just blew the diaphragm in an electic fuel pump in my kitcar by replacing an on leaky gas cap with a nonvented one - doh!

However, diesel has a much lower vapor pressure than gas, so we're probably safe. I bet this mod doesn't work on gas trucks. ...or is quite dangerous.

Whatever the case - a vented cap would probably be a good thing to go with this mod - I anyone finds one - please post it here!

 

Glove, have you read this thread?:

https://www.ford-trucks.com/forums/9...-mod-redo.html

Maybe you need the vent hose in your mod. IDK

 

as I understand it our caps and tanks are vented. There should be no pressure of any kind. My guess is that you are using gas caps from a gasoline engine and not the diesel caps.

richard

 

This thread is funny.