filler cap

cantdecide

I failed the NCT over a missing seal in the temporary filler cap I have in my '98 A6 petrol. it seals around the mouth of the filler pipe.

I've been having a slight 'kick' from my engine lately like a strong 'miss' and today, while investigating, i decided to check the seal on the new temporary filler cap. I had 20 miles done from a full tank and i had to really pull at the cap to open it and it gasped really loudly for a good 3 seconds when it came loose. I also noticed the seal had been heavily indented where it met the filler pipe as if it were being sucked on heavily.

The re-tester commented that he wouldn't have failed it the first time because it is a vented system. I also wondered how you could lose a seal from somewhere it was more inclined to stay on than fall off, unless it had been taken off.

I drove a further 5 miles and it didn't kick at all. I am going to get a proper audi filler cap anyway but could this be the cause of the kick or could it have any other consequences?

Mr.Diagnostic

Hi,

It sounds to me like there has been some confusion with the NCT people here.
Firstly, your Audi is not a vented system, as in the old type with a breather valve open to air. I think you will find the first tester failed it because the seal was not correct, ie the wrong cap. The second tester should probably have failed it too for the same reason but mistakenly thought it was a different system.

You posted about the purge valve before. This system was introduced for emission reasons. Petrol evaporates thus causes polution even when the car is not being used. The system on your car allows the tank to breath through a carbon filter and the fumes are recycled through the purge valve into the inlet tract to be burned in the engine.
If you have a problem with the purge valve that can cause a running concern.

cantdecide

Mr.Diagnostic wrote:

Hi,

It sounds to me like there has been some confusion with the NCT people here.
Firstly, your Audi is not a vented system, as in the old type with a breather valve open to air. I think you will find the first tester failed it because the seal was not correct, ie the wrong cap. The second tester should probably have failed it too for the same reason but mistakenly thought it was a different system.

You posted about the purge valve before. This system was introduced for emission reasons. Petrol evaporates thus causes polution even when the car is not being used. The system on your car allows the tank to breath through a carbon filter and the fumes are recycled through the purge valve into the inlet tract to be burned in the engine.
If you have a problem with the purge valve that can cause a running concern.

the first guy told me specifically that the cap itself is alright but the seal, a rubber ring was missing. i dashed over to a nearby motor factors and bought a €2 filler cap, identical to the first but with the ring included, and the second guy, who used to be my mechanic, passed it but told me he would have passed the first time.

I was really surprised at how much it gasped when i pulled off the new filler cap....

daedalus2097

Seems like you worked up a big vacuum, which will cause bad running as the fuel pump has to work much harder against a vacuum than against normal atmospheric pressure. I'm not familiar with the finer points of the Audi fuel systems, but is it possible that the first (original) cap had a one-way breather valve in it that the temporary cap didn't? That would explain the huge vacuum built up behind it, deforming the seal. ose temporary ones are really a get-you-home job anyway, chances are that when you get a new Audi one that it will work fine and your driving will be back to normal.

dodgyme

I got a temporary petrol cap for the fuel in the car where the other one was nicked. Does that mean I could fail the nct because of it. It seems to fit ok.??

cantdecide

mine passed after i got a new one with the rubber seal intact so you should be fine.