MGB leaking petrol

[adam73bgt]

Hi, bit of an odd one here..

The other night i filled up my tank pretty full. Next day i get a text from my housemate informing me that there's petrol coming from the fuel cap

By the time i get home its night time and when i have a look inside the filler cap, the petrol level seems to be right down near the bottom of the neck piece which connects the filler cap to the tank.

So I assume the fuel just expanded in the heat..

Next day, the fuel is coming out again, i open the filler cap and a fair bit comes out as it looks as though the fuel level was about an inch up the filler cap itself, so i run the engine for a bit, the fuel level again drops down to about level with the top of the tank and i think problem solved.

Now the other day, i go out to the shops, come back home, no problems. after 20 mins, i go to go out in the car again and sure enough, fuel is coming from the filler cap as soon as i turn the engine on however, the fuel level drops back down and i have no issues with leaks over the rest of the day.

So I think Ive narrowed down my problem to either

• Me being a clot and overfilling my car (though i didnt think i particularly overdid it)
  
• Fuel expansion due to heat (is it really that much warmer?)
  
• My car is parked on a slight hill (its not a massive hill)
  

So I'm a bit confused, as i got through last summer (heat, hills, full tanks) without any issues

Is there anything else I should check?

Is there something that stops fuel from returning to the tank from the carbs once the car is turned off?

Ive now taken the precautions of parking nose downhill, in shade where possible. Also ive done 30 miles since i filled up so i must have used a gallon of fuel at least so id be surprised if any overfilling was still having an effect..

Sorry for the huge, rambling question any help is muchly appreciated

[cobblers]

Fuel doesn't expand much at all when warm (in the order of 1%) but the level won't rise noticeably
because the tank itself will also expand with heat.

It sounds as though you might have a blocked/airlocked tank breather and the base of the fuel filler neck is below the level of the fuel.
Fuel vapours expanding in the tank or fuel boiling in the fuel lines/or carb once the car is switched off is pressurising the tank and forcing fuel up the spout.

Once the level is reduced so that the vapours can work their way up the filler neck you'll have no issues, but it's worth looking about for any tank breathers that are blocked (or more likely bunged up with a bolt by a previous owner)

[adam73bgt]

ah thanks for that, i was always a bit suspicious of thinking it was just fuel expansion as it was going a long way! I guess ill go have a look for tank breathers then

Its odd though because the filler neck looks as though it connects to the top of the tank, so i would have thought any vapours would have worked their way out that way, the cap certainly isnt airtight as the escaping fuel has shown lol

There is a little 'T' hose junction that pops out into the spare wheel well which ive noticed before, perhaps this could be it?...

[adam73bgt]

After a small bit of googling it seems that the fuel cap is the only venting mechanism for the tank.. ill have a look to see if its blocked somehow, but surely id be having running problems with the engine if the tank was pressurised?

[killerbee]

when the car runs does it run fine? if there's no other vent besides the cap i can't see why fuel would be pulled up towards it? Because the minute you open the cap the fuel would go back down, also if a fuel tank does get a blocked breather it will shut the car off on something as old a your B. Had that with a friends spitfire and it took a while to work out why after 5 minutes his engine died! Check the fuel pump it working correctly and not leaking, also inspect the fuel lines just to be sure. Even on hot days when pushing the car my fuel tank has never felt hot so i doubt it's just heat causing the problem.

[adam73bgt]

Yeah the car runs fine. As soon as you start the engine, the fuel level in the filler neck drops right down (i assume due to the pump working) ive been parking with the nose pointing downhill for the last couple of days and it seems to be stopping leaking for the moment. I'm just curious as to what could be causing such a big fuel level rise

[killerbee]

mmm confusing. I'm no expert besides owning one so i can take a pic of my tank cap or inspect it to see how it vents if that helps you? You know the tanks are prone to rust on top where it meets the boot floor? It might be worth draining the tank and dropping it and inspect the entire unit, clear out the crud at the bottom, refresh and hoses and clips that are buggered, and clean the fuel filter. I have a fuel pump rebuild kit and i notice it has a few very small (2cm) fine mesh filters in it, so work even checking that out to see if something is blocked and causing pressure / vac.

[adam73bgt]

I'm gonna have a look at my cap soon and just give it a little clean, its a bit of a confusing one, but it seems to have stopped leaking now. I guess ill just have to not fill it too full and make sure to park nose downhill until i can afford to properly investigate

[ChasR]

A new cap for one of those is not much.

How many clicks/how do you fill your 'B? My white chrome bumper never got overfilled using a 1 to 2 click method, and the brown 'B was similar (although if the pump was not put in correctly (quite easily done) it would be a royal pain to fill up (read always clicking off despite me having an empty fuel tank!).

The brown 'B also used to suck its tank into itself it sealed that well. I believe it may have had the wrong filler cap mind you.

