Fuel cap venting

[Speed 3]

Since I bought it in November I've noticed a considerable whoosh when removing the fuel cap which would suggest a vacuum being generated in the system (pretty sure its vacuum rather than pressure as there is sometimes a plastic creaking sound suggesting the tank is returning to a more normal shape). After I first noticed it, I have been releasing it every time I go in the boot. I don't want to damage the tank or stress the fuel lines and pump - is this normal ? I have seen discussions about the polished aftermarket caps having too tight a seal and some have cut a slot in them, but this is the original TVR factory fit I believe. I don't want the boot filling with fuel fumes by creating a vent if its not necessary. I do get a slight whoosh on my other two cars but only after drinking a full tank and to much less px differential correction than the Tuscan.

[keith24]

Hi Rob,

Mine occasionally suffers from the same symptom.

My understanding is as follows :
- The car is not vented, so I suggest no cutting!
- It may be a symptom of a blocked charcoal can, mine was clear however
- I've also read that "overfilling" can contribute. I now stop at first click. Might be worth a try.

Cheers Keith

[Paulgooner]

I have this problem too, and was lead to believe it was the carbon canister. However when I checked this, it looks like a sealed unit. Keith how did you check yours. I try now to release filler cap each time I use the car just to relieve any vacuum building up.

[Speed 3]

keith24 said:

Hi Rob,

Mine occasionally suffers from the same symptom.

My understanding is as follows :
- The car is not vented, so I suggest no cutting!
- It may be a symptom of a blocked charcoal can, mine was clear however
- I've also read that "overfilling" can contribute. I now stop at first click. Might be worth a try.

Cheers Keith

I know about the overfilling issue so always put the nozzle fully home and stop at first click or just fill to about 7/8 ths if I can judge it right.

[Paulgooner]

Went to the car today and opened the filler cap, a big whoosh of air. Drove to the garage to refuel, opened it again another slight sound of air, put 25 litres in, so only half full, got to my destination about 20 miles away checked again and another slight sound, so over filling in my case is not the cause. Anyone know more about this carbon canister

[wongthecorrupter]

I have ordered a new carbon canister off ebay, it is a ford part.

[S6 Devil]

http://www.pistonheads.com/gassing/topic.asp?h=0&a...

[nrick]

There is also a technique that uses a nick in the fuel cap filler sealing ring as the car uses fuel it creates the vacuum. A quick search of the forum especially on chim's should bring it up. I have done it and not caused a problem, and since I went to the alloy filler haven't had a problem. Carbon canister is a good shout as well.

[ratboiler]

Mine does it all the time, but have not had any ill effects.

[ochten]

drilled a very small hole in the cap. No problems afterwards

[arbydb7]

Also drilled a small hole in the top centre of the plastic cap. Problem solved.

[bhippy]

OK, so here's the dumb question of the day:

Why is this a problem?

Mine has always done this and I've never noticed any ill effects (I think). It's been regularly to Str8Six who do a fantastic job of going over it for me to get everything that needs doing, done....

It starts easy, goes fast, makes a nice noise and stops when I need it to... Am I missing something?

[arbydb7]

Too much vacuum could cause damage to the tank I guess. I could hear my tank contracting and expanding after a journey. Drilled a hole to release the pressure. Now no metal tank noise and the filler cap does not hiss when I undo it.

I could be wrong but it was what I was advised to do. It certainly solved the problem and hasn't seemed to caused any other problems.

Edited by arbydb7 on Tuesday 10th June 12:55

[ochten]

I thought I had the Answer by drilling a 2mm hole in the cap. But NOT! Yeasterday after a long and warme ride I made the Tank full (almost) After some Minutes a strong Petrolsmell in the Garage. Petrol everywhere on the Floor. The Petrol was expanding in the Tank and pushed it out through the little hole in the Cap.

Could clean the Paintwork in time!

[cuzzaluva]

Mine hisses too, never had a problem and I quite like the sound.

I never fill my tank more than three quarters full.

Read some nightmares on ph over the years about people that have filled to the top so it's put me off.

Hissssssss