johnh

that's evidence that vacuum in the tank is bad

infidel

It's been my belief that the only reason for the tank to hold vacuum/pressure is a misguided design by Chrysler engineers to mimic gasser tanks where the law says no fumes can escape.
I don't know how the law applies to diesel fuel but I've never seen fumes come off it like I have with gas. It must give off some though since you can smell it.

cowboykilla235

just bumping this thread, cuz the problem is still there

oobudoo

Thanks for bumping this back. I too still have the problem. Did you check the tank vent? If so how did you do it? I am not dropping the tank unless I am sure it will fix the problem. For now I just loosely screw the gas cap on so it cannot build pressure. I might look into pulling the o-ring off the cap. Did not think if it before. Sounds simple enough.

I will note that before I did anything about the tank pressure, every now and then I would have hard starting issue. i would have to crank it for ten seconds or so to start. This truck has always started right up since new (11 years now). Have not had a single hard start since loosening the cap a year ago.

cowboykilla235

i have a negative pressure in my tank, if you have a positive pressure then its prob your fuel cap is bad.

oobudoo

I'm sorry, I guess I don't understand the term "Negative Pressure". Does this mean vacuum? Does the tank suck in air when the cap is removed? As opposed to "positive pressure" which is when air is forced out of the tank, similar to when opening a carbonated soda (Or a beer).

cowboykilla235

yes negative pressure is a vacuum. my tank actually sucks air in.

what is yours doing?

oobudoo

Mine is doing the same thing. If my gas cap is on tight. When I remove the cap to fill the tank it sucks in air. I think it contributed to my occasional hard start problem. I don't know if there is a problem leaving the gas cap loose, but so far I have no issues with the vacuum or hard starts. So unless I have another reason to drop the tank, I am leaving well enough alone for now. Can you tell me about the o-ring in the cap? Do you damage the o-ring removing it? Do you think it will do the same thing as leaving the cap a little loose? It's not like I leave the cap just sitting on the filler neck. I screw it on until I feel contact and just leave there instead of turning until it clicks. I know this doesn't help fixing your problem. Just wanted to add my .02

cowboykilla235

well i was deployed and just got back, so im going to try and pull out the vent, i can see it from under the truck just didnt have the time to mess with it today!

i will say that after storing my truck for over 6 months...(disconnected the batteries, and put some additives in a FULLY topped of tank) that truck started RIGHT UP!!!!

my wife said i was smiling more when my truck started up, then when she picked me up :>)

infidel

You should be able to connect a hose to the vent and blow though it without dropping the tank.
Make sure the fill cap is removed first.

I mostly see vacuum/pressure tank problems when people replace the cap with a generic locking cap that doesn't vent.

Not real sure about the statement that there should be no pressure or suction on the 24 valve tanks because they use the same OEM fuel cap.
For sure the 12 valve caps are designed to hold 3 psi +/-.

The rollover valve is something entirely different. It's a large ball in the tank fill hose designed to block flow if the truck is up side down.

patdaly

Such is the love of a good Cummins...........

cowboykilla235

yup, i told her im allowed one mistress, so i got my truck

on a serious note, i took the rollover valve off (turn towards the electrical connection, pull up, then turn back towards original position and pull out)

tried to blow on it (were all adults here, so let the puns roll!!!) and it was plugged

used some weed wacking string to unplug it, banged it on the ground several times. anyways it seemed it was working good, put the O-ring back into the fuel cap

cowboykilla235

well see how things look next fill up!