steiner43511 made a recent post about how much vac should be on the fuel tank and started me thinking. I wouldn't think there should be vacuum with the returning fuel but i'm not sure myself. I have a slip tank in my box thats tied into the filler neck of the truck so my fuel tank is gravity fed with fuel always keeping positive pressure on the fuel tank. Positive pressure is good, no?


This also keeps my fuel alot cooler as its always being fed from the cool fuel in the box.

The vacuum comes from depleating the fuel cells volume and having a cap that won't vent over a psi of vac or pressure.

Well that kinda, sorta, maybe, maybe not, answers part of the question??

Positive pressure couldn't hurt.

Here is my two cents on this. Had a friend with a 6.5 (1997) that had the tank colapse like a smashed pop can. Cost him around $300 in parts alone. He had vaccum. My 6.5 (1998) started to collect enough vaccum that I heard a whoosh of air when I cracked the cap. That was a lot of vaccum. Is my tank colapsed? No, I drilled a hole in it. Is it the best cure? Probably not, but it worked. I would like to know from the experts a little more about this if there is anything to add.

yeah i definately have a whooshing sound when i crack the fuel cap.

ok this question goes out to all the 6.5 owners here.


how much pressure do you have when you unscrew the filler cap on the fuel tank?

i was told by a dealer tech that our diesel fuel cap was rated with a diaphragm that would allow air in at -1psi or less.

ie. as soon as there was negative 1 psi of vacuum, it would draw air in thru the cap.

i can suck air thru mine ( awfule taste http://www.dieselplace.com/forum/smileys/smiley11.gif....and
no nasty comments pls) easily, but i still get a vacuum whoosh when i
open the cap after a short 20km or so drive. the whoosh lasts a
few seconds (3-5) only

A new - or functional used - cap should be venting at 1" hg vacuum or 1psi pressure.


Actual volume which requires venting will depend on fuel level in tank.


Warm fuel from in-ground tank will contract when pumped into frigid vehicle tank - winter - creating vacuum - 1" hg max.


Cool fuel from in-ground tank will expand when pumped into warm vehicle tank - summer - creating pressure - 1psi max.


Word up.....

so is it ok to have vaccum?

Tank vacuum will occur anytime fuel contracts, mainly in cold climate conditions.


Fuel exposure to damp atmospheric conditions results in water absorbption, which results in bacteria growth, which clogs fuel lines, filters, pump valves.


Which is why your Diesel fuel cap does not have an open vent hole, which would allow any and all contaminants to freely pass thru the cap as fuel volume decreased. And other conditions....


Factory determined that the Lift Pump and Inj Pump could function at full power with 1" vacuum on tank in cold climate conditions.


Other condititions - off-roading, construction-site, unpaved access road usage requires close attention to the fuel cap to ensure the venting system is not clogged.


Again, vented VOLUME depends on remaining fuel level, but the negative pressure LEVEL should not exceed 1".


2" hg = 1psi


This can be checked with the commonly available gaged plastic hand pump used for radiator cap, and emissions and vacuum motor testing.