Petrol tank vacuum
My petrol tank gets quite a vacuum build up (when I remove the petrol filler cap there's a big whoosh).
I'm only an amateur - so may be a silly question:- should there be any kind of vacuum build up? If not then I take it there's a breather pipe or hole somewhere that needs to be unblocked.
Is that easy to locate and clear? If so, simple directions would be much appreciated.
Are you sure it's a vacuum and not pressure?
For the 300GE, the tank venting is controlled though a carbon cannister leading back to the engine intake manifold though a controlled valve and also through a vent-valve leading to some pipes in the rear somewhere. There is also a pressure accumulator on the second fuel pump and if its membrane breaks, this can also affect tank pressure. Does the engine start OK?
Ok, so fule- pressure accumulator is fine , then,
Tanks usually build up some positive pressure, not least from evaporation.
Only managed O Level Chemistry and Physics (and both a LONG time ago) but if petrol is evaporating from fuel tank wouldn't that indicate that (a) air breather working (can't evaporate in a vacuum) and therefore there shouldn't be any change in pressure or (b) if fuel is evaporating in a vacuum then it would have to produce even more of a vacuum?
Still thinking of a foolproof way of working out whether whoosh sound when removing petrol filler cap is positive pressure or vacuum (other than holding lighted match close by to see whether flame sucked in or blown out (only joking 
).
Petrol evaporates easily at lowish temperatures, so it is possible that a change in ambient temperature from warmer weather and sloshing in the tank can cause enough vapor pressure to cause the whoosh you are hearing, when you open the cap. (In vacuum conditions, volatile liquids such as petrol will evaporate even more quickly, decreasing the vacuum, so i doubt it's a vacuum, which if it were, would also make the engine harder to start.) The management of fuel vapor in the tank is controlled by feeding it back to the engine after passing thorough the carbon canister. Excess tank pressure, however, is vented through another valve. To reduce hydrocarbon emissions, there is no open venting of the tank, which is why I am inclined to think that it positive pressure in the tank, which you are hearing. If this is very high, there may be a problem with the vent valve to atmosphere.
Mark,
did you ever determine whether you had a partial vacuum or positive pressure in your tank, because:
http://www.benzworld.org/forums/w124-e-ce-d-td-class/1712285-screwed-charcoal-canister.html
?
Arnie,
Thanks for your continued interest - no I haven't.
Interesting link - thanks for that.
Just had a decent service carried out (and forgot to mention this problem to them). They did replace the fuel filter for me but that doesn't appear to have made any difference to the tank pressure.
Does my 300 G Petrol have a charcoal filter and if so how can I locate it so I can check on it's condition?
Mark
the charcoal filter is located under the right-hand, rear wheel arch, near the fuel-filler - opposite arch to the fuel filter and pump.. It will look similar to that in the previous link, but it's a plastic-cased unit, rather than metal-cased, so it should not rust away like the other example. But i understand that they are a 70,000 mile maintenance item and they can clog and gum-up internally, when they get old and then restrict flow.
Thanks Arnie,
Mine has done 111,000 miles and I'm guessing chances are that charcoal filter never been changed. But is there some way of me telling whether that is the case or not?
How straightforward is it to replace the filter?
I think in practise, these charcoal canisters are rarely, if ever replaced. They are clipped to the underside of the rear wheel arch and they have, from memory, 3 small hoses connected. Probably about 30 mins to change - same as a fuel filter.
But, I guess they can be checked with air and if not blocked, then probably OK to leave it.
JOIN THE GWOA
This website and forum are provided for the benefit of enthusiasts and owners of all Mercedes Benz off road vehicles, in particular the G Wagen. The costs of development, upkeep and provision of the site are met entirely by the annual subscriptions of club members. If you are using the site and are not a member of the G Wagen Owners Association please consider joining. You will have access to areas on the site that are not open to non members, to club trips and meetings and to technical assistance. In addition your contribution (only £25 annually) will help us maintain and develop our website for future users.
Who's new
- Jaffa_JIM_23
- Wrabstar
- GG
- KTMSCANIA22
- 280ge
Ah - good question. Will have to wait a couple of days, undo filler cap and try and work out if it's vacuum or pressure.
Turn key, no accelerator, engine fires straight away.