2,000 Miles from home and dead (solved, quiz time)

[drober23]

So... It's been a glorious vacation! We had hit Mt. Rushmore, Jewel Cave, Rocky Mountain National Park, and were visiting Arches National Park (in Moab, Utah). The bus has been running well, if fighting through the hills and altitude a bit.

I left my gas cap in Wyoming, then got pulled over by one of Utah's finest on my way to Arches National Park. No license plate light. This is when I discover that my driver's license did not make it to Utah either. So, after a polite discussion and presentation of my passport (which I had for some reason), he decides we are productive members of society and sends us on our way without a citation or even a written warning. I got a new gas cap, and fixed the license plate light the next day.

Now the fun starts. We get to Arches National Park, and we decided to go look at one more set of arches before calling it a night. It is nearly dark, and we start going down a big hill toward the Window Arches. Cough, cough. Uh oh! As I was thinking of what to do, cough, cough again, then stall.

Anyone want to guess what caused me to stall? It was an easy fix, and I was on my way again quickly. But... for a couple minutes I was absolutely sick to my stomach!

[glasseye]

new gas cap vent not working.

[drober23]

You guys are too smart!

I went through my roadside troubleshooting procedure.

1) Did anything fall off

2) What is the last thing I messed with

In this case, that is as far as I had to go

[Amskeptic]

You guys are too smart!

I went through my roadside troubleshooting procedure.

1) Did anything fall off

2) What is the last thing I messed with

In this case, that is as far as I had to go

I was under the impression that the late model VWs did not use vented gas caps. They are supposed to be fully sealed save through the evaporative charcoal system.

Perhaps your gas tank vent line is plugged up and therefore needs to have an incorrect vented cap?
ColinTheMysteryRemains

(p.s. I am still disappointed that I missed you out there in South Dakota)

[SlowLane]

I was under the impression that the late model VWs did not use vented gas caps. They are supposed to be fully sealed save through the evaporative charcoal system.

Hmm, I was thinking the same thing, but I had the feeling I was missing a piece.

I think I may have it now: the charcoal canister EVAP system is designed to let fumes OUT of the fuel storage system in a controlled manner to be absorbed and eventually burned in the engine. It isn't designed to let air INTO the system. On the Vanagon, at any rate, there are one-way check valves to ensure the fumes only move in one direction.

Which leads to the question of how the system keeps from drawing down a vacuum as the fuel level decreases. One logical way is to have a one-way check valve in the gas cap itself. Keeps fumes in, prevents vacuum from forming in tank.

Just speculation, of course. I'll take another look at my gas cap when I get home.

[Amskeptic]

I was under the impression that the late model VWs did not use vented gas caps. They are supposed to be fully sealed save through the evaporative charcoal system.

Hmm, I was thinking the same thing, but I had the feeling I was missing a piece.

I think I may have it now: the charcoal canister EVAP system is designed to let fumes OUT of the fuel storage system in a controlled manner to be absorbed and eventually burned in the engine. It isn't designed to let air INTO the system. On the Vanagon, at any rate, there are one-way check valves to ensure the fumes only move in one direction.

Which leads to the question of how the system keeps from drawing down a vacuum as the fuel level decreases. One logical way is to have a one-way check valve in the gas cap itself. Keeps fumes in, prevents vacuum from forming in tank.

Just speculation, of course. I'll take another look at my gas cap when I get home.

All of the VWs I have seen blow fresh air into the canister from the fan housing. I have not run into any check valves as I have blown through lines checking for blockages. I will experiment at the next lay bay with intact evaporative system. The BObD gas cap is tight, no one way valve in that sucker. My Mercedes has a vacuum when you pull off the gas cap. It is described in the owner's manual.

Air to replenish a dropping fuel level or rapid cooling comes right through the vent lines from their source, the evaporative tank which has an inexhaustible supply of fresh air from the fan housing, running or not. Blow into the charcoal tank with that fresh air hose, I don't think it will give you resistance. I am very curious about the drober symptom resolution.
ColinInBellinghamWA

[SlowLane]

Just speculation, of course. I'll take another look at my gas cap when I get home.

All of the VWs I have seen blow fresh air into the canister from the fan housing. I have not run into any check valves as I have blown through lines checking for blockages. I will experiment at the next lay bay with intact evaporative system. The BObD gas cap is tight, no one way valve in that sucker. My Mercedes has a vacuum when you pull off the gas cap. It is described in the owner's manual.

Air to replenish a dropping fuel level or rapid cooling comes right through the vent lines from their source, the evaporative tank which has an inexhaustible supply of fresh air from the fan housing, running or not. Blow into the charcoal tank with that fresh air hose, I don't think it will give you resistance. I am very curious about the drober symptom resolution.

My understanding is that aircooled VWs blow air through the charcoal canister to purge the stored gasoline fumes out of the canister into the air cleaner box. I would hope that there wasn't a path from the fan housing to the fuel tank, because 1) it would pressurize the tank, possibly upsetting the fuel pressure seen by the fuel pump inlet on carbureted models, 2) it would introduce outside air into the fuel tank, which really needs to be an oxygen-poor environment to remain safe, and 3) because I don't think we want dirty carbon dust being blown into our gasoline.

The water-cooled Vanagons, which of course don't have a fan housing to provide a supply of blown air, take a different approach by drawing air in from the air filter box, through the charcoal canister and out into the intake manifold just downstream of the AAR. They also incorporate a "charcoal filter valve" (Bentley's name), which appears to provide the same function as the EEC valve on the late bus air boxes. It regulates flow from the charcoal canister to the intake manifold.

Having said all that, the cursory inspection of my gas cap this morning didn't turn up anything that looked like a one-way valve. I do have an old gas cap here. Maybe I'll dig it up and perform an investigative dissection.

[drober23]

Hmm... I don't have my evap canister connected. The PO had it disconnected, and I have not taken the time to sort it back out. I will have to investigate what is going on down there.

[SlowLane]

So what was the ultimate solution for you? Are you still using the new gas cap? Do you just loosen it off every couple of hours to relieve the vacuum?

[drober23]

The new gas cap had a rubber seal. I poked a couple holes in it to allow it to vent. Haven't had trouble since.

[SlowLane]

Okay, upon further investigation it turns out that Colin is (once again) correct.

By the simple method of blowing and drawing through the gas cap key-hole, I found that my old cap allowed air to pass in both directions, but that my new cap prevented any such passage, at least at the modest pressures I was able to apply.

Dissassembly of the old cap revealed a dried out rubbr o-ring rattling around in a dirty groove, so it;s no wonder that it leaks.

As it happens I still had the packaging from my new cap, so I googled "Blau T85", the make and model of the cap, and found this link, which clearly states "without ventilation". Further searching on the Blau site turned up a link for the model T16, also a non-ventilated cap, usable for the 1967 - 1979 Transporter.