Cannot refuel!!

[Albourneboy]

Thought I’d top up with diesel having driven 150+ miles with caravan in tow but found I couldn’t. Inserted fuel nozzle as normal and pulled the trigger but the auto cut off stopped the flow of fuel just as it does when the tank is full. But it isn’t full!!  Tried another pump but with same result. Took the car to a local garage and persuaded a guy to have a quick look. He used a funnel to open the flap and found that the pipe down to the tank was full of fuel so that is what is triggering the nozzle cut out. What on Earth could be trapping the fuel in the pipe. Is there another flap at the bottom where it enters the tank?  Has anybody else experienced this?  Is there anything I can do?

[ApertureS]

How sure are you the car isn’t full of fuel and the gauge reading incorrect?

 

If there is an airlock in the fuel filler neck then this should be cured by either a blocked relief that goes to the top of the filler neck (if that’s blocked with debris your tank will be full of rubbish)

 

My second suggestion is void as you have a diesel so it won’t have a vapour return.

 

Id be confirming you definitely have an empty tank first before thinking there is an air lock.

[Albourneboy]

Not empty but definitely 30 litres or so short of full. Like I said, I’ve just driven 150+ miles with my caravan in tow. I doubt that my car has somehow become so incredibly economical, it would not use any fuel on that journey 

[Gammyleg]

If there is an air lock, could you not run a length of hosepipe down the filler neck and burp it out?

[Albourneboy]

We’re off down to Builth Wells tomorrow. I’m hoping that once the fuel level drops further that the problem will go away. If not, a call to Green Flag might be needed to get us home to Darlington next Saturday

[Kenny R]

Rocking the car might burp out an air lock if that’s what it is.

[AGFalco]

The fuel filler cap has a small hole it it to let air in to the fuel tank as it empties.

If this is blocked then air will not get in to the fuel tank.

 

What can happen is the fuel pump when working will make the fuel tank shrink.

A bit like you blowing in to a crisp bag ( to pop it ) but then sucking the air back out. 

 

Has your fuel tank shrunk?

 

Thanks. AG Falco

[Albourneboy]

Can’t see that happening. Besides, when I removed the fuel filler cap to refuel, surely that would let plenty of air in

[SurreyJohn]

Is there an anti-siphon valve that is stuck closed

 

The petrol station pumps have a pressure sensor and if fuel looks like it can't go in, will shut off the flow, to avoid spillages

[Albourneboy]

Fuel pump nozzles have a very simple system that shuts of the fuel once the tank is full. It’s just a small tube incorporating a Venturi. As you refuel, escaping air from the tank flows up this tube to a diaphragm keeping it inflated. Once fuel starts to flow up the tube, the diaphragm collapses shutting off the fuel delivery. I don’t know what is fitted to a Superb to prevent siphoning. Perhaps someone else out there does. 

[J.R.]

It wont be an air lock in a filler hose of that diameter.

 

Something must be blocking the filler hose, think back to the previous fill up, did anything abnormal happen? Did you fill to the top of the neck? Was the pump nozzle hard to remove? Could something have fallen off of the filler nozzle?

 

When you tried to refill did you get any fuel in like 1/2 - 1 litre or was the filler neck overflowing when you removed the cap?

[Albourneboy]

Didn’t seem to be anything wrong with the previous fill up. Nothing abnormal happened. No fuel came out when I removed the cap on this attempt to refuel. Didn’t get any fuel in at all really. The fuel nozzle tripped off as soon as I pulled the trigger

[Paws4Thot]

2 hours ago, Albourneboy said:

Can’t see that happening. Besides, when I removed the fuel filler cap to refuel, surely that would let plenty of air in

Yes and no. Yes it will allow air to enter the fuel tank, but no it will not re-expand a collapsed tank.

[J.R.]

Why not, it will be the force of atmospheric pressure that would re-expand this mythical collapsed tank holding a partial vacuum which allegedly collapsed it. All that would really happen in that scenario is that the engine would suffer fuel starvation

 

Also the contributor who suggested this scenario has overlooked the fuel tank venting system, in fact I bet the filler caps no longer have an air bleed hole.

