Gas cap trouble (?)

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SteveEdmonton
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Gas cap trouble (?)

Postby SteveEdmonton » 15 Sep 2016 20:07

I've never thought much about a gas cap. If you've got one, good; if not, bad. That's it, right?

Or maybe not. I've had a problem with fuel odour for the last few months, especially in the trunk but also of course wafting into the cabin from there. Last weekend I tackled the issue by pulling all of the lines in the back half of the fuel system.

Found one pinhole leak in the hard-plastic fitting at the top of the filler neck, and plugged it with JB Weld. Otherwise, I just put a turn or two of yellow teflon tape (the gas-rated stuff) around each of the joints and re-assembled. Naturally the residual fuel odours lingered for a while after opening up the system like that, but naturally I hoped I had it licked.

And naturally I was wrong. The smell is worse that ever. Here's the scoop.

1) First day after "fixing" all the lines, the smell was pretty bad, especially after cornering, and after getting home I found a bit of gas inside the filler-door. So I wiped it up, dried off the gas cap, and decided to check it again the next day.
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2) After the next drive there was more gas outside the filler-cap. I also noticed some staining on the fender below and behind the gas-cap, suggesting quite a lot of spillage. Maybe gas was somehow sloshing past the cap because I had taken a few tight, fast corners? Again, wiped it up and dried the cap.
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3) On yesterday's drive home (an 11-mile commute, in rush hour) I took it really easy, no hard cornering, just gentle and low-speed turns where necessary. Again, lots of odour-- and now there's a lot of fresh gas visible on the fender itself. Wiped everything dry.

4) This morning, the smell was bad after just 3 blocks , so I stopped to have a look. The fender was just covered with fresh gas. I hadn't gotten past 3rd gear yet-- maybe, just maybe touched 30 mph. I'd turned 5 corners but none of them were faster than an idle in 2nd gear.

So: My first conclusion is that the seal on my gas cap must be toast. The rubber looks perfectly fine, but obviously it's letting fuel right on by. Guess I'll try to to track down another cap to see what difference it makes.
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However: I'm also wondering why so much fuel is coming up the filler neck in the first place-- especially this morning, on a short and low-speed drive with ZERO fast cornering. There's not a lot of elevation difference between the gas-cap and the top of the tank, but they're certainly not at the same height. So I've always assumed (!!) that fuel would only rise up through the neck to reach the level of the gas-cap very rarely, like maybe on hard left-hand turns. Under normal circumstances I would have assumed that the fuel would stay in the tank!
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(By the way, I've been keeping the level of fuel in the tank quite low for the last few months because of this very issue. Rarely more than 1/4 tank. Yesterday morning, though, I filled it just above 1/2. And it's since doing this, that I've had the visibly wet fender.) (##3 and 4 above).

Does this sound like a normal state of affairs, meaning that if I merely replace the gas cap I should be good? Or does it strike you too as strange that fuel is finding its way back up the filler tube under "tame" conditions, with half a tank of fuel?
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'71 4-door
'74 MGB-GT
'04 Miata

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Byron510
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Re: Gas cap trouble (?)

Postby Byron510 » 15 Sep 2016 20:23

The problem is often the metal lip on the inside of the photo above. You'll notice that it is (or is supposed to be) a screw by nature. as you turn the cap, it pulls the cap down tight against the rubber seal. This "ramp" wears out over time, then not providing enough crush.
I have also seen the top surface of the metal face deform, which really doesn't help the situation.

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Three B's Racing
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Re: Gas cap trouble (?)

Postby Three B's Racing » 16 Sep 2016 04:42

The top surface deforms like Byron says and will cause fuel to leak past the rubber fill seal and not the cap or both. Also, does your fuel system have a good vent? if not a vacuum can build up and when a weak seal (top surface deformed) opens it may cause fuel to shoot up the fuel tube,,maybe? I'd pull the fill cap and mounting ring and check that the sealing surfaces on the body and ring are flat and not deformed which is a typical issue when those mounting screw that pull on the sheet metal which is what deforms it in the first place. I once made a ring with through holes and nuts spot welded in place in behind the top fuel hose up against the sheet metal. It keeps the thin metal from deforming and actually flattens it some or make a gasket from fuel proof rubber to go behind the sealing ring filling any little inperfections letting fuel out or both.
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defdes
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Re: Gas cap trouble (?)

Postby defdes » 16 Sep 2016 04:58

My solution was extreme, but I got so tired of this problem and the lack of availability of new parts that I modified the whole thing. I made a new stainless receiver on the inside of the filler door, and moved the filler location to the top of the tank and used marine grade hose as the filler.
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okayfine
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Re: Gas cap trouble (?)

Postby okayfine » 16 Sep 2016 05:57

Gasoline is very mobile and will move all over the place just in normal driving. Even a low-speed left-hand turn will have fuel move all the way up your filler tube if you gas tank is even half full.

Steve, when you inspected the hoses and such, did you remove and examine the upper rubber flange/tube piece? Typically you'll have some deteriorated rubber causing the leak. Byron's suggestion of examining the metal flange is also very good.

