Gas overflow after filling up.

[eaglescout526]

On 3/9/2020 at 6:55 AM, Ωhm said:

At the charcoal canister remove the vent line from the fuel tank and let it vent to air. Try fuel fill that way.

Wow mustve been a day where I missed your comment, its an extra step in the process but its worth a shot.

[gogmorgo]

My long bed used to spew gas at me when it clicked off. It would take fuel as fast as you shoved it, and you could hear air rushing out that vent tube, the “secondary fill tube” if you will. Then once it got full it would kick the pump off, then a gush of fuel would come up out of the vent tube. Almost as if there was an air pocket that somehow compressed and then once the pump shut off it released, and there was less pressure head in the vent tube than the filler neck so that’s what blew out. 
For me filling it on a slower notch seemed to reduce the effect enough to stop the outward gush of fuel, but there was still some that burped up the vent tube after the pump shut off, just not enough it splashed back out.

I never really thought much about it until I saw this thread, which inspired me to check out a photo or two of an MJ fuel tank to try to see if there even was somewhere to trap air. And I think there is a reasonable explanation.

That’s my MJ tank next to... the wrong tank for an MJ. I thought it was an XJ tank at the time but now I’m thinking maybe it’s a ZJ tank. Either way it was a real long saga that’s not relevant. 
You can kinda tell in the photo that the tank vents are recessed. A new tank photo makes it obvious. 

So if that’s the case, when the tank gets full, the vents will have gas push up and trigger the rollover vents with air still trapped at the top of the tank. If the rush of fuel into the tank right next to the filler vent tube is enough to keep fuel from pushing back up the vent once the fuel level rises high enough to stop allowing air back out, then the air probably would pressurize, and act like a spring once the pressure was released.

That or the vent tube is a restriction for fuel pushing back out compared to it pumping in, so that’s what compresses the air pocket and then it keeps pushing out the vent anyhow? 
I’m not sure. 
I’m pretty sure the rollover vents are clogged because it’s a right pain in the cunning linguals to put gas into it with the stupid sealy-thing filler nozzles in California. That or they were only ever intended to account for fuel vapours, not fill venting. There’s also a pretty good chance I’m somehow overfilling the tank, because I know for a fact it’ll take 25 gallons from still not quite empty.

My shortbed still burps a bit, but as far as I’ve noticed doesn’t splash like the long bed did. Other than the smaller tank size, the one other difference I can think of is the long bed has the typical driver’s side sag to a much greater degree than the shortbed does. This would put the fill neck slightly lower, making for a bigger air pocket in the tank? Maybe?

 

I don’t really have a good answer unfortunately, just trying to make sense of something that doesn’t. I could be way off base. 

[eaglescout526]

4 minutes ago, gogmorgo said:

I can think of is the long bed has the typical driver’s side sag to a much greater degree than the shortbed does

This got me thinking. Prior to my ownership my grandfather asked me how I liked his "replacement J-hooks" for the gas tank straps. I will have to snap a pic of his genius work but All it is is a long threaded rod, threaded into where the hook would sit. Anyways this makes me think that I could have the nuts tightened just high enough that the tank could be slightly tilted enough and with that sag only exaggerates it further. This has me really thinking, good stuff gogmorgo!

[gogmorgo]

The quickest and easiest way to test it would probably be to park the truck on a couple blocks on one side or the other when filling. 

That reminds me of something else that probably has nothing to do with it. My parents had a late ‘90’s GM minivan in the early 2000’s, and the gas station in town was a bit run down and the concrete pad was cracked and tilted a bit on the one side. There was a thing that would happen if we filled up that particular van on the cracked side, but not on the other, more level side of the pump. But I’m not 100% on what the thing was. It didn’t spit gas, though... I think it might have been more that it wouldn’t fill past 3/4 tank. My memory’s real fuzzy there... we got rid of the van more than 15 years ago.

[BeatCJ]

10 hours ago, gogmorgo said:

The quickest and easiest way to test it would probably be to park the truck on a couple blocks on one side or the other when filling. 

This was a typical thing for Ford Diesel pickups for a long time, like '83 to '06. Since many diesel pumps pump faster than gas pumps, the same venting wasn't enough. The guys that cared carried a chunk of 4x6 to run the back tire on.

[gogmorgo]

Interesting. 

I worked at that gas station in the mid 2000’s after they fixed the slab, and pumping diesel was frequently an ordeal. We had a big pump mostly meant for filling heavy trucks, so the nozzle didn’t physically fit into older fillers like the gas nozzle would. It wasn’t an issue for the locals because they’d all punched out the fill necks on older trucks to allow for the big nozzle, but occasionally you’d get a truck in from the city with an undamaged filler and we’d really struggle. VW TDI’s were obnoxious because there’s a little valve you have to push with the nozzle to open an additional tank vent and they’d still only take fuel at about 1/3 squeeze. Then with a newer generation (2006ish?) VW did away with the valve and they just wouldn’t really take the diesel very quickly. There was one guy with a smart ForTwo as well that was a bugger, because it wouldn’t fit the nozzle into it more than about 1/8”. And trickling in the diesel was a PITA because if you went too slow the pump assumed it was leaking and shut down. 
Now I’m kinda curious about parking things on blocks. Trucks with service bodies on them are almost always a pain, because the fill necks were never routed very well, damn near horizontal in most cases, and the result was diesel constantly trying to flow back out instead of down. Turning the nozzle upside-down in the filler somehow helped with that, but you still couldn’t put it in too quickly without tripping the nozzle shutoff.  I should try it out with the service truck at work next time I’m filling it.