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Fuel smell but no apparent leak

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Just spent some time looking at this - all my hoes are leak free.

Just looked at the piping diagram and noticed an issues...

...technically there is always a leak from the tanks !

The charcoal cannister has 3 pipes attached to it.

1) from the tanks

2) from the air filter

3) to the inlet manifold

The idea being to draw vapour in from the tanks and then when it purges the system to open a valve to the inlet manifold drawing air through the carcoal to dry it out so to speak and purge the vapours

Thing is - the air filter line is always open, and directly in line to the tank so is this fuel smell a complete red herring ?

With no leaks the only place the fuel vapour is coming from is the air filter....seems a bit pointless to me having the thing in the 1st place

I've just cut the line from the tanks to the charcoal canister and you can blow air through the in-port (fuel smell then comes out the air intake) - now I also have a line to the tanks, this has much more resistance to blowing air down it but does not hold a true pressure for long...should it ? I mean in theory it should hold steady - just wondering if one of my tanks has gone southwards :)

Is it worth drainging the fuel down as much as possible and doing a full pressure leak test on it ? Is there a relief valve in the cap ?

Any opinions / thoughts ?



Just to add, pressure tested the tanks using about 0.5 bar no leaks at all in the system (held pressure for over 10mins)

The cap however does have a positive pressure releif on it, cant tell about negative - couldn't test it, although you can suck on the cap and it will breathe.

Charcoal cannister seems to be the issue here, the tube to the air filter is open to the tanks via the charcoal canister so can in effect breathe away quite happily.

Fix would be to fit a 1 way valve from the airbox facing the charcoal canister so air can still be sucked in for the purge, but not vented to atmosphere 24/7

I actually thought of blanking this pipe altogether, if the filler cap has a vacuum and positive pressure relief system on it (which it looks like) - what is the point of the tube to the airbox ?

Edited by Jonathan

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Jon I had a strong smell last year and had no apparent leaks it turned out on my s4 to be the fuel filler cap rubbers had gone. I replaced these and the smell disappeared.

Worth a try and easy fix :)

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How is the air intake for the cannister hooked to the air filter housing? On my car that hose just runs down under the car and is strapped to the chassis for fresh air...

1995 S4s

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Wayne (or should I say Buford T. Justice LOL) - I swapped mine over with the GT2 and the GT2 seals a bit better, I replaced the seal with an O ring but it seems to make little difference. The smell seems to be confined the the engine bay - we'll see.

Jim - My air breather for the charcoal cannister comes back through the engine side wall and pops into the airfilter case from there (ie the fixed half if you like).

Thinking of ripping it out, patching the hole and then doing something similar but with a 1 way air valve on it so the bloomin system can't leak any more pong into the car.

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If your system is the same as mine then you have the one-way valve which allows air & vapour mix to be moved in to the induction manifold when the manifold is below atmospheric pressure, pushed by the atmosphere (there is no such thing in reality as suction, just a lack of pressure on one side causing a differential). Thus, the canister is only vented to the manifold when there is a pressure differential, and that's not happening for a lot of the time due to turbo pressure with the throttle not closed.

The open-to-air pipe must be there to allow the pressure that does build up in the tanks (fuel expanding and air expanding due to temperature increase) from time to time, to vent. A one-way valve on the open-to-air pipe would stop one of these two actions, which one depends on which way you orientate it.

There are roll-over valves on the tank vents, they allow air to pass at low volumes when orientated correctly (I suspect one f these was shifted to cause my ongoing issue), these can also be made to go to the closed sate by a large air flow in one direction (into the tank) as it would be enough to suspend the metal item on the air flow, closing the hole off inside. When checking for leaks etc, don't forget their presence as it may give false readings if you have one of these closed off by mistake.

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