When I built the aluminum gas tank I made a spring gate covering the filler hole into the tank so that it would close off the filler to cornering gas forces.
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SteveEdmonton
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Re: Gas cap trouble (?)

Postby SteveEdmonton » 16 Sep 2016 15:12

Thanks everyone for weighing in on this. I'll see what I find out as I investigate the present situation a little more carefully. Sounds like either a thorough overhaul of the filler neck assembly, or a complete re-design and re-build, is in the future.... :(
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Re: Gas cap trouble (?)

Postby Byron510 » 16 Sep 2016 15:30

SteveEdmonton wrote:Thanks everyone for weighing in on this. I'll see what I find out as I investigate the present situation a little more carefully. Sounds like either a thorough overhaul of the filler neck assembly, or a complete re-design and re-build, is in the future.... :(


I had a slight leakage problem in my Bronze car a couple years ago. Tried a new cap that I had laying around in a box and it didn't solve the issue. Then I tried a couple old caps with "modified" engagement tang and one did the trick - for now. So I watch this thread with great interest as I feel a modification to what I have is also soon in the works.

I have seen a few people lately modify the whole filler cap area, adapting later model parts. I remember seeing a post recently where an unleaded flapper/restriction valve was used. Certainly this is a good idea, as long as you never need to fill at the track. What I do like above is John's use of flexible rubber hose between the tank and the 1/4 panel mounted fitting/cap. As I track my car, having this flexibility between the two I feel is a great safety advantage. If I ever tag a wall or get tagged by another car at speed one day, I'd hate to rip the fuel supply open coupled with a spin. It could make for a rather dangerous situation.

I'll watch with great interest.

Byron
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SteveEdmonton
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Re: Gas cap trouble (?)

Postby SteveEdmonton » 16 Sep 2016 19:34

Byron, I was thinking exactly the same thing about the value of a significant amount of rubber hose in the line.

Dadzsun in Ottawa did a nice job adapting a Subie filler neck: clean work, carefully thought out, and well documented. It's here: viewtopic.php?f=3&t=23691&p=254070&hilit=subaru+neck#p254070

I'm toying with doing something similar with my setup, but not "all metal" like his. As you say, too much rigidity can definitely be a bad thing.
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Re: Gas cap trouble (?)

Postby SteveEdmonton » 21 Sep 2016 19:45

Ahh, the sweet smell of..... nothing!

Project is done and yes the fuel smell-- and splash-- is gone. Couldn't be happier. And it wasn't that tough either.

Started with a "new" filler neck from a '90 Subaru Legacy courtesy of Pick-N-Pull, which cost about $14 and took about 10 minutes to remove. Dadszun in Ottawa used pretty much the same one in his conversion, and I can see why. I looked at dozens of different styles of fillers on Japanese and European cars and didn't see any other ones-- besides Subies-- that use a bolt-on cover plate on the outside to secure the neck. Most others are attached somewhere inside the trunk or fender, which I assume would be a harder mod on our 510s.
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So if you're thinking about doing this too, I suggest looking for a Subie unit. The cover plates on the older ones (early to late 1990s) use 3 Philips screws; somewhere in the early 2000s they switched to Torx fasteners instead. Should work the same, but just make sure you take the right screwdrivers to deal with them (I didn't).
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As you can see, I cut the neck a couple of times before I got it short enough to be usable, and also at a wide enough point to maintain maximum flow. (The Subie filler pipe narrows down to about 1.75 inches, within about 4 inches of the filler cap.) An off-the-shelf exhaust adapter from Canadian Tire worked perfectly-- 2.25" OD to connect to the 510 gas tank X 2" ID to attach to the upper part of the Subie filler neck.

I also added a small piece of an old fuel line (including its barb) to the Subie breather tube, which lands in a very good place on our cars. This additional small line connects to the vapour / expansion tank in the trunk, letting me maintain the stock EEC system.
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I chose to solder both of these joints for simplicity's sake. I'm not that great of a welder and didn't want to deform the filler tube with too much heat. Not quite as strong as the original brazing on the rest of the Subaru tube, but I figure it should be good enough. At least I've never had any issues with soldered joints elsewhere in my fuel systems (Datsun and MG).

Painted her up and installed it using the Subie rubber gaskets, inner and outer, that sandwich around the quarter panel. A decent length of 2.25" ID marine fuel-rated hose connects the modified Subie filler neck to the stock Datsun tank inlet.
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One more detail: There wasn't enough room for the fancy Subie gas cap that flips down on a hinged mechanism, but I foresaw that and snagged a cap with a cord (so it doesn't get lost) from a 90's Jeep Grand Cherokee. I didn't know that modern-style gas caps were as universal as they appear to be-- I tried 3 or 4 makes and they all fit, but then spotted this Jeep one with a good attachment system that was easy to adapt for my purpose.
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I haven't driven very far since hooking this all up, but a few rather sporting left-hand turns resulted in ZERO odours or spillage. I'm calling this project DONE!
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'74 MGB-GT
'04 Miata

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Re: Gas cap trouble (?)

Postby Byron510 » 21 Sep 2016 22:05

Awesome!

Thanks.

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