Fuel tank pressure - alternate theory (not the CC) (1 Viewer)

[bloc]

I have an LS in my rig and I haven't worked out every little detail yet.. the fuel tank evap system being one. For the time being, the line from the tank normally to the charcoal canister is vented to atmosphere. I'm not proud of this, and it will be fixed when I install the GM tank pressure sensor.

I've put a lot of miles on like this, and usually the rig stinks of gasoline and there is no tank pressure. Clearly the line is clear.

Sometimes though, no smell, and a large whoosh when I open the gas cap.

When I did the body swap I noticed that the fuel tank vent isn't just a line off the top. There is a somewhat complicated valve that seems to be to stop fuel tank flow in the event of a rollover. It also seemed to have some low density plastic in it.. like a float.. to stop off the vent if fuel gets that high.

I wonder if this valve isn't working correctly all the time (closing too early, then tank pressure keeping it closed) and trapping evap pressure in the tank in rigs with otherwise healthy charcoal canisters.


Thoughts?

 

[Haspin FJ]

Interested as well. I just repacked my canister with charcoal and made sure the ports allowed airflow but have no change in the tank pressure.

 

[Cruzilla]

This valve has been a theory of mine for some time, I was thinking that the ethanol gas is making the float in the valve stick.
The valve is only 10 bux, but you have to drop the tank to swap it out.

 

[Cruzilla]

One guy in the charcoal canister thread mentioned this valve and another guy forced carb cleaner down the vent line to free up this valve. His fix worked for a while but it did reclog. At this time I have a small hole in my gas cap vent, I do not want my tank to crack so it bleeds off pressure when it gets to a few pounds.

 

[LandCruisinMy93]

I have assumed it was the vent not doing its job for quite a while, it's why I'm always fighting everyone who jumps to the charcoal canister conclusion for tank cracking.

 

[shocktower]

It's a chevy motor you alway smell unburnt gas

 

[rc51kid]

I also drilled all the valves in my CC. The other day still had a bunch of prrssure in the tank. I unplugged the cc to vent to air just for diagnostic and have not had pressure return. But i also have not gone on a long drive yet. I actually. Think the CC is good.

I was thinking of another thing. Doesnt the cc vent through a valve before it vents into the intake manifold? I think the valve opens only at certain timea or keeps vapors flowing in one direction. It is near the mess of vacume lines in the intake manifold. Coukd that valve clog or fail and prevent fumes from leaving tank and entering intake?

 

[rc51kid]

...

 

[bloc]

Well.. the valve is doing SOMETHING. Just went to fill up. Popped hood, lots of gas vapor flow out of the vent line.. AND a large whoosh from the tank fill cap. So maybe the line just isn't keeping up with the volume of vapor produced by the hot underbody and especially at altitude where the atmospheric pressure is lower.

 

[GeoRoss]

I have an LS in my rig and I haven't worked out every little detail yet.. the fuel tank evap system being one. For the time being, the line from the tank normally to the charcoal canister is vented to atmosphere. I'm not proud of this, and it will be fixed when I install the GM tank pressure sensor.

I've put a lot of miles on like this, and usually the rig stinks of gasoline and there is no tank pressure. Clearly the line is clear.

Sometimes though, no smell, and a large whoosh when I open the gas cap.

When I did the body swap I noticed that the fuel tank vent isn't just a line off the top. There is a somewhat complicated valve that seems to be to stop fuel tank flow in the event of a rollover. It also seemed to have some low density plastic in it.. like a float.. to stop off the vent if fuel gets that high.

I wonder if this valve isn't working correctly all the time (closing too early, then tank pressure keeping it closed) and trapping evap pressure in the tank in rigs with otherwise healthy charcoal canisters.


Thoughts?

I've suspected this check valve too

 

[rc51kid]

I've suspected this check valve too

I am too lazy to look in the fsm. Is there a way to tast the check valve that allows it to vent into intake system? If it is cheap, like $15 it might be easier to just drop a new one in. I hate testing the little electronic valves. The valves also sometimes fail kind of intermittently od sorts. I guess what i mean is electronic valves get half clogged and then the actuates get gummed up moving slowly and stuff lile that. So in my very limited experiance with them they can be tricky to test.

 

[BILT4ME]

I recently (in the last 6 months) replaced my charcoal canister (VC120) and my fuel tank with a salvage tank. Since having done these, I have not had the WHOOSH at regular fill-up using the whole tank with shut-offs in between or even long drive on one tank.

Last week, I went to a job site and it was VERY hot out (98'F) with high humidity (90%+), so I left the truck to idle for 6 hours, keeping the interior nice and cool. During that time, I burned about 9 gallons of gas. When I went to fill it up, I had the WHOOSH for a few seconds. This surprised me. I don't know if there is any info there that will help, but it is info.

