Venting tons of gas fumes while offroading in SoCal heat

[totallypro]

2002 4runner, v6. When I'm offroading, especially on a hot day, a significant amount of gas fumes vents out of my filler cap. Enough that when we stop the vehicle there is a very strong smell of fumes around the truck. There is absolutely no smell inside, only outside. And there is an audible hiss as the pressure vents. The engine is running fine and there are no error codes. I understand that the cap is designed to vent excess pressure, but this seems to be way too much. In the right lighting you can physically see the fumes almost pouring out. Is this just normal? I would guess that the heat and bouncing around would lead to the buildup of pressure in the tank, but should it really be venting this much? It's so much that I'm pretty sure it's effecting my mpg because so much of the fuel is being released into the atmosphere. Anyone have similar experiences or a suggestion?

 

[alumarine]

I don't think the cap is supposed to vent.
Maybe try a new cap.

The evap system is supposed to eliminate fuel vapors.

 

[octanejunkie]

Gas caps are supposed to vent, otherwise gas tanks would deform from either excess pressure or vacuum. There is a pressure valve in the gas cap that allows air exchange but it's supposed to vent via a breather line to the evap system, not directly into the atmosphere.

In CA, when you get a smog test, they will test your gas cap to make sure it functions properly. A failed cap is a failed test. Many shops offer a pre-test. See if one of them near you will test your gas cap real quick.

 

[pluton]

Is this just normal? I would guess that the heat and bouncing around would lead to the buildup of pressure in the tank, but should it really be venting this much? It's so much that I'm pretty sure it's effecting my mpg because so much of the fuel is being released into the atmosphere. Anyone have similar experiences or a suggestion?

No, it is not normal. There should be no visible vapor or audible noise. You should not smell gas. It is supposed to be a closed system for smog reasons.
Check the evap system and/or the tubes that feed it.

 

[baja1]

wow sounds dangerous too! as far as the OP suggesting poor mpg because those fumes are actually gasoline......is that true?

 

[cbr900boy]

Correct, gas caps are definitely NOT supposed to vent. This is why you get hissing from fumes escaping when you undo the cap. The fumes are recycled into the evap canister then back into the air intake. If you smell gas outside of the rig, you should first inspect the tank for any leaks then trace the line that feeds the fumes into the evap canister. My 98 has the canister located under the hood. I believe on newer 3rd gens the canister is located adjacent to the spare tire. Depending on how strong the gas smell is, you might have a leak as supposed to fumes escaping.

 

[totallypro]

I don't believe I'm losing a significant amount of mileage yet, but it is definitely gas fumes. I have a new oem filler cap on its way to try it out, but in the meantime I guess I'll check the evap/charcoal canister system. But does anyone have any pictures or a guide to find it? When I search I can't find anything particularly descriptive or with pictures, and I tried the FSM but it's literally too big for my PDF reading software to search through.

I know for sure it is not a gas leak, it is definitely fumes coming out of the gas cap. However it is only when there is a lot of pressure, most of the time and anytime I'm just driving around town it does not vent any fumes at all.

 

[Kanoe]

I know for sure it is not a gas leak, it is definitely fumes coming out of the gas cap. However it is only when there is a lot of pressure, most of the time and anytime I'm just driving around town it does not vent any fumes at all.

There shouldn't be a LOT of pressure. Some, yes but not a lot. There is a valve system in the charcoal canister that allows excess pressure to vent through the charcoal canister, that is it's function. If you are building up too much pressure then perhaps there is a restriction in your system somewhere. Or perhaps it's time to try a used canister and see if anything improves.

I believe on one of my old corollas I found a procedure to blow out the canister with moderate air pressure. This seemed to help a lot with it building up high pressure when hot. Not sure if that would apply to the t4r..

 

[gamefreakgc]

I don't believe I'm losing a significant amount of mileage yet, but it is definitely gas fumes. I have a new oem filler cap on its way to try it out, but in the meantime I guess I'll check the evap/charcoal canister system. But does anyone have any pictures or a guide to find it? When I search I can't find anything particularly descriptive or with pictures, and I tried the FSM but it's literally too big for my PDF reading software to search through.

I know for sure it is not a gas leak, it is definitely fumes coming out of the gas cap. However it is only when there is a lot of pressure, most of the time and anytime I'm just driving around town it does not vent any fumes at all.

On some years and models, the gas tank filler next is known to crack or leak. If the cap does not fix the issue, that's the next spot to look at.

 

[Jhelms]

Rollover check valve on the 01 / 02. Very common. When they fail -you get a nasty gas smell at the back of the car or around the car. Needs replaced. While you have the tank down to replace it - change your fuel pump.

Your situation may be different - never seen it flow out the filler area. But the stink will be all around the car, primarily at the back IF it is the check valve.

Info starting at post #84
http://www.toyota-4runner.org/3rd-gen-t4rs/193922-my-4gunner-restore-refurb-re-work-6.html

 

[cbr900boy]

Rollover check valve on the 01 / 02. Very common. When they fail -you get a nasty gas smell at the back of the car or around the car. Needs replaced. While you have the tank down to replace it - change your fuel pump.

Your situation may be different - never seen it flow out the filler area. But the stink will be all around the car, primarily at the back IF it is the check valve.

Info starting at post #84
http://www.toyota-4runner.org/3rd-gen-t4rs/193922-my-4gunner-restore-refurb-re-work-6.html

Sounds like we got a winner here!

 

[totallypro]

Rollover check valve on the 01 / 02. Very common. When they fail -you get a nasty gas smell at the back of the car or around the car. Needs replaced. While you have the tank down to replace it - change your fuel pump.

Your situation may be different - never seen it flow out the filler area. But the stink will be all around the car, primarily at the back IF it is the check valve.

Info starting at post #84
http://www.toyota-4runner.org/3rd-gen-t4rs/193922-my-4gunner-restore-refurb-re-work-6.html

Awesome thank you so much! Finally something with pictures! I have ordered a new gas cap, and will try that out first. If that doesn't work my next step will be to follow your guide. I have also heard of a smoke test, do you guys know anything about that? Do they just pump the smoke into the filler neck of the gas tank? If so it would be super easy to come up with a much cheaper DIY solution.

 

[19963.4lsr5]

You have no EVAP codes?


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[totallypro]

You have no EVAP codes?


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Nope, no error codes or dash lights at all which is what I find odd, it also only happens when offroading. Offroading also happens to be the only time I'm driving the vehicle in temperatures above 100 degrees, and I can imagine the excessive sloshing around and shaking of the tank helps create some extra fumes. But still it never happens while just driving around town.

 

[totallypro]

The new gas cap is here, it visually looks identical to the old one but is obviously in much better condition since it's not 15 years old. It seems to work and is holding pressure, when I went to fill up this morning I could hear pressure releasing when I unscrewed it, so hopefully that has fixed it! I'll report back after a trail run I'm doing on Sunday to see if it truly fixed the problem.

 

[19963.4lsr5]

The new gas cap is here, it visually looks identical to the old one but is obviously in much better condition since it's not 15 years old. It seems to work and is holding pressure, when I went to fill up this morning I could hear pressure releasing when I unscrewed it, so hopefully that has fixed it! I'll report back after a trail run I'm doing on Sunday to see if it truly fixed the problem.

You had no EVAP codes for leaks?


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[alumarine]

It seems to work and is holding pressure, when I went to fill up this morning I could hear pressure releasing when I unscrewed it

Actually it probably wasn't releasing pressure.
If working properly there is vacuum and the noise you hear is air going into the tank.

The purpose of the evap system is to stop fuel vapor from going into the atmosphere.