Fuel cap venting

gmctd · **Posted:** Wed Feb 18, 2009 1:28 pm

Barometric Pressure is Atmospheric pressure, measured in pounds per square inch actual, or psia, 0 - 15psia (rounded up from 14.696psia mean) - 14.696psia BP, or one atmosphere, = 1 BAR

Vacuum is any reduction in atmospheric pressure, measured in inches on the mercury column, or "HG, where 0"HG vacuum = 15psia, and 30"HG vacuum = 0psia - 2"HG vacuum = 1psia, so 2"HG would be (15psia - 1psia) = 14psia

Gravity and atmospheric pressure on the surface of a liquid enhances an effect known as surface tension - aeration is ability of a liquid to foam - thus water boils at ~212* at sea level* - water boils at lower temperature as altitude increases, higher temperature as altitude decreases - your radiator cap is proof enuff of that, as greater pressure increases surface tension, raising coolant boiling point - also ensures the t-stat opens at the rated temperature, as IIRC, water boils at ~202*F in Denver, the mile high city - always use a thermometer in the pan of boiling water when verifying t-stat operation in the kitchen

Foaming is aeration you can see, the bubbles being large enuff to break thru the surface tension of the liquid - as the bubbles decrease in size, they have greater difficulty in breaking surface tension to emerge thru the surface, thus remaining submerged within the liquid

Diesel fuel foams easily - the hotter the ambient temperature, the warmer the fuel, the greater the foaming, the greater the evaporation, so the tank pressurizes easily in warm climate, but never more than 1psi above BP, if the fuel cap is functional - conversely, Diesel fuel foams less when cold, so the tank pressurizes less easily in cold climes

Actual fuel cap venting is (+/- 1psia) - 1psia = 2"HG vacuum, so your fuel cap will vent at (rounding Barometric pressure to 15psia) 16psia overpressure, also at 14psia underpressure

When tank is full of fuel, little free-air volume remains between the surface of the fuel and the top of the tank, so any hiss will be small - as fuel level drops, free-air volume above the fuel increases, so the hiss will occur over longer period of time as all that air escapes - this indicates greater volume of air in the tank, not greater pressure or vacuum, where 1ft3 @ 1psia will vent quicker than 18ft3 @ 1psia

The tank and cap was designed to also maintain ~2"HG vacuum, intended to reduce aerated fuel, which is foaming you cannot see, the bubbles being too small to break thru fuel surface tension - since each bubble below the surface of the fuel is a tiny microcosm of atmospheric pressure, any reduction in BP above the surface will reduce surface tension, allowing the smaller bubbles to break thru, thereby reducing aeration

The fuel cap should be checked regularly to ensure it meets correct pressure\vacuum venting specs - can use a radiator-pressure hand pump and a vacuum hand pump

*Note: the boiling point of H20 is ~212*F\100*C at mean sea level, tho the actual temp depends on altitude, which relates to Barometric pressure - the downer you go, the upper the temp, for instance Death Valley - with global warming, the increasingly warm atmosphere is expanding, becoming less dense, such that mean Barometric Pressure is decreasing - AGA mean BP standard reference of 14.72xpsia at sea level was changed to 14.696, accordingly - mean sea level will be changing as polar ic caps melt, further decreasing BP as less ice means warmer air

nescosmo · **Joined:** Sat Jul 14, 2007 11:17 pm **Posts:** 202

I thought that on the CRD fuel cap pressure did not mean anything, I thought that if you let the cap off the CRD will work fine.

gmctd · **Posted:** Wed Feb 18, 2009 7:04 pm

The fuel cap is intended to vent excess vacuum\pressure, Nescosmo - the excess is bad for CRD operation - yes, the engine will run without the cap - but, the cap also prevents moisture entering the fuel tank, which it will do even in humid without precipitation conditions, as Diesel fuel attracts water, such that algae forms at the fuel\water interface, and you gotta mess in yer tank and fuel system - best not to defeat the vent valves in the cap, plus install the cap till the ratchet mechanism clicks to seal out moisture

nursecosmo · **Posted:** Wed Feb 18, 2009 7:10 pm

Respectfully, in my experience the cap wont make a lick of difference in modern D2 with anti foaming agents. Also, cold diesel foams MUCH worse than warm diesel (When the temp is <0 I can not fill up to the neck unless I want to take 10 minutes for the last gallon).

gmctd · **Posted:** Wed Feb 18, 2009 7:23 pm

If pressure is allowed to build up in the tank, excess pressure in the tank can cause the return-fuel system to not function

If vacuum is allowed to build up in the tank, excess vacuum in the tank can cause the fuel supply system to not function - barometric pressure is what pushes the fuel outta the tank and up to the lift pump vanes on the injection pump - if you have an auxiliary electric lift pump, you still need atmospheric pressure in the tank to make it work

The cap has definite purpose in limiting pressure\vacuum to +/-1psi, in addition to sealing out moisture and silicates - if your KJ CRD fuel cap is not currently residing tightly on the fuel filler neck, put it back on now, no matter what else you may have heard

Trust me - would gmctd lie to you??

nursecosmo · **Posted:** Wed Feb 18, 2009 7:55 pm

Water and dirt are definitely not good additives to the fuel tank.

