Pressure in fuel tank.

[Karl_mac_]

Just a quick one, i've been battling trying to fix an exhaust warning light, i've had this looked at and i've been told its running lean at part throttle (fine on idle/WOT). Today the light turned itself off but i heard some strange noises from the rear. I removed the filler cap and there was a massive escape of air and petrol vapour.

A quick google says possibly fuel filter blocked or something to do with the carbon filter?

Is there anything else worth checking?

[Teutonic_Tamer]

Just a quick one, i've been battling trying to fix an exhaust warning light, i've had this looked at and i've been told its running lean at part throttle (fine on idle/WOT).

Who told you this, and more importantly, what equipment did they use to form their said diagnosis?

It would seem whoever told you this is verging on the incompetent - because the whole ethos of the FSI system is to run 'lean' at part throttle!

You really need a print-out of the fault codes (DTCs) from either a generic fault code reader, or the full manufacturer codes from VCDS or the genuine kit at a VW stealer.

Today the light turned itself off but i heard some strange noises from the rear.

The light will turn off when not detecting an error - but will re-light if the problem re-develops.

Can you describe the noises in more detail?

I removed the filler cap and there was a massive escape of air and petrol vapour.

'Massive' - as in it forced the fuel cap out of your hand as you were unscrewing it . . . ?  Or was it more like just a noticeable hiss?

If the former - you got a problem, but if it was the latter, then that can occur in certain circumstances.

A quick google says possibly fuel filter blocked or something to do with the carbon filter?

And did google state how, exactly, a blocked fuel filter can pressurise the tank??? 

It could be the carbon canister, but then this would be shown via a correct DTC from the scan tool.

Is there anything else worth checking?

VW Golf Mk5 specialist UK internet forums - rather than google!!! 

[Karl_mac_]

Thanks for your input!

The car was looked at by JBS, who also checked pcv and dv. They suggested throttle body could be at fault.

The light is back on now and the pressure in tank has gone, not sure if these two are related.

I originally thought I'd parked over a blocked drain, when I got out of the car i realised the gurgling was coming from my car. When i opened the cap I could feel air passing over my hand as it escaped.

My own pet theory is it could be sensor related, probably in the exhaust. So I'm booked into Cobra next week for them to take a look, i'm also taking the car for a service and they've offered to have a look for me too.

[Teutonic_Tamer]

I thought JBS were VAG specialists . . . so should have VCDS or similar.  Can't you fone them and ask them to e-mail you the scan results?

I'm a little worried that JBS 'checked pcv and dv. They suggested throttle body could be at fault.'  They should only be checking those components and suggesting others ONLY if the scan results indicated said parts were at fault.  I hope they didn't charge for that part of their work!

I also really don't see why they are even suggesting the throttle body?  The exhaust warning light relates to emissions, not throttle operation.  Basically, the problem could either be a faulty exhaust oxygen sensor or a failed catalyst - but again, these will be indicated by fault codes - rather than just guess work.

Re the fuel tank.  Say you filled up the tank late one night when it was very cold, and the following day, after the car had been baking away in the sun all day - you might get a noticeable hiss when removing filler cap - along with a very strong smell fuel.  You may also get a similar hiss if the pressure is just below the limit required for purging vapour pressure to the carbon cannister.  Check the fuel cap say on a daily basis - if the hiss is intermittent, or varies, you have nothing to worry about.

Does Cobra have VCDS?

[Karl_mac_]

I thought JBS were VAG specialists . . . so should have VCDS or similar.  Can't you fone them and ask them to e-mail you the scan results?

I'm a little worried that JBS 'checked pcv and dv. They suggested throttle body could be at fault.'  They should only be checking those components and suggesting others ONLY if the scan results indicated said parts were at fault.  I hope they didn't charge for that part of their work!

I also really don't see why they are even suggesting the throttle body?  The exhaust warning light relates to emissions, not throttle operation.  Basically, the problem could either be a faulty exhaust oxygen sensor or a failed catalyst - but again, these will be indicated by fault codes - rather than just guess work.

Re the fuel tank.  Say you filled up the tank late one night when it was very cold, and the following day, after the car had been baking away in the sun all day - you might get a noticeable hiss when removing filler cap - along with a very strong smell fuel.  You may also get a similar hiss if the pressure is just below the limit required for purging vapour pressure to the carbon cannister.  Check the fuel cap say on a daily basis - if the hiss is intermittent, or varies, you have nothing to worry about.

Does Cobra have VCDS?

They didn't charge me, bit to be honest after I left and had a little think I realised that the throttle body theory made no sense, if its running lean then it has to be fuelling right? But I was told fuel pressures were normal.

Cobra don't have VCDS, but we're going to have the car and ramps and do a physical inspection of the exhaust system/cat (mainly because I don't know what was fitted by the previous owner).

As mentioned, the car is going into another specialist who does have VCDS for a full service. I'll let him have a good look over the car and see what he reckons.

I noticed that it start building pressure when i ran the fuel quite low (showed <5 miles on the range) which I don't normally do. Not noticed any sounds coming from the tank but will check pressure tomorrow.

Really annoying me as I want to go to RTech!

[Teutonic_Tamer]

I noticed that it start building pressure when i ran the fuel quite low (showed <5 miles on the range) which I don't normally do. Not noticed any sounds coming from the tank but will check pressure tomorrow.

Ahhhh . . . . when the fuel is really low, the lift pump in the tank will aireate the fuel as it tries to intermittently pump air rather than fuel.  This can then result in a pressure build-up.

[Karl_mac_]

I noticed that it start building pressure when i ran the fuel quite low (showed <5 miles on the range) which I don't normally do. Not noticed any sounds coming from the tank but will check pressure tomorrow.

Ahhhh . . . . when the fuel is really low, the lift pump in the tank will aireate the fuel as it tries to intermittently pump air rather than fuel.  This can then result in a pressure build-up.

Seems like this. Regular 'bleeding' of the tank seems to have cured it.

Took the car out for an italian tune up today and I'm almost certain eithery exhaust is blowing or i've got a boost leak somewhere. Can hear the air spooling at high rpm.