Fuel Tank Pressure (1 Viewer)

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enox

enox

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Changed the vapor canister aka charcoal canister 2 years ago with the VC120.

I have no issue with tank pressure around normal driving, however with climbing elevation and in the heat it will vent for a long time 5-10 min + when I stop and loosen the cap.

Yesterday was particularly hot and I left the cap loose most of the day, it was still venting when I would stop and take it off.

Replace with another vc120 or connect those hoses together, or I read somewhere about adding a filter to the end of the hose but not sure about that either?

 
Do you regularly top of your gas tank? Read that will require a new vapor canister fairly quickly.

I just replaced my original OEM with a VC120. Now wondering if the VC120 may need replacing more often than the OEM model? Seems like they should last at least half as long as OEM…12 years or so :meh:

Sub’d for more info.
 
What shape is your gas cap in?
I had that very same experience 3 years ago. Stopped for a break and heard cracks and pops coming from the fuel tank area under the truck. Cracked the fuel cap and fuel vapors and liquid came flying out.
Immediately went thru the canister cleaning the ports and check valves. Never had another issue. Then I replaced the fuel filler cap early this spring as PM.

And THEN...one day last month I was standing near the drivers-side rear taking some pics and hear it...a slow, lengthy, barely audibly exhale. Like the kind one does before 'squeezing a trigger'. It stopped for 10 seconds, then did it again. Then stopped, then again. I'm usually solo on my rides, so the sound was unnerving but decided it was my imagination that 'Squatch' was just behind a nearby tree.

Finally, on the way back down from a trip to 12kft, I stopped for lunch and hear it again. This time zeroed in on the fuel filler and popped the door open to check. Sure enough, it was venting from the tank thru the cap...as it should do presumably...in 10 or so second intervals. This went on for 20 minutes or so before it eventually quit.

Guessing a properly operating system includes a good cap and canister resulting in venting both back thru the can and thru the cap when necessary.
 
enox said:
Well - If I replace this VC120 with a new one, and dont have an issue then ill know they are only good for about 2 yrs or 15k miles...
Click to expand...
Standing by…inquiring mind want to know
 
Initially, I had great luck with my newly installed VC120, drove from 1000’ above sea level to CO/Utah 4 days on the trail at Cruise Moab, temps were in the 50’s/60’s and never a hiss. I later spent 3 days on trails back in TN in 90+degrees and back to hiss/whoosh and even some gas smell, however during normal driving in warm weather there isn’t a hiss or any smell.

I am wondering if the VC120 is a little susceptible to failure when exposed to high ambient temp trail riding? Anyone else experiencing the same?
 
I just had this issue in Colorado over the weekend driving from Denver to Ouray and also going over Imogene pass. I pulled the hose off the hard line in the engine compartment leading to the charcoal canister (a VC-120 for me) and let it vent to the atmosphere for 20 minutes. There was noticeable pressure trying to blow my finger off the line if I blocked it.

I will note that I attribute this to elevation change combined with high chassis temps/ low air movement…. Here in Texas I never get the tank pressure anymore since replacing the stock canister with the VC-120, even on the hottest of days.
 
desmocruiser said:
I am wondering if the VC120 is a little susceptible to failure when exposed to high ambient temp trail riding? Anyone else experiencing the same
Click to expand...
Yeah exactly my case too - I've done a few trips this summer at elevation.

Specifically Saturday it was anywhere between 96° - 117° outside temp and I climbed from 3300' to 6700' in a few hours and then back down.

The tank vented for at least 10 min when I arrived to the top, the video I posted was during a lunch break about 1/2 way down. I decided to video it after it had been venting and glugging for a while.

I did read another thread that others have had the same issue with low speed crawling in summer temps with a full tank. However there has to be something going on with the VC120 as I havent had any issue with it during the previous summers.
 
