Gas tank pressure, how much is normal?

[1973Guppie]

Was driving the FJ40 tonight and I noticed quite a bit of pressure coming from the front tank when I went to fill it up at the station. I had just finished coming down a large, long hill for about 25 minutes, probably a 1000 foot drop of elevation. I am trying to figure out if after a drop in elevation like this, is it normal to have some pressure built up in the tank? OR should the charcoal/vent system take care of this completely so you have no pressure in the tank? I have the tank routed correctly, I am sure of it. I am using the stock 73 FJ40 fuel separator and it is routed to the stock chevy charcoal canister and then to the engine so the available vapors can be used (the stock chevy canister has markings for each port and where they go). The only change I have made is to pull the beisch fuel filter looking thing which I believe allows vapor to pass but not gas, I replaced this with a piece of metal fuel line. The truck drove fine, so not worries there, and I did not really get any in cab fuel vapors but was wondering how much pressure is normal. I did notice that the vented cap I have mentioned to open slowly as fuel pressure can cause fuel to spray. Anyhow, any opinions appreciated.

Noah

 

[1973Guppie]

bump, anyone, or am I just a retard?

 

[FJBen]

bump, anyone, or am I just a retard?

OCD?



















sorry I have nothing to add about how much pressure is normal

 

[Mace]

The tank should be vented. So no pressure buildup if it is performing properly.

If you do not have a vent on the tank. A significant increase in ambient temp + a full or near full tank can actually fill your motor up with fuel..

 

[jm599]

many times when filling up a near empty tank I also hear a rush, but seems to be more of a vacuum situation. I am not sure why the vents would allow this to build up

I wonder if...next time, you were to unplug your hose to charcoal canister.... or at the valve whether it would depressurize.

I remember the one way valve, that looks like a fuel filter, that is in-line to canister, after the separator, being confusing to me... not really understanding why it needed to be there...maybe to keep water from getting in the fuel in high water???

 

[Colorado Boy-74-FJ40]

I drilled a hole in the gas cap on mine!

 

[Mace]

I drilled a hole in the gas cap on mine!

Good idea until you have a off camber situation

 

[Strait Cruisin]

The idea is that when a sealed fuel tank pressurizes, it pushes fumes into the charcoal container where it is stored until purged by a dump valve after start up (works off ported vacuum so it doesnt just gulp a bunch of air and die). Having the canistor is a good thing, IIRC, it can store up to a cup of fuel. On the down side you have fuel pushing against the float valve and fuel line connections. Some smog checks look at your gas cap too and how many HC's you are leaking there.

 

[1973Guppie]

The idea is that when a sealed fuel tank pressurizes, it pushes fumes into the charcoal container where it is stored until purged by a dump valve after start up (works off ported vacuum so it doesnt just gulp a bunch of air and die). Having the canistor is a good thing, IIRC, it can store up to a cup of fuel. On the down side you have fuel pushing against the float valve and fuel line connections. Some smog checks look at your gas cap too and how many HC's you are leaking there.

so for what it is worth, I have a vented gas cap AND I am sure that the tank is vented properly. According to the description above, isn't it possible that enough pressure built up quickly enough from me coming down in elevation that the purging system did not have enough time to vent the pressure buildup? If it doesn't just gulp a bunch of air then I would guess that some amount of pressure would be normal when coming down some in elevation? I will have to test it a few more times to see if it continues to happen.

Noah

 

[Mace]

If it builds pressure it is not vented..

1000' is not that much elevation change.

 

[stumpy]

and if your tank isnt vented, you can split your tank at the seams. trust me.

check your carbon canister and your evaporative control box. the later 40 evap box had a vent to fresh air too.

 

[Colorado Boy-74-FJ40]

Not too bad

Good idea until you have a off camber situation

I had it nearly 30 degrees withe a pretty full tank and it leaked a little but not too bad. Anyone with a 40 knows they don't stay full for very long!

 

[1973Guppie]

something must be going wrong then, I will check back through everything (lines, canisters, etc) and post back up. Do the charcoal canisters go bad? Does the evap emmission fj40 stock thing amajig go bad? to the point where they won't pass air/fuel? and how long do they usually last?

Noah

 

[Strait Cruisin]

Carbon canisters usually have some sort of filter on them or draw from the air cleaner. They dont draw air (or should not) from the tank itself when running (you'd be doing the opposite of lowering the pressure in the tank from ambient) GM's, for example, have a filter pad on the bottom of the canister (stick your fingers under one sometime). Yes they do go bad (dust and dirt usually) but check for collapsed hoses, operation of the purge valve, etc.

 

[John McVicker]

Carbon canisters usually have some sort of filter on them or draw from the air cleaner. They dont draw air (or should not) from the tank itself when running (you'd be doing the opposite of lowering the pressure in the tank from ambient) GM's, for example, have a filter pad on the bottom of the canister (stick your fingers under one sometime). Yes they do go bad (dust and dirt usually) but check for collapsed hoses, operation of the purge valve, etc.

Maybe taking the line off @ both ends & running some air thru it ???

John

 

[KOOTENAY KRUZER]

When you come down in altitude the ambient pressure increases so the tank would show a relative vaccuum if it was still at the pressure of the higher altitude, also the engine removes fuel from the tank, these two would combine to give you a vacuum in an unvented tank, not pressure. Also eventually the engine would quit because the fuel pump couldn't pull fuel against the vaccuum.

 

[FJFOOL]

I was concerned about this also, and determined it was indeed vacuem, not pressure. I'm wondering if the vented cap I have is like a one way valve, letting air out, but not in as fuel level is depleted? Just a thought, haven't gone farther than that.

 

[scouthead]

My '76 fj55 builds up pressure in the tank also. A few times it has actually projectile vomitted about a pint of gas onto me. And that was after filling the tank, driving a few gallons out of it, and THEN it belched gas out when I went to top it off and check milage...I have checked and all lines are clear, can blow air in and out of canister easily, replaced fuel filter looking vapor seperator-no change (and yes it's in the correct way), exhaust is not too close to tank, etc.
But even if something isn't working properly, where is the pressure coming from? I could understand a vacumm, but what could possibly be pumping pressure into the tank? A coup[le of times it's been bad enough that I've worried about the tank splitting or leaking...WTF !!??????

 

[1973Guppie]

I have a vented gas cap as well, not sure if the vented caps allow only air OUT and not IN? I think for me it was probably vacuum as well, but not sure. Either way, vacuum or pressure, it should be equalizing if the system is vented correctly. Something must be wrong in my system. I am thinking it is somewhere in the engine area, charcoal canister or ?, I may pull the canister and leave an open line in the engine bay for venting and see what happens.

Noah

 

[camcruiser13]

where did you get your vented gas cap?

i think i need one