The humidity was so high, that with the AC on the whole time, the EXTERIOR of the windows on my truck looked like it was raining because it was condensing on the outside of the glass.

 

[GeoRoss]

I am too lazy to look in the fsm. Is there a way to tast the check valve that allows it to vent into intake system? If it is cheap, like $15 it might be easier to just drop a new one in. I hate testing the little electronic valves. The valves also sometimes fail kind of intermittently od sorts. I guess what i mean is electronic valves get half clogged and then the actuates get gummed up moving slowly and stuff lile that. So in my very limited experiance with them they can be tricky to test.

I'm pretty sure it is a simple check valve. I can't recall with certainty, but I think there are two. One drone the charcoal canister and one from the fuel rail. I really can't remember. I've only experience boiling gas, 5 minutes of whoosh when doing extreme elevation changes in hot weather. A new gas cap has helped, I have an OEM VC to put in someday. If I ever have to drop the tank I'll check those valves.

 

[dallen341]

I recently (in the last 6 months) replaced my charcoal canister (VC120) and my fuel tank with a salvage tank. Since having done these, I have not had the WHOOSH at regular fill-up using the whole tank with shut-offs in between or even long drive on one tank.

Last week, I went to a job site and it was VERY hot out (98'F) with high humidity (90%+), so I left the truck to idle for 6 hours, keeping the interior nice and cool. During that time, I burned about 9 gallons of gas. When I went to fill it up, I had the WHOOSH for a few seconds. This surprised me. I don't know if there is any info there that will help, but it is info.

The humidity was so high, that with the AC on the whole time, the EXTERIOR of the windows on my truck looked like it was raining because it was condensing on the outside of the glass.

idling for 6 hours? I gotta ask...why?

 

[BILT4ME]

idling for 6 hours? I gotta ask...why?

It started out that I was going to let it run to keep it cool for 30 minutes or so while I was on the ground with a crew doing a structural steel roof installation. We were working on getting everything set up to fly (use a crane to install) some support structures to complete a temporary building. I ended up being 30 feet in the air on support steel with a harness on when I remembered my truck was still running (I could see the red charger light from my vantage point way up high). The only way up or down was a manlift that was currently set on the OTHER side of the building (I wasn't driving it) I didn't come down for about 5 hours while we flew everything up. Then, when I got down, I had other things I had to take care of to get the crew taken care of rather than walk 300 ft over and back to shut off my truck. It wasn't that important.

 

[bloc]

The valve I'm referring to is built into the fuel tank where the vent line comes off the top, eventually going to the charcoal canister. Whoever mentioned that you'd need to drop he tank to get to it was correct.. I only saw it when pulling the tank from my old smashed body and the complexity of the "tank vent" piqued my interest. It is just a check valve, only I suspect it is only supposed to actually close when the truck is upside down.

It may be that this whole system is designed as a system and needs the .5psi or whatever backpressure that the charcoal canister provides to help hold back fuel boiling as it heats up under the truck. I'd bet sheer heat is what happened for @BILT4ME that time. Lots of really warm fuel returning to the tank from the fuel rail. No airflow under the truck to cool it. And for those wondering, warm fuel returning is a big part of why GM and others have moved to return less fuel rails.

My story is going to diverge from you guys because I will be installing the fuel tank pressure sensor that GM needs to run he evap system. Still.. that valve has me curious, and I don't know when I'll get back to where my spare tank (200 miles from home) is to have another look.

 

[Haspin FJ]

Anyone have a part # for the valve on top of the tank? Can't find it on the online microfiche diagrams on a dealer website

 

[Cruzilla]

I believe the valve is designed to close when you over fill the tank, that way you dont saturate the charcoal canister with fuel when over filling the tank... ie filling after the pump handle clicks of...

 

[Nay]

Happy Happy Joy Joy. New Murphy Express near me, $.30/gal premium for mid-grade ethanol free.

My wife saw this a few weeks ago and told me about it. Been through about 4 tanks now. My 80 is running as well as it has in years, and although I've also done a bunch of PM, she sure seems very happy sans ethanol.

Not saying ethanol is the cause here, but to me it's hard to eliminate as a variable in a system not designed for it. I will be visiting that blue pump for every possible fill up - hope we see more retailers making ethanol free a standard option.

Please take this as a PSA if you've been looking for ethanol free. It's not my closest station and I doubt I would have stopped there and discovered this.

 

[ajax1]

I got rid of all the vacuum lines and the vsv switches under the intake. I left the charcoal canister and vent it to atmosphere I don't have any boiling or pressure this summer at high altitude