Fuel filler cap

[John Dean]

Have I got a standard ex factory filler cap or what?

Its light grey in colour and made of plastic WITHOUT a breather hole in it. The reason I mention this last point is the terrific vacuum felt when I come to fill it.

Does anyone else have this problem

John

[Ian Linden]

Have I got a standard ex factory filler cap or what?
Its light grey in colour and made of plastic WITHOUT a breather hole in it. The reason I mention this last point is the terrific vacuum felt when I come to fill it.

John,
Mine is exactly the same, including the vacuum. I think its designed that way, to stop petrol fumes getting out of the tank.

Ian

[Chris S Palmer]

Does anyone else have this problem
John

John
Only a problem if there is "Whoosh" when you remove the filler cap, as the cap incorporates a vacuum relief valve instead of a breather hole to stop fuel escaping if overturned.

csp

[Ian Linden]

Only a problem if there is "Whoosh" when you remove the filler cap

Chris,
Several fuel-injected vehicles I have used have all "whooshed" when the cap was removed. There is no data in the manual on the relief valve setting, but I think it reasonable to assume that this is a substantial value, but obviously not large enough to cause the pump a problem, i.e. the relief valve releases excessive vacuum, but always retains a minimum value. If correct, this would suggest that, if there is no whoosh, the sealing gasket in the cap is defective.
Ian

[Chris S Palmer]

i.e. the relief valve releases excessive vacuum, but always retains a minimum value. If correct, this would suggest that, if there is no whoosh, the sealing gasket in the cap is defective.
Ian

Ian
There is a mean value i.e. try taking the cap off just after the valve has dumped and you will still release a slight vacuum.
If there is no equalization it indicates that there is an air leak somewhere within the tank instalation not just the cap O-ring, it could be corrosion,welds,neck sleeve etc. and I have even seen minute cracks!

csp

[John Dean]

Thanks Ian and Chris

I have'nt noticed any fuel supply problems even when I have run low on petrol. So, I guess air is getting in somehow!

Nonetheless, there is a definite "whoosh" when I unskrew the cap which sometimes seems to resist - and this I put down to the vacuum.

Then I thought with concern the fuel pump might be under strain because there is too much vacuum?

Is that the relief valve in the top of the inside of the filler cap and if so can it jam or stick??

John

[Ian Linden]

Chris,

Sorry ; misunderstood the "no" icon - we were both of the same opinion

John,

Clearly the valve in the cap could stick, but I would then expect the pump to have problems in delivering. Next time we get together, let's have a whoosh-comparison session, to set your mind at rest.

Ian

[Murray Betts]

The tank pressure is regulated by the control valve, which connects the tank vapour to the "carbon can" or Evaporative Loss Control Canister. When you refuel the fuel is generally cold (from underground) and releases vapour as it warms up. The recirculating fuel also picks up heat from the engine bay and heats the bulk fuel in the tank. The vapour is adsorbed onto activated charcoal in the ELC can, which is then purged with fresh air by the running engine to take the hydrocarbons into the engine. Tanks usually run at a slight positive pressure; this helps suppress vapour generation to some extent, and helps avoid cavitation and vapour locking in the pump. It's possible (in theory) for the tank to achieve a slight negative pressure as fuel is used up, or as the temperature drops if some vapour condenses, but it's usually positive. I can't find the +/- values for the valve in the manual (anyone?), but the postive is more than the negative (if you see what I mean!). When you remove the cap you release this pressure; what you describe sounds perfectly normal to me.
Incidentally, the worst time for vapour handling problems (vapour locks etc) is typically 15mins after refuelling, due to the warm-up effects described. I've not experienced any problems with the Capp.