[adam73bgt]

Usually i only go for an extra click or two when I'm filling up, but i think i might have held the pump a bit higher in the filler neck last time, so it took a while before it clicked

This has also highlighted the inaccuracy in the fuel gauge, it was just below full even as there was petrol flowing out of the filler haha

It seems to be ok for the moment, I'm going to monitor it and if it starts doing it again i might get a new cap

[killerbee]

if i top mine up and a few clicks for long trips, the needle goes over full a little, but then is pretty accurate the whole way down, unlike my old mini which was pretty casual on how it reported fuel... "maybe i do... maybe not".
I tend to not fill the neck on mine because I know it will leak a little! But the design isn't great to start with. Keep us updated anyway so when we get the same problem we know the solution ;D

[ChasR]

Mine when brimmed used to sit just below the bottom of the full mark.

[adam73bgt]

Well i finally had a look at the fuel cap today and I don't think it looks as if its vented...







It looks as if the black rubber ring inside the cap is held tight against the filler neck by a spring in the cap. I can't immediately see any obvious venting, I don't know if anyone knows if this looks like a vented cap or not?

For what they cost I might just buy a new cap and see how it gets on.

oh and no leaking problems at the moment as ive been driving for the last 20miles or so with the fuel gauge hanging around Empty ;D trying to see how brave I can be/testing the gauges accuracy haha

[killerbee]

looks the same as mine. when you put your cap back in does the lock only turn a small amount, with them being so old and worn out anyway mine barely moves to lock.

[adam73bgt]

seems to lock pretty well, its a full 90 degree turn as the tabs you can see in the bottom pic all line up when in the 'unlocked' position

[penrhynrallycentre]

I look after 3 MGB rally cars and to be honest with you I am baffled with what is going on here as I have never come accross this problem before,As your fuel guage is not working properly you will definatly need to replace the sender as it is usually the culprit with low readings.
One thing I would DEFINATLY look into is the EXHAUST has it moved and it touching the tank which could very well be the route cause of the problem.
If it is to close to the tank it could very well be the problem.....I know you have said hte car is running well but do yourself a favour and check that your timing is spot on as if it is far to advanced then it would cause the exhaust to be very HOT even to the extend of hte EX manifold glowing Red.Check it and see


Dave

[adam73bgt]

Mar 13, 2013 21:34:00 GMT penrhynrallycentre said:

I look after 3 MGB rally cars and to be honest with you I am baffled with what is going on here as I have never come accross this problem before,As your fuel guage is not working properly you will definatly need to replace the sender as it is usually the culprit with low readings.
One thing I would DEFINATLY look into is the EXHAUST has it moved and it touching the tank which could very well be the route cause of the problem.
If it is to close to the tank it could very well be the problem.....I know you have said hte car is running well but do yourself a favour and check that your timing is spot on as if it is far to advanced then it would cause the exhaust to be very HOT even to the extend of hte EX manifold glowing Red.Check it and see


Dave

I'm 99% sure the fuel gauge is fine, just a little inaccurate. Ive never noticed the exhaust being too near the tank as its in the standard position but ill give it a check, the manifold has never been that hot and I'm fairly sure that kind of heat would show up on the water temp gauge surely? its been a little while since the timing was set up but it couldnt go that far advanced on its own surely? I'm gonna be filling up the tank at the end of this week so ill see if the problem re occurs but i have a feeling the problem was possibly a combo of an overfilled tank, parked nose up a hill coupled with the car having a bit of reverse rake, and possibly a too well sealed/non vented fuel cap..

But as i say ill fill the tank and keep a close eye on it

[ChasR]

It would not necessarily show up on the temp gauge since more of the exhaust gasses would be getting entered into the exhaust system later than necessary due to too advanced timing. Retarded timing OTOH can cause overheating since the exhaust gasses are present within the engine a little longer than they should be (in theory).

Unless you have set the timing up dynamically via a strobe light it is possible that your timing is too far advanced. I know that my MGB ran fine on way too advanced timing (it was stuck at 20 degrees from idle all of the way to the top). Considering that you have fitted an Accuspark (esque?) kit on in the past these do tend to thow the timing out by quite a margin.

I would say the cap is at fault though. Even on the Stag I will have to replace the fuel seal on the filler cap this year (after only changing it a couple of years ago : Don't you just love cheap rubber?).

[adam73bgt]

ah fair enough then, well the timing was set up by strobe at a garage after i fitted the accuspark last summer so unless something inside the dizzy is a bit broken id hope its not that..

It'll get a cap before i start dropping fuel tanks and things, (gives me an excuse to get a nice shiny new thing for the car )

[ChasR]

A knackered dizzy could cause too advance timing in the lower part of the rev range but this would have been obvious when the garage set your distributor up (it probably would have causing timing scatter).

I'd check the exhaust mounts anyway. MGB rear box mounts can vary in quality with people bodging all sorts of rubbish together since the final back box hanger can be awkward to assemble for a few (my white car had a bit of coat hanger in place along with the sidewall of a tyre :/).