I've had a VC120 canister in my 80 for at least 5 years. I was noticing the ethanol smell that I used to get from the original OEM canister. I swapped to a brand new VC120 and still have the smell. I attribute the smell and the increased pressure to the increase in ethanol in the gas. In 2019, the EPA allowed the summer blend to go from %10 to %15 ethanol. You might try a tankful of non-ethanol gas to see if the pressure drops noticeably.
 
i swapped my OE with a VC120 right before a trip knowing that i had issues in previous trips involving elevation and hot weather. Unfortunately a new VC120 made no difference. Except this time the smell is now coming on both the gas cap and on the canister. I think the VC120 works, but it seems it (or even the OE) just doesn't have capacity to vent that much pressure with this high-blend ethanol gas.

It only did it once coming up schneby hill in sedona, and for those who know the track, it's just pretty rocky long stretch, so at >100F and sloshing the gas around was a great recipe for over pressurized tank. It is unnerving to vent that much fume/pressure!
 
I'm in the same boat. Put the vc120 in a handful of years back. Seemed to work better but not perfect. Coming back over the pass a few weekends back (0 to 5000k ft) fairly quickly in 100 degree heat pulling a 3000k camper and at the top, the tank vented for probably 10 minutes. I think hard problem to deal with in extreme conditions as the logic in the vsv/egr/tvv (I believe) needs to sense some time at not-full-throttle to put a vacuum on the tank and try to reclaim vapor....so long periods of time with high heat/pressure have to be handled either by venting or the charcoal canister itself. I may try a new vc120 or if I have the time rebuild the old one.
 
Update - put an extra hose on the tank side of the VC120 blew air by mouth through it.

I can hear air passing through and it vents through but it takes some pressure to get it through.

I ordered a new VC120 and will give it a try to see how easy air passes through a new one.
 
I have this issue too and I don't think a new charcoal canister will fix it. I believe the venting you (and I) are seeing is the gas tank boiling. The charcoal canister is only designed to hold vapors from say filling up the tank. If the gas tank is boiling for several minutes then there is no way the canister can hold all of those vapors and it comes out the bottom of the canister (routed from a hose into the fender well near the firewall). I've found using ethanol free gasoline makes the issue go away. Opening the gas cap and letting it vent is probably the wrong thing to do, as it will just boil even harder (until it all cools down).
 
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WCBlueSky said:
I have this issue too and I don't think a new charcoal canister will fix it. I believe the venting you (and I) are seeing is the gas tank boiling. The charcoal canister is only designed to hold vapors from say filling up the tank. If the gas tank is boiling for several minutes then there is no way the canister can hold all of those vapors and it comes out the bottom of the canister (routed from a hose into the fender well near the firewall). I've found using ethanol free gasoline makes the issue go away. Opening the gas cap and letting it vent is the wrong thing to do, it will just boil even harder (until it all cools down).
Click to expand...
This makes sense, it does sound like it's boiling, at times for a really long time.

Now I'm just not sure how to distinguish which stations have ethanol free gasoline.

I only run 91 usually from one of the top tiers which would include, Shell, Chevron, 76 - etc.
 
I think there is a common misconception here that if there is any "whoosh" when you remove the gas cap then the charcoal canister is bad. The OEM canister has check valves in both directions (in and out) that by design open at around a very low psi, which holds in a small amount of vapor in the gas tank. I tried drilling out the check valves and got terrible fuel smell and would not recommend it. I later found a used canister at the salvage yard so I could get check valves back.

What does it mean for the gas tank to "boil"? Every liquid including gasoline has a vapor pressure, and when the vapor pressure equals atmospheric pressure, then it boils. The vapor pressure depends on the temperature and the chemical composition of the liquid (ethanol vs no ethanol). The atmospheric pressure depends on the elevation. So hot days and climbing in elevation make it easier to boil the gas tank.

I'm still trying to figure out *why* the gas tank gets hot enough to boil: if my cats are clogged, or the fuel in the return line is super hot, or if it is just normal and due to the ethanol in the gas which lowers the boiling point.
 
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Last edited:
WCBlueSky said:
why* the gas tank gets hot enough to boil: if my cats are clogged
Click to expand...
Yeah interesting, I also have new cat. Works great and doesn't get nearly as hot as the OEM.

But I was in LOW climbing a few thousand feet, steep and on a nearly full tank @ 96° +++ I think 96° was the lowest outside temp I came across..
 
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