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Discussion Starter #1
Hey guys. Got a strange question. When my truck is just sitting there idling or running around with the windows down, it lets out a farting noise from the fuel cap. The tank is obviously becoming pressurized somehow. I've tried new caps so thats not the issue. When i go to fill it up and pull the cap off a big rush of air comes out the neck. Somehow i gotta unclog the vent system or whatever that allows pressure to not build up in the tank. Any ideas or ways to troubleshoot it?

Thanks,
NK
 

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The factory setup vents to a 'purge' tank mounted under the bed of the truck (above the spare tire) and is relieved whenever the engine is started. You may have a vent hose pinched or maybe some previous owner took out the purge setup and plugged the vent valve / line at the tank.........

Bird
 

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Discussion Starter #3
Any way to troubleshoot other than just using the old eyeballs? Could be the purge valve is stuck shut perhaps and not letting vapor be consumed by the engine? Just throwing ideas here. I'll have a look but as far as I know, the truck is pretty dang virgin.
 

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Stock block 11 second club
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To build off what Bird mentioned... there are two vapor vent valves on top of the fuel tank that allow pressure in the tank to vent. Those vapors fill the lines of the evap system. The vapor vent valves are mechanical (i.e. ball-check-valve with a spring). Their secondary purpose is to close in the event of vehicle roll over so fuel can't empty from the tank. IIRC the vapor vent valves only close in the event of roll over when the weight of the fuel overpowers the spring.

On the firewall by the brake booster you will see a sensor with large, green and white connectors and a few vacuum lines. That sensor is the evap purge solenoid.

At the rear of the truck is the charcoal canister and a purge solenoid (the proper name escapes me at the moment.

On top of the tank is a sensor called the fuel tank pressure sensor.

Its been a while since I've gone through the operation of the evap system so bear with me if I list any inaccuracies... when the truck is off the purge solenoid in the rear of the truck by the charcoal canister is open so any excessive vapors can bleed off so the system doesn't become pressurized. These vapors pass through the charcoal canister to eliminate the odor of fuel.

When the truck is running the ECU is monitoring the evap system pressure via the fuel tank pressure sensor. Periodically the ECU commands the purge valve at the rear of the truck closed so pressure builds within the system. This allows the ECU to verify proper operation of the evap system (via the fuel tank pressure sensor). Once the system is determined to be satisfactory the ECU commands the evap purge solenoid on the firewall to open so the vapors are introduced into the combustion process and burnt off.

With all of this in mind it sounds as though you have a pinch line, faulty sensor, or the previous owner improperly deleted the evap system.
 

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Discussion Starter #5
Thats helpful. Thats kind of how i understood it to work as well. And since this only happens when its running i am kind of assuming that it should have something to do with the purge side that goes to the intake rather than the vent side and canister. Does that make sense to anybody else? It only seems to build pressure when its idling or just chugging around. I'm waiting on parts to arrive so I can build up my engine but once its ready to go, I'm going to try and compare it to my 98 Expedition. They should have similar systems if not identical ones. I'm assuming getting to the fuel tank pressure switch is a pain in the ass. May have to do some more snooping. Any other ideas? Thanks TJ and Bird. I'll see where i get with that info.
 

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Quit feeding it chili with beans!

T
 

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I just recently deleted my evap. Wasn't having any issues but noticed upon removal of the lines that every single rubber part was nearly degraded to powder. I don't know what effect air leaks in the evap hoses might have if any. But thought I'd share.
 

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Come to think of it mine from time to time makes a gurgling noise from the firewall area near the EVAP. It tends to do it right after startup (or sporadically when warm) so I've never been able to pin point the possible issue. Now that I better understand how the EVAP system works I may try to inspect the lines.
 

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...i am kind of assuming that it should have something to do with the purge side that goes to the intake rather than the vent side and canister. Does that make sense to anybody else?...I'm assuming getting to the fuel tank pressure switch is a pain in the ass.
The lines for the two vapor vent valves route to a y-fitting so there aren't "sides" going to opposite ends of the truck, its all one system.

Here's a pic of the fuel tank. #1 and #3 are the vapor vent valves. #2 is the spot where the fuel tank pressure sensor is located.

OEM Fuel Tank Numbered.jpg
 

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Discussion Starter #10
I just recently deleted my evap. Wasn't having any issues but noticed upon removal of the lines that every single rubber part was nearly degraded to powder. I don't know what effect air leaks in the evap hoses might have if any. But thought I'd share.
Thanks. Might be something to this.

Come to think of it mine from time to time makes a gurgling noise from the firewall area near the EVAP. It tends to do it right after startup (or sporadically when warm) so I've never been able to pin point the possible issue. Now that I better understand how the EVAP system works I may try to inspect the lines.
I'm 100% sure mine is making its noise by reaching blow off pressure of the gas cap. Our problems could be similar though.

The lines for the two vapor vent valves route to a y-fitting so there aren't "sides" going to opposite ends of the truck, its all one system.
This and the pic are helpful. Once i get it pieced together I'm going to try some stuff. I haven't noticed it doing it when the vehicle is off but its not like I ever just stand there looking at it when its not running. Maybe some of you do but not me lol.:smt042 Thanks guys!
 

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I'm 100% sure mine is making its noise by reaching blow off pressure of the gas cap. Our problems could be similar though.

Mine is definitely coming from the drivers side between the dash and engine compartment. I was going to dig under the dash to inspect, but after reading your post and Allen's explanation I'm betting it's EVAP related. Hopefully I can replicate the noise while I am under the hood to pin point it.

I hear mine once in a blue moon and when I do it's usually right after startup. There have been a few times also after the engine is warm at idle and cruising at a low speed. It wouldn't surprise me if it's a hose issue as I've already replaced many of the hoses due to age.
 

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Dude
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It hasnt been mentioned but this can cause problems with the Evap. I had an issue with vapor lok on a long trip on mine. Its never done it again except on this trip. I always filled my tank all the way to the top and maybe then some more fuel especially on along trip. Well after going thru the whole thing I came across an article about how filling up the fuel tank to the top is not a good idea because it allows some of the fuel to go into the charcoal canister and mess things up with the evap system. Anyways I stopped doing full fill ups and haven't had a problem since. I do 3/4 or a little more now.
 

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Discussion Starter #13
So I have done a bit more investigation. I swapped purge valves with my 5.4 expedition. There was no meaningful change. My Expedition is still silent and my Lighting is flatulent. I also noticed that it never does any of this farting business when the tank is above half full. It only does this when the tank is about 3/8 to half and will continue doing it until i fill the tank back up. I will try and get a video sometime here.

Thanks again guys.
NK
 

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how full do you usually fill the tank?

if you're one to try filling right past full and into part of the filler neck you could be mucking up the evap system by simply filling the evap system with gas
charcoal cans full of gas were not uncommon back when..

and a line full of liquid could be a plugged line
 
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