Fuel Tank Pressure

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The gas cap will allow air into the tank, as will the canister, if there is vacuum in the tank.

If I remember, the VCV switches at 3000rpms to open the Evap to intake vs being absorbed into the CC.
It also purges at idle, see the note at the bottom of the page in the FSM: "The PCM controls the on/off period of the VSV..."
 

enox

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Also curious as to what % of ethanol is in the gas and it that varies by brand or state?
It's 10% at least here. Asked my friend who owns a gas station and here's a photo of the sticker they are required to put on the pumps.
IMG_20210719_125120.jpg

Has anyone tried this with leaving the gas cap loose?
Yeah - I left my cap loose last Saturday, seems to fix the venting issue but then it just smells like gas...
 

vegasfj40

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I've experienced this fuel boiling in my 95 as well. I also use a VC120. Last summer I was following a friend up a steep trail to a fire lookout in the Jemez mountains going from about 6k feet to 9k feet and my tank was boiling but my friends 16 4Runner was not. I wonder what the difference between the 2 trucks evap system is? Maybe just higher underhood temps in the 80? I didnt know that the added ethanol boils at a lower temperature. I'll have to start paying attention to gas station pumps.
 
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I wonder what the difference between the 2 trucks evap system is? Maybe just higher underhood temps in the 80?
I would like to know this as well. I did a smoke test of my exhaust and found some very tiny exhaust leaks at the joints, not sure if that is contributing to the gas boiling or if instead the fuel returning to the tank is just super hot. My purge VSV definitely works, I think it gets overwhelmed when the tank boils. When I was in Moab with 109 deg F temps, using ethanol-free made a HUGE difference. As soon as I left Moab and put in 10% ethanol it was venting like crazy again.

With 10% ethanol when the engine temp gets on the hot side, I had trouble starting/vapor lock. One other time it was so bad I had gas spewing out the gas cap.
 
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I've experienced this fuel boiling in my 95 as well. I also use a VC120. Last summer I was following a friend up a steep trail to a fire lookout in the Jemez mountains going from about 6k feet to 9k feet and my tank was boiling but my friends 16 4Runner was not. I wonder what the difference between the 2 trucks evap system is? Maybe just higher underhood temps in the 80? I didnt know that the added ethanol boils at a lower temperature. I'll have to start paying attention to gas station pumps.
5th gen 4runner has more heat shielding around the fuel tank than an 80 series.
 
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Straight from Mr. T (1986 LC Owners Manual):

"Gasohol, a mixture of unleaded gasoline and 10% ethanol, is available in some areas. If you use gasohol in your Toyota, please take the following precautions: If drivability problems are encountered (poor hot starting, vaporizing, engine knock, etc.), the use of gasohol should be discontinued."
 

vegasfj40

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I'm doing the climb up the back side of Sandia Wednesday am. Not sure if the boiling will happen as the temps will be in the 70s but I'll be checking for it. Looks like an issue we'll just have to deal with as unfortunately ethanol isnt going away. Wonder if there is an additive that helps against the boiling?
 

enox

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I think another factor that plays into the tank pressure/boiling is your engine RPMs. If I remember, the VCV switches at 3000rpms to open the Evap to intake vs being absorbed into the CC.

This makes sense in that the most common time I get tank pressure is when I am doing a climb on a trail 2000’+ where rpms stay low. A hot day doesn’t help the situation also.

Also curious as to what % of ethanol is in the gas and it that varies by brand or state?View attachment 2735400View attachment 2735401
So basically under 3k rpm's that will stay closed hence needing to just vent straight into the CC. I would bypass the CC all together, but I dont want to have a line with just a filter on the end and have fuel vapor at all times.
 

Broski

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It's 10% at least here. Asked my friend who owns a gas station and here's a photo of the sticker they are required to put on the pu
Find another station.
I know when I go to Moab there only one station that doesn't have the Ethanol in it. You have to pay more.
I can't speak for others but when I run straight Gas no issue, when I put gas /ethanol blend in the smell of gas returns.
 
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Certainly not popular but I have been no charcoal canister for years. Back when I had the charcoal canister I drove up Imogene pass in the summer and at the top I loosened my gas gap because it was hissing and it vented and sputtered out gas for a solid 5 min. It has not done this since I ditched the charcoal canister, but yes you can occasionally smell fuel, but much less than when it vents for 5 min. Lol
 

enox

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Just saw this in the 100 series thread: post #206, has some interesting suggestions/data.
Yeah - so looks like that poster is fixing it by running heatshields and a fuel cooler. I just don't see how this "all of a sudden" became an issue when in the years past I have not noticed this phenomenon.

Leads me to believe:
  • Need a new VC120 (even though its fine normal conditions)
  • Need to check the vent tube @ the tank and check for a possible pinch?
  • Checked through some diagrams, cannot find if we have a "check valve" for rollover situations.
  • Check to see if the Purge VSV is working properly.
I will say the only nice thing about the pressure in the tank is the 80 fires right up with like 1/2 engine crank...its really great. Otherwise it has to turn 1ish cranks before it fires. that's the only positive I've noticed.
 

enox

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@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 ?
 

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