Fuel Tank Pressure

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@GeoRoss, any resolution in that long thread?

@WCBlueSky meh not really, a handful of people saying they have that issue and others stating with Gas boiling. Heat from exhaust and T-Case/ trans. One person report that they had a bad fan shroud and a new one fixed it, talk avout the fan clutch function, cooling aspect. My 80 runs cool and does not get extremely hot (coolant wise) that would trigger an problem... Nothing definitive.

I ordered a new VC120 (will have it tomorrow)
Checked to make sure return line was clear.
Checked operation of the purge valve, appears to be working properly.

I really don't want to have to re-pipe my exhaust (again) but may move some stuff if I can't seem to get a solution with the CC/ Vent components.

Thoughts @GeoRoss ?

The 1990 Clean Air Act requires summer gas blends to have a lower level volatility and that goes for ethanol containing blends also. In theory, our mid-90's Cruisers should be able to handle these levels of summer gas blend volatilities. Maybe the 1FZ-FE design team didn't have enough time to fully address the Clean Air Act. I don't know if ethanol is to blame or our EVAP systems are just designed on the edge. All I know is that different parts of the gas vaporize before others and maybe our system can't handle the modern blends or our EVAP systems are just getting old and need refurbishing.

I haven't had the problem in 12 years and only coupe of really bad ones. It was after a long highway run coming from Tucson to Flagstaff (2300-7000ft) in the summer and the next day going across the four corners region headed to Dolores, CO. This may have been because I had filled up a week earlier and the pump shutoff didn't work and I overfilled the tank. Maybe I saturated the charcoal canister. I have replaced all the easily accessible rubber lines going to the charcoal canister, vacuum lines in the engine bay, check valve under the intake and a new gas cap. I haven't had this problem since and now just have the normal "whoosh". I do have a new OE charcoal canister that I bought back in 2009 if the one in there ever fails. Fingers crossed as I'm headed up to the White Mountains in a few days and there is a long sustained climb that should test the system.

Bottom line is I think we are just running into PV=nRT and unless our EVAP system is working really well it just can't handle whatever blends they use in the summer when there are dramatic elevation changes.

The times it has happened to me were where I ran the tank down to 1/4 tank in the summer with significant highway miles and elevation change. I have always been curious what the temperature of the gas is coming back to the tank from the fuel pressure regulator. Is it heated enough to affect tank gas temperatures enough to start boiling when you remove the gas cap (lower the pressure dramatically).

There is the fuel cut off valve (check valve) that is a part of the fuel tank breather tube that goes the charcoal canister. I guess if this isn't working it would build up pressure in the tank too.
 

60wag

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So we've got a 2 speed fuel pump in the tank. Is that common? Does the pump create much more heat when on the high speed. When is it on high vs low? I would expect a tank that is low on fuel to show more heating of the gas than a tank that is full from the fuel pump.
 
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A timely thread, indeed... Did a trip to Sawmill lake in the Sierra last week. Stopped on I-80 at elevation to top off the fuel tank, and it vented like mad.

On the way back down the hill on Thursday afternoon, it was 104 in Auburn where we stopped to grab some food and the engine idle speed in Park was at 1100 RPM, would not drop down.

If the VSV is venting to the intake, will this affect the idle speed? It stayed high all the way home to Santa Rosa. The next day, started up as normal, running higher until the coolant temp came up, then idled away at 650 RPM.

My vent system is all stock, stock canister, VSV, etc.

I am running the Wit's End turbo kit and I have the 8.5 PSI spring pack in the wastegate. The rig weighs 6500 lbs fully loaded on 37's with 4.88's. It can hold 70 mph up I-80 with the cruise control set. On a few hills it had to shift down to 3rd, but pulls like mad. Temps were nothing more than 210 on the coolant and 220 on the oil all the way up.
 

Road Apple

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Topped up the fuel in Springdale, UT before grabbing lunch and driving through Zion Canyon. Grabbed Thai Sapa to go (great Pad Thai), headed up the road, and start smelling gas in the cabin climbing the grade.

Quick check under the hood and a text to @Broski to confirm. Yup…vapor pressure was through the roof. Loosened up the gas cap and it vented for over 5 minutes. Tried to take the cap off, but gas was burping out with the cap very loose.

With 10% ethanol fuel and 95 degree temps @ 4000’, the vapor recovery system on our 80 couldn’t keep up. This was with a new VC120 vapor canister. Will try to find ethanol free fuel in Moab. @Broski said there’s at least one gas station in Moab without ethanol.

picture of us waiting for the tank to vent.

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Road Apple

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Topped up the fuel in Springdale, UT before grabbing lunch and driving through Zion Canyon. Grabbed Thai Sapa to go (great Pad Thai), headed up the road, and start smelling gas in the cabin climbing the grade.

Quick check under the hood and a text to @Broski to confirm. Yup…vapor pressure was through the roof. Loosened up the gas cap and it vented for over 5 minutes. Tried to take the cap off, but gas was burping out with the cap very loose.

With 10% ethanol fuel and 95 degree temps @ 4000’, the vapor recovery system on our 80 couldn’t keep up. This was with a new VC120 vapor canister. Will try to find ethanol free fuel in Moab. @Broski said there’s at least one gas station in Moab without ethanol.

