W124 Rush of Air from Fuel Tank Query

[300CE]

Hi All,

I know this may sound like a silly question to some, but do you any of you guys know whether the sudden rush/whoosh of air that escapes from the fuel tank when you take off the petrol cap is normal

I also get a 'popping' sound (best way of describing it is like a metal can that is dented being popped back out again). It only happens once and after about 5-10 minutes of driving. My mechanic friend seems to think that this noise maybe pressure within the fuel tank and air being released at some point and this is what is making the sound?

Any ideas/thoughts on this would be great.

Cheers, Justin.

 

[d w124]

Hi All,

I know this may sound like a silly question to some, but do you any of you guys know whether the sudden rush/whoosh of air that escapes from the fuel tank when you take off the petrol cap is normal

That`s normal

 

[tippandy]

Hi, sounds to me like the fuel evaporative cannister is blocked and not venting the tank when it should.

Andy.

 

[MBK]

Hi, sounds to me like the fuel evaporative cannister is blocked and not venting the tank when it should.

Andy.

Is this a fuel tank out job or can you check this with the tank in situ?

K

 

[grober]

YEP sounds like that. The evaporative canister is situated under the nearside front wing behind the wheel arch liner if you need to renew it [ nowhere near the tank!]. Its evacuated by a vacuum pipe leading from the inlet manifold and controled via an ECU controlled solenoid valve mounted on the inner wing in the engine compartment..

 

[tippandy]

Same as that ,grober beat me to it.

 

[macabethiel]

All my W124 Mercs tended to make the same noises you describe I think its the nature of the beast.

 

[300CE]

As always, many thanks for all your help and info chaps! I'll go for replacing the canister behind the wing to see if that sorts it out - any ideas as to how much they are at all?

Cheers, Justin.

 

[nick mercedes]

As always, many thanks for all your help and info chaps! I'll go for replacing the canister behind the wing to see if that sorts it out - any ideas as to how much they are at all?

Cheers, Justin.

A defective filler cap is a more likely suspect.

It's not unknown for 124 tanks to implode if the caps do not vent as they are supposed to do.

 

[bouncer]

I agree Evap valve sticking, have you noticed any changes in the fuel eonomy.

sTeVe

 

[tippandy]

Hi, are there any other symptoms, strong smell of petrol in the car or lumpy idling. As far as the petrol cap is concerned (and i may be wrong ), I dont think it has a vent because it is a sealed system vented by the evap purge valve and cannister filter.

Andy

 

[Ian B Walker]

any ideas as to how much they are at all?

Cheers, Justin.

£56 plus vat from your dealers. I dont think that ECP or GSF supply them.

 

[300CE]

Thanks again everyone! She does about 50 miles on a tenner which i think works out to about 23mpg and I only use the car at weekends. I can't smell any petrol and she idles at about 700rpm - it does drop ever so slightly once the car has warmed up.

Do you think it's worth changing both the evap valve & cannister filter? Cheers Ian for advising on the cost of the canister.

Cheers, Justin

 

[grober]

Check the operation of the REGENERATION [EVAP] VALVE valve first. It should be open under the following conditions.
coolant temp >70c
> 2mins after engine start
engine not in de-acceleration mode

Its pulsed on and off at about 7.5 hertz while operating.

 

[kth286]

fuel cap is not vented.

a worry is the pop mentioned - because if the system is blocked, the fuel tank could be imploding as the petrol reduces - expensive.

several instances of it happening on various forums Merc forums.

charcoal canister needs replacing every 70k ish miles as a service item, but because it is hidden behind wing, is often not done.

 

[300CE]

Thanks again fella's - I've got the car booked into Merctech on Thursday for an ATF change and i'll ask Andy nicely to see if he can check the operation on the valve whilst it's in there.

I'll definately get the cannister changed though - the car has done 158k and i reckon you're right kth in that it's probably never been changed. I'll provide an update as soon as!

Cheers, Justin.