Dual gas tanks - interesting observation

This site may earn a commission from merchant affiliate
links, including eBay, Amazon, Skimlinks, and others.

alia176

SILVER Star
Joined
Aug 21, 2003
Threads
789
Messages
15,398
Location
Tijeras, NM
Background:
Bunch of us were at Death Valley running around last week and I had to transfer fuel from the aux tank to the main tank many times. This transfer usually took place while the vehicle was bouncing around. As you know, the fuel is dumped into the main tank vent line from the aux tank.

Observation:
My co-driver and I detected a faint hint of fuel vapor while driving with the windows open. The gas cap was opened and lots of fuel started to spew out. So, the gas cap was only cracked a little bit to let the vapor escape the tank. It took about ten minutes to completely evacuate the vapor. :eek:

Theory:
I think transferring fuel while jostling the main tank didn't allow it to breathe. The main tank vent line was being used for fuel transferring and not venting at the time. Perhaps this caused extra pressure to build up in the main tank and that's what we smelled. Does this sound right to you folks?

Does the gas cap have a release mechanism of some sort if the fuel pressure exceeds a setpoint? Did the vapor odor come from that?

Thanks.
 
Nothing to contribute here, but that would be kinda scary!

Hope some of the experts would chime in to offer suggestions.

Mot
 
you could have a bad fuel cap

I have dual tanks with the factory dual filler neck and have had no issues with fuel transfer. I have had a filler cap go bad once or twice. I smelled the fuel as you described changed the cap and all was good. How do you have your aux tank plumbed? If you don't have a vent from it to the main tank (mine is via the dual filler neck) you could build up enough preasure in the stock tank to pop the filler cap preasure release.
 
How do you have your aux tank plumbed? If you don't have a vent from it to the main tank (mine is via the dual filler neck) you could build up enough preasure in the stock tank to pop the filler cap preasure release.

This is the first time this situation occured to me and I've transferred fuel hundreds of times in the past. However, never transferred during trail runs so that's the real issue. Again, this is just a simple observation and not a problem.
 
Do I understand correctly that you had the gas cap clicked tight and it started to spew fuel through a sealed (or thought to be sealed) gas cap?
 
Do I understand correctly that you had the gas cap clicked tight and it started to spew fuel through a sealed (or thought to be sealed) gas cap?

Incorrect. I opened up the gas cap to vent the tank and lot of gas began to spew out so the cap was tightened back up and cracked only a little.
 
Incorrect. I opened up the gas cap to vent the tank and lot of gas began to spew out so the cap was tightened back up and cracked only a little.

Ali

I havent smelt or seen any gas leakage with my setup BUT there is A LOT more pressure when i open the cap then before i installed the subtank.

No fuel has come out, but after driving the rig for a bit, and i open the cap there is a very strong rush of air that comes out of the filler neck. Had this setup for almost a year, no vapor lock, no emissions problems (just aced smog), and no smell of fuel.

When i first installed my subtank i posted up about it, and no one really had any ideas what was up.

Anyway, i in no way shape or form answered your question, but the above is my experience!
 
Ali,
Talk to Amando (i4c4lo) about this- maybe you two can solve the issue. I was having flashbacks watching you vent the gas cap. He has a huge secondary tank and it seems to be more pronounced.
Maybe it's all that DV washboard? dunno...
 
<snip> BUT there is A LOT more pressure when i open the cap then before i installed the subtank.

That is because you have more than doubled the ammount of fuel you are packing. The evaporative emissions system was not designed to deal with that much fuel vapor.
 
Do I understand correctly that you do not have a secondary vent on the main tank? If you do not, the gas cap seal is failing and becoming you secondary vent from your discription.

Think of it as a simple displacement transfer. For evey cc of fuel tranfered a cc of air in the tank must be transfered/compressed. If you transfer the fuel without a vent then you are compressing the air in the tank. This is a very bad idea.... One simple way to solve the issue could be to run a line (about .5 inch) between the tank.

Btw, the fuel vapor concentration from one tank vs two is not an issue. The partial pressue of a gallon of gas is the same as 100 gallons.
 
Do I understand correctly that you do not have a secondary vent on the main tank? If you do not, the gas cap seal is failing and becoming you secondary vent from your discription.
.

There is a secondary vent (original vent per oem), please re-read the original post. I deduced that this situation will occur anytime one is transferring fuel in the middle of a eight hour washboard trail when the fuel is sloshing the most. This has never occurred while transferring on the hwy.


Quote: That is because you have more than doubled the amount of fuel you are packing. The evaporative emissions system was not designed to deal with that much fuel vapor.

I'm thinking that the evaporative system in OZ isn't much different than ours. Curious, how do their system handle the extra fuel?
 
Background:
As you know, the fuel is dumped into the main tank vent line from the aux tank.


Theory:
The main tank vent line was being used for fuel transferring and not venting at the time. Perhaps this caused extra pressure to build up in the main tank and that's what we smelled. Does this sound right to you folks?

Does the gas cap have a release mechanism of some sort if the fuel pressure exceeds a setpoint? Did the vapor odor come from that?

Thanks.


You said nothing about a secondary vent in addition to the one you are transfering through on the main tank to the secondary tank. If I am missing it in your first post please point it out.

If you do have a secondary vent then it might be clogged, full of gas from the washboard running, or too small. If possible please draw a picture of your fuel system and post.
 
My secondary vent, I assumed you're referring to the main gas tank vent line or the gas cap. In any case, they're still functional as usual except for the time when the fuel is transferring through the tank vent line. During this time, the main vent line isn't able to "breathe" the main tank and this is where the problem is created during washboard driving. Hopefully this makes it clear somewhat!
 
That is because you have more than doubled the ammount of fuel you are packing. The evaporative emissions system was not designed to deal with that much fuel vapor.

Thanks Dan

Is this going to have any long term negative effects and damage anything?

Anything i should change?
 
In any case, they're still functional as usual except for the time when the fuel is transferring through the tank vent line. During this time, the main vent line isn't able to "breathe" the main tank and this is where the problem is created during washboard driving. Hopefully this makes it clear somewhat!

That statement Cleared it up for me! Thanks
 
Thanks Dan

Is this going to have any long term negative effects and damage anything?

Anything i should change?

I don't know that you can change anything really. The most likely problem would be a saturated charcoal canister. As far as I know the fix for that would be to replace the canister, and then saturate the new one all over again.
 
As far as I know the fix for that would be to replace the canister, and then saturate the new one all over again.

Sounds Easy! And Expensive! :D

On a side note, how are the NON US rigs that have the Aux tanks setup?
 
Since the fuel capacity is not as great as what you have they may be the same as a single-tank setup. I do not have an OZ catalog so I have no way to compare the EVAP systems to see if there are any differences.
 
Since the fuel capacity is not as great as what you have they may be the same as a single-tank setup. I do not have an OZ catalog so I have no way to compare the EVAP systems to see if there are any differences.

Gotcha!

Thanks Dan!
 
Sounds Easy! And Expensive! :D

On a side note, how are the NON US rigs that have the Aux tanks setup?

My 93 was one of the first 40 FZJ assembled in my country, it was very close to US specs but came without the evaporative charcoal tank, only the brackets to hold it :D. The next lot were completely stripped of any anti smog part.

I never had any serious problem with my dual tank setup. Of course after a serious off-roading trip if I open the cap it release some gas vapor but nothing to be afraid.
 

Users who are viewing this thread

Back
Top Bottom