[Albourneboy]

No, they don’t. And I was wondering about the venting system but I can’t imagine how it would malfunction in such a way as to retain fuel in the filler pipe. It’s all very weird. Drove another 90 odd miles this morning to Builth Wells so now I have less than half a tank. I’m hoping that the problem has gone away but won’t know until I try to refuel. Will give that a go tomorrow 

[langers2k]

What's the tank arrangement like on a 4x4 Superb?

 

Assuming it has saddle tanks, could it be a transfer pump issue?

[Albourneboy]

It has just has the one tank. Had a bit of a look underneath and can see the tank, part of the filler pipe and a clear pipe which could be part of the vent system. All looks good apart from some staining which probably happened when I tried to refill yesterday and the clear pipe that looks to have diesel in it 🤔. 

[J.R.]

1 hour ago, langers2k said:

What's the tank arrangement like on a 4x4 Superb?

 

Assuming it has saddle tanks, could it be a transfer pump issue?

 

Smart thinking! 👍

 

Do saddle tanks have transfer pumps, twin pumps or is it a parallel feed system with some sort of isolating valve so that when the first runs dry it can suck the remainder from the other?

 

Whatever it is will be overcomplicated and if the OP does actually have a saddle tank then I reckon you have come up with the most plausible solution, some sort of blockage in the balance pipe, transfer pump or whatever meaning that the vehicle was running on the left tank only, assuming that the filler neck is on the right.

 

To the OP, a saddle tank is one single tank but with reservoirs either side of the differential and twin float guages, you probably do have one, all the other 4x4's do, VCDS would show a disparity between the fuel levels of both tanks.

[skomaz]

For reference the Octavia 4x4 we had definitely had a saddle tank with a 'hump' in the middle and a transfer pump to even out the level across it.  However i understand it was a sngle tank that was connected at the top so when it was filling up fuel would naturally flow into the far side.

 

Not sure if the Superb is the same but if it is that upper link would mean the level in the filler would drop as fuel was used...   So im still betting on a blocked anti Syphon arrangement 

[Albourneboy]

Well, I’m pleased to report that the problem has disappeared. Have just filled up at a local co-op. Can’t tell how relieved and happy my wife and I are. We were having visions of it spoiling our holiday but we need worry no more. Might actually get a full night’s sleep tonight. Thank you everybody for your suggestions. It would be nice to know the cause in case it happens in the future to me or anyone else for that matter but I suspect we’ll never get to the bottom of this mystery. 

[J.R.]

I don't want to spoil your sleep but a problem not currently manifesting does not equal a problem disappeared.

 

Skomaz, I am going to have a good look at mine soon, sounds like I have an as yet undiscovered potential problem item!

[Albourneboy]

That’s true but I should now have enough fuel for general trips in this area plus enough to get us home next Saturday so I shall sleep soundly for the remainder of our holiday at least. If it happens again when I get home, I can at least take it to our local very good independent to have it looked at and we can use my wife’s car if mine is off the road. I don’t work any longer so once home, the car spends most of it’s time parked outside the house 

[Warrior193]

2 hours ago, Albourneboy said:

That’s true but I should now have enough fuel for general trips in this area plus enough to get us home next Saturday so I shall sleep soundly for the remainder of our holiday at least. If it happens again when I get home, I can at least take it to our local very good independent to have it looked at and we can use my wife’s car if mine is off the road. I don’t work any longer so once home, the car spends most of it’s time parked outside the house 

Take careful note of what happens the next time you refuel, particularly as you're getting close to full tank.

When you first found this issue, was the fuel gauge indicating normally for expected fuel use at that point? 

[Albourneboy]

Yes it was. I estimate I’d used about 6 galls. Had towed the caravan about 160 miles so about right (average consumption was 28 mpg). 

[J.R.]

That points to a failure of whatever the system is to maintain an equal level in both tanks or perhaps to draw fuel from the RH tank first.

 

Next time I use VCDS, if the fuel guage shows about 1/2 a tank I will look to see what the individual volume of fuel is on both sides.