Cowcatcher · **Posted:** Thu Feb 19, 2009 1:04 pm

Ahhhhh, unless others here are mistaken the cap is only a dust cover. Others have reported that the tank actually has another overflow tube. Seems to me I have used it once too, inadvertantly while agressively filling my tank. It drains on the other side of the tank from the filler if memory serves. Of course some of you have had the tank out to install the tank pump so you would know best.

Joe Romas · **Posted:** Thu Feb 19, 2009 8:56 pm

Cowcatcher wrote:

Ahhhhh, unless others here are mistaken the cap is only a dust cover. Others have reported that the tank actually has another overflow tube. Seems to me I have used it once too, inadvertantly while agressively filling my tank. It drains on the other side of the tank from the filler if memory serves. Of course some of you have had the tank out to install the tank pump so you would know best.

That's what I thought too :shock:

that tube is on the drivers side and as I recall clipped the fill pipe. When mine went back in I was able to elevate it some and it sure cut down on it running out on the ground when I fill up :roll:

gmctd · **Posted:** Thu Feb 19, 2009 10:58 pm

Fair enuff, cowpattie* (and Jose') - let's examine the KJ CRD dust-cap covering the fuel filler-tube neck thingy - if you'll go outside, remove yer green oem fuelcap-type dust cover, and obscrutinize the business end, I'll explain - the rest of you, just talk amongst yerself(s) - we'll be right back...........

Note the center section assembly, looks sorta like a little plastic sport wheel with spokes'n'stuff, right? That little wheel contains the bi-directional, two-way, in-out, pop-off valve, intended to vent-off dusty vacuum as well as dusty pressure, particularly if the dust-cap had never been screwed down tightly, and the fuel tank is indeed full of dust - you can test functionality yerself by wiping Diesel fuel off with a clean, not dusty, rag, then (and don't let the neighbors see you doing this, you'll never live it down!!) put a lip-lock on it and draw thru it sorta like a large odiferous milk-shake straw until you hear the comical flatulent sound (snicker, snicker!), whereupon it just vented vacuum - same effect can be snickered at with pressure by blowing thru the little wheel - the valve emits the f**ting sound both ways if fully functional - and that's the vented fuel cap, in all it's green tighten to three clicks -> Diesel fuel only splendor

Ok, it's prolly not as cold here as where you are, so screw the vented (dustless) green fuel cap back onto the filler neck, close the fuel filler door, and let's go back inside and join the others before they get bored and start wandering off (I get paid according to the number of attendees that finish the class, whether they pass, or not - being subject to the EOC\ADL\ACLU 'no student left behind, none shall fail' mandate, each student achieves a simulated-gold plated ball-point pen with a close approximation of his name spray-stenciled on the plastic barrel, along with an official-looking diploma, also personalized with his name, indicating, not that he passed, but that the student successfuly attended the seminar)

Now, as to the anti-foam diaphragm vent tube attached to the puck mounted at the back of the tank and adjacent to the fuel filler neck port - I fill my tank up into the neck, higher than where I extended that vent into the filler neck cavity in the fender, and it has never dumped fuel - further, I do not see how the tank could meet EPA specs if it had an open vent that would spill fuel on the ground at over-fill, or steep angle, or roll-over, or severe fender bender crash - it is part of the anti-foaming fill system, but cannot vent the tank as it is on the atmosphere side of the diaphragm valve, intended to vent diaphragm displacement just like any diaphragm regulator, be it natural gas, propane, butane, acetylene, oxygen, air compressor tank, water tank, etc - a leak to the fuel side could be disastrous, and is a definite EPA no-no

So......questions.........discussions.........humor........ridicule?

Bring it on..............

BTW, I installed a lawnmower fuel filter when I extended the vent-hose up into the filler neck cavity, just like the one on the vacuum regulator solenoid, up front - also on the differential, trans\xfer case, and front diff breather-vent hoses - no dust fer my KJ criticals, eh........

Note both inter-connected white pucks, part of the anti-foaming fuel-fill system, first-use on yer KJ CRD

Sorry 'bout that - it looks better if you're laying down.........

*ibid., Jim Stafford

Joe Romas · **Posted:** Fri Feb 20, 2009 8:21 am

I'm glad you put in all the disclaimers :lol:

I get paid by the number of attendees that finish the class, whether they pass, or not - being subject to the EOC\ADL\ACLU 'no student left behind, none shall fail' mandate, each student achieves a simulated-gold plated ball-point pen with a close approximation of his name spray-stenciled on the barrel, along with an official-looking diploma, also personalized with his name, indicating, not that he passed, but that the student attended the seminar

nescosmo · **Joined:** Sat Jul 14, 2007 11:17 pm **Posts:** 202

back in the old country our house had tile floor and my grandmother use to have a mop, spray diesel fuel on it and put it in the sun to dry late took it and mop the floor; well the mop pick all the dust of the floor so well tha you could see the mop full of it .
My point is that it seems the the diesel love dirt and dust, so GMCTD is on track.......

gmctd · **Posted:** Sat Feb 21, 2009 12:39 am

Hmmmm - looks like I also confused Cowcatcher with Cowpie - this must be Friday the 13th, 'cause that's three bads for me, today - good thing it's already 2240hrs, 'cause I don't think I could stand another one :oops:

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Fuel cap venting

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