Excessive fuel tank pressure

[MaddBaggins]

I disagree, would make the “problem” worse.



The only process that I can see causing this is fuel off gassing/boiling, two ways to stop it are; lowering the fuel temp or raising the system pressure. As the fuel temp increases, the vapor pressure is lowered and it starts off gassing. The vapor has more volume than liquid, so pressure goes up. The system is designed to hold pressure, this is a good thing, it raises the boiling point of the fuel, like how the cooling system works. If the pressure is lowered, like you saw by removing the cap, it will boil more.

A vented system would boil off more fuel, more often. Our gas is a cocktail of ingredients, like with any distilling process the ingredient with the lowest vapor pressure will go first. With an open system much more fuel is going to be lost as vapor, at least this way most/some of it is being burned as fuel?

Is this causing a real problem or is it more is a noise/smell issue? Is it setting a code, making the motor run rich, etc? I agree that the system appears to be overtaxed and is operating outside of it's design, so is venting, but is it more than a temporary problem under certain conditions?

How about sitting on top of a gas tank holding so much pressure it vents for 10 minutes. Sound safe and fun to you? The idea of a tank bursting scares the s*** out of me.

 

[landtank]

as far as the topping off idea, how much stink bug if any do you guys have?

 

[MaddBaggins]

turbocruiser, both Ross and I do not top off. We discussed this while we were scratching our heads. I'm running a new fuel pump as of sunday. I'll see if that does anything.

 

[MaddBaggins]

as far as the topping off idea, how much stink bug if any do you guys have?

I'm level

 

[GeoRoss]

I wonder if brittle vacuum lines down stream of the canister reduce the vacuum enough by letting in atmospheric air into the system and compromising the ability of the canister to purge.

I am also completely ignorant about the fuel pump system. Could the pump or
relay fail in a postion of too high of pressure causing more volume of gas to return via the fuel pressure regulator?

 

[GeoRoss]

The pressures called for in the FSM to test the charcoal canister are on the order of .5psi. That is hard to achieve on a regular shop compressor. Alvin's one year old canister "failed" this past weekend but we may of been using to high of psi. I don't recall Alvin if you smelled gas in the engine bay during your last gas boiling event.

It may be useful to just relace all to rubber lines in the
HVAC loop in an attempt to solve this. I just wish there was an easier way to do this other than removing the air intake. Granted, I don't know how filthy the intake gets but a
thorough cleaning of it and the associated ports may help as well.

I would think they might have similar problems in Aus unless they don't have a simlar HVAC system as they don't have our EGR.

I'm as concerned as Alvin yet feel like I am grasping at straws.

 

[turbocruiser]

turbocruiser, both Ross and I do not top off. We discussed this while we were scratching our heads. I'm running a new fuel pump as of sunday. I'll see if that does anything.

Okay, so scratch that off the list then ... but ... do return to testing the system using sub 2psi pressures (IIRC the gas cap itself is designed to hold two pounds or 2.5 pounds so anything more than that assuming a properly working cap is hindering your search for solutions).

Another thought just to cross another easy thing off the list ... do any of your regular parking positions include a decline on the USDS of the vehicle? If so right after a fill-up, even one carefully controlled so as to prevent topping-off, you might have some gas seep into the overflow tube causing the "pre-load problem" in the charcoal canister. Hope this helps.

 

[raydouble]

Can anyone post up the hose routing diagram?

 

[Tenzig Norgay]

Bad gas Tank?

 

[turbocruiser]

Bad gas Tank?

Not likely.

 

[MaddBaggins]

Giving more thought to what Tools posted

If the boiling point of gas is lowered and gets lower still at elevation then yes my system is slightly over pressurized, but not enough to blow hoses or seals anywhere. But when I open the gas cap to relieve that pressure I cause a major boil off and thats why it vents for 10 minutes. If I just left the cap on and let it all cool off, it would be fine in a little while.

Make sense to anyone else?

 

[a.mus.ed]

Make sense to anyone else?

Yes, but I think it would take a while for the temperature to go down enough to relieve any significant amount of pressure. Keep driving? Burn the gas? The way our trucks suck the gas, maybe that would be the best solution.

 

[turbocruiser]

Giving more thought to what Tools posted

If the boiling point of gas is lowered and gets lower still at elevation then yes my system is slightly over pressurized, but not enough to blow hoses or seals anywhere. But when I open the gas cap to relieve that pressure I cause a major boil off and thats why it vents for 10 minutes. If I just left the cap on and let it all cool off, it would be fine in a little while.

Make sense to anyone else?

Again, unless the cap isn't working well and is itself "clogged", it isn't going to hold anything above about 2 to 4 psi tops, you need to be testing section by section with those types of pressures. Secondly, I think you need to start with the end in mind and the end is actually how the rig was when it started its life; there was no worrisome pressure-up or boil-off then everything within the evap system was working and working well. That's how it was then , that's how it needs to be again. Again I know for a fact that rigs driven regularly at altitude on hot days don't demonstrate this problem. Please continue chasing this problem until you ultimately find a solution. For grins and giggles do you park with your USDS declined? Just to cross another easy thing of the list.

 

[a.mus.ed]

For grins and giggles do you park with your USDS declined? Just to cross another easy thing of the list.

