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I have been having for the last twelve months an intermittent but increasing ECU fault on lean mixture on bank 1 and or bank 2 (P1071 & P1074). This can be caused by a small air leak. Dealers told me to ignore it & to cancel the fault via the OBDII dongle I have as they could find nothing.

A chat with Simon Daunt over dinner at the Anglemog trip to Scotland helped track down a possible cause to the seal on the filler cap.

Mine was badly perished & a loose fit in the groove. I had assumed the locking cap was the pressure & venting valve so didn't think it an issue and that this seal just allowed the cover to lock down.
But it seems this seal & the spring loaded inner pad do act as the pressure & venting valve.

A replacement was sourced from Melvyn Rutter, but they commented they were over size. This new seal has stopped the error most of the time. But fails when you stop and refill the tank.

I can obviously cut the seal to make it shorter but I am unsure how tight it should be. I am concerned that it needs to be loose to let air back into the tank when / if it pulls a vacuum Is there a kind soul out there who has a newer car & can let me know their seal diameter?

I just took the o-ring off my old cap and the diameter of the ring section is 3.5 mm so I guess it puts the nominal id of the ring at around 70 mm ( 78 - (3.5 x 2) )

Will have look to see what is available. Yes it is a good fit, not overly tight.

Simply Bearings have a selection of o-rings in Nitrile and Viton at 3.53mm cross section.

The 3.5mm cross section selection is very limited doesn't have any suitable bore sizes, 72mm is closest but may end up a loose fit.

https://simplybearings.co.uk/shop/Products-O-Ring-Seals/c4747_4515/index.html

Select Nitrile or Viton and 3.53 cross section, then select the appropriate bore size

such as

3.53mm Section 69.44mm Bore Rubber O-Rings
Pack Size Pack of 2
3.53mm Section 69.85mm Bore Rubber O-Rings
Pack Size Pack of 2
3.53mm Section 68.26mm Bore Rubber O-Rings
Pack Size Pack of 2

I have ordered a selection in Viton, but probably Nitrile is OK as both are resistant to petrol and diesel, Nitrile is cheaper and has the lower temperature spec.

Will let you know the outcome

Dirt?

Inside the cap?

Pfft that's a tie break fail...…...

I think it is clear that Janice hasn't kept on top of things and cleaned around the bends

Mike,
I make no comment on cleaning abilities. Car cleaning is an outside AKA man job.

But I have cleaned the seal face on the locking cap and now it holds pressure. It vents as you open it & the fault is radically reduced but annoyingly still there.

I may buy a new locking cap or seal.
John

Is your locking insert the older rubber band type (rectangular section), or later o-ring type ? The hex bolt securing the disc suggests it may be the older style, later style seem to have a slot headed screw.

I found the rubber band style insert a PITA, once it was out it was often difficult to get it back into the cap. I tried some 'rubber grease' on the seal but it didn't solve the problem. The original insert had been attacked with a knife by the previous owner, easier to get in, but not sealing to the extent that fuel vapor entered the cabin when the tank was full

It is the older square one.
I gave it some TLC today , cleaning the surfaces etc and now the tank is staying pressurised but I think it may need replacing as the fault still happens but I must say it is far less often now.

there may of course also be another air leak elsewhere.

Our (I say "our" meaning the wife's) plus 8 has that annoying idiot light after the first refill after the dealer turns it off. We just ignore it, but now I have an idea what to look for

I understood that "They all do that Sir!" (mine's a 2011 SIII 3-litre). I was given to understand that the compromises in the adaption of the Ford ECU with external Lambda sensors was the presumed cause - and by definition unfixable. Certainly I get the engine management light on intermittently - always the same fault - and turn it off with a "dongle". It's an irritation - but not worth losing any sleep over, I feel!

Re the filler cap: may be worth talking to Newton Equipment . My cap is their "Classic 300". They appear to offer a spares service.

The o-rings have arrived, and in practice any of the above sizes will do the job. I will bring a 69.44 diameter o-ring up at the weekend and we can try it on your cap.