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Cannot fill fuel tank

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dipstick Avatar
dipstick Kenny Snyder
La Center, WA, USA   USA
1941 Ford N-Series
1958 MG MGA 1500 Coupe "Rosie"
1970 MG MGB GT "Pat's GT"
1971 MG MGB "Gifted To Me"    & more
Back in the day yellow goop was sold to coat the inside of fuel tanks to seal small leaks. After some time the yellow goop coating separated from the metal and plugged things up?confused smiley

Be safe out there.

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ddubois Avatar
ddubois Gold Member Dave DuBois
Bremerton, WA, USA   USA
You might check that the vapor reduction system is not clogged. There should be a round canister (vapor separator) in the trunk, attached to the left rear fender with a hose going into the fuel tank and a second hose going to the engine compartment and into the charcoal canister. If that line from the tank through the vapor separator on to the charcoal canister is clogged, it could well be the cause of the problem you are experiencing as iti is the vent for the fuel tank.

Dave DuBois
1953 MGTD
1966 MGB

Edited 1 time(s). Last edit at 2017-05-16 10:25 PM by ddubois.

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crgrbrts Avatar
crgrbrts Gold Member Craig Roberts
Wheaton, MD, USA   USA
1970 MG MGB MkII "Bumblebee"
This evening I illuminated and inspected my tank's filler neck and inlet tube. There were no apparent obstructions and my bent coat hanger probe -- as suggested by Mr. Johnson -- hit the bottom of the tank with a reassuring clunk. When I first removed the gas cap a bit of air escaped and I could see the fuel level near the top of the filler neck. I then bounced the car's rear end to be met by a sizeable escape of air and a fuel level drop to the bottom of the filler neck just about even with the top of the tank itself. Obviously a lot of air had been trapped in the tank.

My theory: until this past week and three months before, the weather has been cold, sometimes very cold. The air warmed considerably a few days ago, thus raising the interior temperature of the fuel tank with a coincident increase in vapor pressure inside. My fuel cap is apparently non-vented and seals very efficiently, thus trapping vapors. Perhaps these vapors have created sufficient air bubbles inside the tank to decrease the available space for liquid fuel and push fuel out of the tank through the filler neck when the pressurized cap has been removed.

Perhaps drilling a vent hole in my fuel cap will cure the problem. Whaddya think?

late 64 Terry O
Milton, Ont, Canada   CAN
are you not suppose to have a vented cap in the first place........? if not then you do not need to "drill a hole" but instead find out why your system is not "breathing"
but regard less of this once you remove the cap to put gas in , its hard to believe that trapped air would be present with such a large opening for the gas nozzle.....confused smiley

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crgrbrts Avatar
crgrbrts Gold Member Craig Roberts
Wheaton, MD, USA   USA
1970 MG MGB MkII "Bumblebee"
My Brit/West Indies friend and adviser - a former Jaguar troubleshooter for BL -- says the problem is the fuel cap. It needs to be vented.

Although my Roadster was built in 1970 (February) I keep finding 1969 parts on it. They're probably previous model year leftovers, I'm thinking. At any rate, one of the 1969 parts on my 1970 MGB is, apparently, the fuel tank. It's not vented -- nor are they any U.S. mandated "save us from emissions" bits on the car.

So, says my friend, the proper gas cap for this filler neck/tank setup is a vented one with the internal "ears". Y'see, I recently replaced the beat-up original cap with a correctly fitting cap that was NOT vented. (Silly me -- I didn't even look).

While I look for the correct cap, I'll be driving around with a borrowed cap from an E-Type Jag. Pretty snazzy, eh?

jmart1419 Avatar
jmart1419 Silver Member Jim Martin
Vancouver, WA, USA   USA
I will be interested in hearing if this works. I don't understand how this would interfere with filling the tank.

PlasticPete Peter L
Bradenton, FL, USA   USA
I've met the same problem, but only once, at a pump I had never used before (non-ethanol.) I'm starting to blame the pump. (Oh, and, by the way, my gauge is hopelessly wrong, as warned to me by the guy who sold me the car.) It sounds like a common problem.