None of us Tucson folks (MaddBaggins, GeoRoss, me) with this problem do.

 

[TrickyT]

Interesting thread.

I have a hard time believing that the temperature of an entire tank of fuel, 25 gals when full, could rise appreciably to the point where the fuel, regardless of atmospheric pressure, could boil. It would take a lot of kcals of energy to raise that much mass even a few degrees. Unless there's a direct heat source warming everything up I just don't see it. One way to tell, of course, is to measure the temp of the fuel in the tank or (easy with an IR) the tank itself just after filling and then again when the problem occurs. What temp difference is involved here?

If the fuel is "boiling", then I agree that opening that gas cap and thereby reducing the pressure, even by a couple psi, is going to worsen the problem. Same principle as why water boils at a lower temperature at higher altitudes. Better to just leave the gas cap on and the tank pressurized.

It may be buried in the location lines of all the posters in the thread, but I'm wondering if this problem is localized to a particular region of the USA, and therefore possibly fuel type/additive related, and/or climate/altitude related. There are several places in the US that are above 6000ft or have rapid changes in altitude over a relatively short distance and that are hot in the summer. Is anyone outside CO and AZ experiencing this problem? In particular, here in California I know that our gas is blended differently because of state air quality standards. But we have several 10,000ft mountain passes and hot weather in the summer, similar to other locales. Has anyone here experienced these symptoms?

 

[AdventureBuddies]

Is anyone outside CO and AZ experiencing this problem? In particular, here in California I know that our gas is blended differently because of state air quality standards. But we have several 10,000ft mountain passes and hot weather in the summer, similar to other locales. Has anyone here experienced these symptoms?

I'm in CA and it happened once to me. I filled tank and aux tank with 87 at Chevron in Olancha and I topped off more than usual to make sure I was totally full. Less than 20 miles later (5 on trail) i had gone from 3000 to 7000 ft and noticed gas blowing past my gas cap. I stopped the engine and it vented for 10 minutes or so.

Even though I don't understand exactly what the charcoal canister does, several people have mentioned over saturation as a problem so I'm assuming that I caused the problem by over filling the tanks.

 

[MaddBaggins]

None of us Tucson folks (MaddBaggins, GeoRoss, me) with this problem do.

Actually I do at home, a very slight decline, only about 5 degrees.

 

[turbocruiser]

Even though I don't understand exactly what the charcoal canister does, several people have mentioned over saturation as a problem so I'm assuming that I caused the problem by over filling the tanks.

In this case that was almost certainly the problem - presaturation of the the charcoal canister which effectively locks up the evap system's efficiency. When I fillup I make a point to put the hose in as far as possible and stop sometimes even before the auto shutoff activates - if you listen carefully you can hear the tank fillup first and then the fuel filling the neck of the tank - right then is when I try to stop the fillup from going further. HTH.

 

[GeoRoss]

He only two times I have experienced this is last summer. Filling up in Flagstaff after driving about 220 miles from Tucson. The overall gain in elevation was about 5000 ft with some steep grades and much of the ambient air temps well north of 100F. I gassed up in Kayenta AZ, about 150 miles. There was some up and down but no elevation changes and the temps were in the 80-90F range. No boiling at that gas up. We then continued on to Dolores CO a trip of 175 miles with warmer afternoon temps. The tank had to vent for five minutes or so during that gas up. It wasn't anything like what had happened a few days earlier in Flagstaff.

I know for a fact that my tank was over filled once when the auto shut off failed and gas was pouring out of the tank. My canister fails the FSM test. I have a 95-97 coming in from Dan. I am also going to replace as many of the vacuum lines as I can which I hope will be easier when I put a new valve cover gasket. I will also clean the throttle body and pay careful attention to the lines that feed the throttle body/air intake from the HVAC.

I have just assumed this past winter that I had a bad charcoal canister and needed to replace it. Alvin has a new canister and everthing else seems ok and that is what has me concerned. Is the charcoal canister to blame or is its premature failure a sign of other problems.

Maybe AZ has crappy gas and CO better stuff, I have no clue.

 

[Jeeper]

Thanks to MaddBaggins, GeoRoss, and others who working thru this for all of us. I have a higher than normal pressure build-up in my tank as it vents intensely for several seconds every time I take off the cap.

I've checked my canister multiple times as best as possible with shop air and also by blowing hard into the lines. Seems to be acceptable. I've verified that my tank line is clear and that the vent line from the CC is also clear.

What I have noticed:

• When I pressurize the tank slightly, I can hear the CC venting and can verify that air is coming out of the vent line at the bottom of the CC. However, this venting process is too slow to vent off the pressure in any reasonable amount of time. I let mine sit 10 minutes, and there was still considerable pressure in the tank. I also pulled the "tank" line from the CC and noticed that it vents rather slowing as well. Note that all of this was performed cold - engine not running.

I'm considering this possibility:

• Fuel cut off valve. This valve is on top of the fuel tank at the vent line connection, and I've never seen the one on an 80. Other vehicle's are basic plastic check valve type things.

Has anyone seen the "fuel cut-off valve" to confirm that it is, in fact, a simple check valve? If this is restricting the flow to the CC, then we simply can't vent fast enough.