Fuel venting woes

[wpicotte]

Troubleshooting a fuel tank vacuum issue; whenever I open the filler tube after driving, a vacuum is equalized, the tank thunks back into shape, and this issue has been exacerbated after resealing the fuel tank.

After the reseal, I drove about 3 miles to add fuel and both at the station and when I got back home, removing the filler cap produced a really audible woosh and of course the tank thunked back into shape.

Meanwhile, I'm scheduled to leave for a decently long trip this Sunday 7/13...

I've removed the emissions canister and can blow through the port that receives the vent line and out the bottom port. Not sure this verifies the canister's integrity? Is this airflow enough to rule it out?

I confirmed that the hose running from the canister's bottom port into the frame is clear.

I've added pressure to the system from the same hard line that plugs into the top of the canister. Pressure is not held, but I'm not sure that I should be able to pressurize?

I've had both rollover tanks off and, with the tank inverted and while plugging one of the two vent ports, I can pressurize the tanks. With the tank held in the mount position (valve side up) the valves release pressure. The grommets look ok on visual inspection, but I've had the van ('87 ~215k) for at least 10 years and haven't replaced them or the valves.

At this point, I'm stumped as to what to do next. I'm not sure the above tests have actually ruled out any issues. Any insight / thoughts / advice is appreciated.

For context, I've read this post and several others, but this one I've referenced seems to provide the most thorough explanation going for the vent system.

[jlrftype7]

With the gas cap off, and low compressed air pressure, you should be able to introduce hissing out of the open filler neck via the hose that runs from the charcoal canister to up front at the evap tanks and fuel tank.
Actual sound of hissing would mean this circuit is clear, not blocked.
Another thing would be trying another Gas Cap, sometimes they go bad.

Last thought, the Tee / breather hose that connects one side of the gas tank to the other, must loop or arc OVER the shift rod on a Manual or the shifter cable on an Automatic, not under it. If routed under, then air doesn’t move or vent correctly-
I’ve done this once to our Vanagon, so I’ve seen the effects first hand.
_________________
'68 Westy- my first VW and vehicle/Bus- long gone.- sold it to a traveling Swiss couple....
'67 Type 3 Fastback, my 2nd car- gone
'69 Semi-Auto Stick Shift Beetle-gone
2017 MINI Coopers, our current DDs
‘84 Tin Top - Hilga....Auto

[wpicotte]

[wpicotte]

[latelogan]

I couldn't get mine apart and I never got around to cutting it open before I moved and tossed it.

mine was completely blocked; no air moving through it at all.
I know you've checked the input and output tubes, but could they be folded somewhere?

FWIW: I had originally thought to replace the charcoal in it my self.. but a very inexpensive used one appeared
_________________
1991 Westy stock 2.1 auto transmission
1978 Westy (Sold)
1971 Westy (Sold)

[?Waldo?]

https://www.thesamba.com/vw/forum/viewtopic.php?t=...mp;start=0

https://www.thesamba.com/vw/forum/viewtopic.php?p=4291885#4291885

[dabaron]

there is a vac line from the throttle body to a valve for the charcoal canister. if the vac line or valve are not functioning properly or the canister is malfunctioning, a vac will occur in the tank. i had an issue with a vac imploding the tank and "solved" it by disconnecting the vent line to the charcoal canister.

make sure the roll-over valves on the expansion tanks in the wheel wells work and the lines to and from are in working order as well.

one day i'll replace the canister, one day.
_________________
1991 Vanagon GL Camper
i had no idea i wanted to be a mechanic

[MarkWard]

To save your trip, leave the gas cap slightly loose. Maybe some foil tape if it’s raining to act like a rain hood. Figure it out when you get back.

[latelogan]

I agree with Mark. the PO had cut off the gasket on the gas-cap to vent it.
In theory you could just cut a channel in the gasket.


I never would have known except that i started parking in the garage and tracking the gas smell. then I replaced the gas cap and then got the tank sucking symptoms and tracked to a plugged carbon filter....
_________________
1991 Westy stock 2.1 auto transmission
1978 Westy (Sold)
1971 Westy (Sold)

[MarkWard]

It’s a stretch, but charcoal may have found its way into the forward line and it’s plugged internally. It’s not rocket science to resolve, but it sounded like there is a time crunch. You’d have to disassemble much of the system to find the “plug” working under the van.