Problem with fuel overflow at my Mk1

[Tomcatciller]

Hello MG friends,

first I wish you all a successful and healthy year 2023. Greetings from Germany.

On of my goals of 2023 is a partially repainting of my Mk1 MGF because I want to fit a MGTF boot lid and a Frontbumper with fog lights and get all the scratches removed from arround the car. Before I can do that I have to solve a problem in my car which eats up my paint at the fuel filler cap:



I only drive my MGF in the summer quiet often with 30+°C outside temperature. After a tour with low tank I drive to the refuel station and refuel the car. The petrol comes from the cold refuel station earth bunker into the hot tank. I guess because of the temperature differences suddenly the fuel increases its volume and overflows after half a minute. The result is, as you can see, petrol overflow which destroys my paint over time. The picture was taken 1 min after full refuelling and the car spits out approximately 5 Litres of fuel through the closed filler cap. I don't have to mention this is dangerous too.

I discussed the topic in the German MGF Forum. The advice I got was to stop refuelling the car in the moment the automatic tank nozzle cut-off happens. In the past I refuelled the car manually after the auto-stop until I saw the fuel inside the fuel filler neck.

I adapted to the advice and it did n´t happen again. But of cause when I now will spend serval thousands of euros in repainting my car, I don't want it to happen anytime in the future. Is this problem known within the thousands of MGF/TFs owners in the UK? Is there a solution for the problem or is it not a problem but a user error? I am used to completely refuel my daily driver (Ford Focus Mk1) to the very top and never had a problem with this.

I also have petrol smell problems inside the car, but this is a well known MGF problem in Germany almost every fifth MGF has to deal by now.

Greetings

Hannes

[Deepfat]

Do you get air sucked in when you release the cap? you should do as the system is designed to run under a vacuum to meet emission rules re venting. I have had no need to work on this yet but as I understand it the vent pipe from the filler goes to a charcoal filter of some kind, then my knowledge runs out. It might be worth googling this and then checking your vent/filter system as this does not appear to be working correctly, hopefully someone with more knowledge than me (not difficult) will explain it's workings better.

[mgtfnut]

Do you get air sucked in when you release the cap? you should do as the system is designed to run under a vacuum to meet emission rules re venting. I have had no need to work on this yet but as I understand it the vent pipe from the filler goes to a charcoal filter of some kind

I don't think the system works under vacuum, inducing air into fuel vapours is not a good idea.
Petrol normally evaporates to vapour in the atmosphere when spilt, petrol will try and evaporate into vapour in a trapped system causing a slight pressure increase. The hiss you can hear is the pressurised vapour escaping from the fuel filler cap. The intent is that the vapour is fed into the activated charcoal container as you say, and released by the inlet manifold vacuum into the inlet manifold under certain conditions as the engine is started from cold and driven off - under the control of the ECU/ lambda sensors. Eventually the petrol vapour is burnt off in the engine and the charcoal container continues to be refreshed with air at regular intervals ready for the next engine start. This routine is part of the mandatory emission controls. Check that your charcoal container clicks at regular intervals.

The impression that there is a vacuum is similar to the effect your hand can feel when checking the engine compartment fan is working the right way round Best checked with a piece of paper

It is not best practice to fill to the brim - I know lots of people do it, but that is why the fuel nozzle cut off is there for. I've done it a few times when on a long journey as the excess fuel is burnt off relatively quickly and there is less chance of fuel leaks in the tank fill tubes and various clamps

Phew - Happy New Year.

[Tomcatciller]

Hello

Thank you for the replies. From the sound of my fuel system (opening the cap) it works. I dont know if it works all the time.

@ Phew: I changed my behavior regarding not to fill to the brim. The question is, will it securely prevent my fuel system from petrol overflow? Because on the event on the picture above, there were 5 litres fuel outside . This is more than the volume of the fuel induction piping.

Greetings from Germany

[Rob Bell]

I've never really had this as a problem on mine. It can get a bit pongy if really brimmed, but like Jerry, I only really do this at the start of a long drive where the fuel level will drop quickly (basically as Jerry has written verbatim!)

I never fill beyond the first click or two. I don't think I've ever filled to a level where the fuel was actually visible in the filler neck. That might be the issue?

Before writing this off, I'd check the condition of the breather pipe - which can get twisted and block off. But otherwise, it seems like the fuel cap is not sealing as it should? But if the tank sucks air in under vacuum when opened, then I think the filler cap is operating as it should, and likely venting excess fuel if the fuel expands in hot conditions...

[Reckless Rat]

There is a rubber sealing ring on the filler cap that should seal against the top of the tank outlet - see pic below. If it is damaged or absent that might be the source of your problem. I have put my thumb underneath it just to show it and how thick it is:

[mgtfnut]

Thought I'd try an advanced experiment today on the petrol cap pressure/vacuum theories.

With help from my neighbour, I covered the area of bodywork with clingfilm, kitchen sort, with enough free material to enable the key to be operated inside the cling film from outside. Held down with extra masking tape and a pair of extra hands, I turned the key and eased the cap loose. Boom, the cling film blew outwards and made us both jump No flames were involved

So, it seems there is pressure in the petrol tank.

If there were to be a vacuum, what causes the vacuum in the first place? I've read many times about older plastic tanks expanding, sometimes making it difficult to get out of the bodywork. Heat and internal pressure? Modern vehicle tanks have to be vented to the charcoal container for hydrocarbon capture. Having an air/ hydrocarbon gas mixture in the tanks is like a disaster waiting to happen.

ps @ Reckless Rat, I've got a green Z profiled seal, not a black rubber washer/seal. Might your original seal have been replaced, as I don't think the green seals are available new?

[Charless]

The fuel pump pressurises the rail which runs through the injectors. There is always surplus pressure after inital priming because the fuel pump is continuously energised whenever the engine is running to cope with acceleration demand. Surplus rail pressure is bled back into the tank by a simple valve which thus controls the rail pressure. Injectors then only need a brief 0v sent to them (of varying duration decided by the ECU) to inject the optimum amount of fuel for current engine conditions. Overpressure in the tank is vented via the charcoal cylinder and delivered back into the engine for emissions control.
So the tank should constantly hold positive pressure once the filler cap has been replaced and the engine run if everything is working normally.

[Reckless Rat]

As far as I can tell that's the original cap seal, but I've only had the car since 2008. Whatever. It works.

[Tomcatciller]

I took a photo of the situation of my car:



Don´t see anything wrong.

Greetings from Germany

[Steve White]

The difference in the colour of the seal rings is because the filler cap on pre 2000 cars is different to post 2000 cars.
The later caps are deeper and have a different seal.

[Geoff.F]

I believe the biggest problem is that the paint should not be effected by the fuel.
Geoff.F

[Peter_B]

I believe the biggest problem is that the paint should not be effected by the fuel.
Geoff.F

That is what I wanted to say too! I never recognized a problem for the painting when fuel was spilled during refilling.
There are some organic solvents or brake fluid for example, which are causing damage for the paint, but not gasoline in normal cases, except something is not to the standard with the paint itself.