Checked the cap after descending the hill and again at Bryce Canyon with no venting. The cloud cover, light rain, and cooler temps at 7,600’ made the difference.

picture of us waiting for the tank to vent.
 
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I recently got fuel boiling after climbing from 5800 to 9300 feet with 10% ethanol. I could hear bubbling noises inside the tank for many minutes after I shut off the engine at the top, and the vapors overwhelmed my newly refurbished OEM canister. It wasn't very hot outside, around 75 deg F, and the engine coolant temperature was perfect. I measured 130 to 140 deg F on the outside of the gas tank with an IR thermometer. Next time I get boiling I'm going to put a pressure gauge on the vent line and see what PSI it reads.

I can think of a few different sources of heat that could cause the boiling:
  • Radiant heat from the exhaust
  • Convective heat (hot air) from the engine bay
  • Hot fuel returning to the tank from the return line
  • Fuel pump generating heat inside the tank
I wish I could isolate the main source of heat. I almost pulled off the fuel sending unit to try to see where it was boiling in the tank, but that seems a little messy and dangerous....
 

vegasfj40

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I recently got fuel boiling after climbing from 5800 to 9300 feet with 10% ethanol. I could hear bubbling noises inside the tank for many minutes after I shut off the engine at the top, and the vapors overwhelmed my newly refurbished OEM canister. It wasn't very hot outside, around 75 deg F, and the engine coolant temperature was perfect. I measured 130 to 140 deg F on the outside of the gas tank with an IR thermometer. Next time I get boiling I'm going to put a pressure gauge on the vent line and see what PSI it reads.

I can think of a few different sources of heat that could cause the boiling:
  • Radiant heat from the exhaust
  • Convective heat (hot air) from the engine bay
  • Hot fuel returning to the tank from the return line
  • Fuel pump generating heat inside the tank
I wish I could isolate the main source of heat. I almost pulled off the fuel sending unit to try to see where it was boiling in the tank, but that seems a little messy and dangerous....
I was just up in the Jemez yesterday driving up 10 in my 80 then took a little side trail near the top. I thought I'd get boiling but never did. Last year I drove up to the top of Cerro Pelado and did get some boiling and a little fuel smell at the top. We hung out up there for about 20 minutes and came down and I never smelled anything after. Really strange how inconsistent it is.
 

Road Apple

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I’ve been wheeling around the San Juan mountains at Solid Axle Summit. Had issues on the drive out, so I downloaded the Pure Gas app to fuel up with ethanol free gas.

During the event, I had some minor vapor pressure issues until the last day. Driving up the Imogene Pass, over 13,000’, we stopped for a break. I started to pull the cap to check, but gas started to come out. Not much venting, just gas looking for a place to escape. Seemed like a straight forward pressure difference between filling up at 7,800’ then climbing to about 13,000’.

Over all, the ethanol free fuel solved my vapor pressure issues.
 

60wag

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Relieving the pressure on the tank by removing the gas cap seems like the wrong thing to do. Lowering the pressure will cause more of the ethanol to vaporize, possibly causing liquid fuel to be forced out the fill pipe. As long as the relief valve on the canister can keep the pressure low enough to prevent tank damage, I'd leave the system closed. If you do vent the tank, then close the cap, it'll go right back to where it was, so why bother?
 

Road Apple

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Relieving the pressure on the tank by removing the gas cap seems like the wrong thing to do. Lowering the pressure will cause more of the ethanol to vaporize, possibly causing liquid fuel to be forced out the fill pipe. As long as the relief valve on the canister can keep the pressure low enough to prevent tank damage, I'd leave the system closed. If you do vent the tank, then close the cap, it'll go right back to where it was, so why bother?
Why bother to vent the tank? To stop gas vapors from entering the cabin, thus relieving my wife’s worry…and mine. Since there isn’t a pressure gauge for the tank, you can’t know or trust that the relief valve on the canister can keep the pressure low enough to prevent tank damage. Plenty of MUD members have reported tank ruptures. I don’t want to be next.

When I smell gas in the cabin, I relieve the tank pressure by partially unscrewing the cap, which allows vapor pressure to escape, but doesn’t allow liquid fuel to burp or boil out. That’s my warning that pressure is rising and an easy way to provide peace of mind. It’s worked so far.

On a 2 week road trip through the southwest, with temperatures in the mid 90’s to 114 degrees, I needed to relieve pressure a handful of times, mostly right after refueling with 10% ethanol fuel. Burning the tank below 3/4 of a tank seems to alleviate the issue. Relieving pressure frees up space to allow more ethanol to evaporate and allow time to burn below 3/4 of a tank.
 
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enox

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Update with new VC120 installed.

Trip yesterday up to 7,500' and of course a fully topped off tank before hitting the trail.

Was in 4LO all day.

Checked cap twice while out; first check approx 85° - no pressure.
Second check towards end of the trail ambient temp upwards of 95° - some pressure which fully vented by the time I unscrewed the cap put it back on.

Interesting result, need more testing but thought Id share from yesterday.
 
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I can think of a few different sources of heat that could cause the boiling:
  • Radiant heat from the exhaust
  • Convective heat (hot air) from the engine bay
  • Hot fuel returning to the tank from the return line
  • Fuel pump generating heat inside the tank
Don't forget to include the radiant heat coming off the highway pavement. Especially if you're a mud member who travels through the desert southwest during summer time.
 

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