Snohomish, WA, USA   USA
I have also had intermittent difficulties with my 1970 Roadster:
Gas filling having experienced backsplashes out the filler neck onto my rechromed bumper, only to have 4 or 5 more gallons available to fill, if I slow the delivery down to a trickle.
I use the same ethanol free vendor in town, varied pumps, though.

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crgrbrts Avatar
crgrbrts Gold Member Craig Roberts
Wheaton, MD, USA   USA
1970 MG MGB MkII "Bumblebee"
Here's what really happened: a couple of months ago I replaced my Roadster's dented, scratched and otherwise unsightly gas cap with -- as it happens -- a Taiwanese copy. All was well for a time BUT after some use the new cap's inner component collapsed into the interior of the outer shell thus sealing off the vapor vent. This, of course, trapped vapors in the non-vented fuel tank with previously reported effects. (Not long ago I surmised that I had erroneously purchased a non-vented cap).

The inability to completely fill the tank appears to have been a separate, local phenomenon having to do with a particular and heretofore unvisited gas station's pump pressure and my own inept attempt to compensate for it.

My previously mentioned borrowed Jaguar gas cap (which is beautifully crafted and apparently crafted of unobtanium) has solved the problem for the moment. At least I have caught the culprits (one of whom is me). I noted very recently that this inferior Taiwanese gas cap phenomenon has been reported before. Too bad I didn't see that posting earlier. Well, as they say (whoever "they" are), poop happens.

Thanks for all the advice and help, fellas. It's much appreciated.

late 64 Terry O
Milton, Ont, Canada   CAN
your welcome and we hope you can breathe a little easier now.......smileys with beer

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crgrbrts Avatar
crgrbrts Gold Member Craig Roberts
Wheaton, MD, USA   USA
1970 MG MGB MkII "Bumblebee"
Yuk, yuk. Yup, I -- not my gas tank - am now sighing with relief.

riley1489 Avatar
riley1489 Gold Member Bruce H
Great White North, QC, Canada   CAN
1953 Jaguar XK120
1959 Riley 1.5 "King George"
1973 MG MGB
Non of this makes sense.

A vented cap fits a particular matching filler tube.
A non vented cap fits a particular matching filler tube.

Not possible to fit a non vented cap to a vented filler tube and vis versa.

The caps & filler tubes have different position locking tabs.


Check your ego Amigo!

barry s Avatar
barry s Silver Member Barry Stoll
Alexandria, VA, USA   USA
1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
Adding to Bruce's problem understanding the offerred explanation, When filling the tank, the gas cap is 'off', regardless of vented or sealed. How adding fuel can be influenced by the tanks condition when the cap is installed is not only not intuitive, it is incomprehensible. As best I can tell, the simple task of removing the filler neck/hose from the tank was deemed unnecessary. Unless you get back to basics all conclusions are conjecture. As to you Jag mechanic, don't ask him to work on your B.

crgrbrts Avatar
crgrbrts Gold Member Craig Roberts
Wheaton, MD, USA   USA
1970 MG MGB MkII "Bumblebee"
Here's what I know: when I employed the defective vented cap that had been compromised so that the vent was blocked, I got a "blow back" of fuel and vapors when I removed the cap. When I switched to the vented cap (from my Jag mechanic friend) no such escape of air and gas occurred. My fuel tank system is sealed, i.e., not vented, so this make perfect sense.

The inability to completely fill the tank was a separate issue unrelated to the gas cap. The linking of the two was my erroneous conjecture in a desperate attempt to make sense of it all. It's not my Jaguar/British Leyland factory veteran troubleshooter who should not be working on my "B" -- it's ME :-) .

Fairfield, CA, USA   USA
I agree, probably the only thing you can do.

In reply to # 3512658 by late 64 might be easy enough to remove the filler neck from the hose inside the trunk and have a look see whats up...?? as you say does not make sense since this a big opening.........confused smiley

1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed

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