EVAP system fault code

[Motorista]

During a recent scan I got the following fault code:

"Address 01: Engine Labels: 4D0-907-560-BGH.lbl
Part No SW: 4D0 907 560 DD HW:
Component: 4.2L V8/5V G 0030
Revision: 00000000 Serial number: VWZ3Z0E7300196
Coding: 0007873
Shop #: WSC 01065 000 00000
VCID: 6BDE56EF499EEF6

1 Fault Found:
16840 - EVAP System
P0456 - 002 - Very Small Leak Detected - Intermittent
Readiness: 0000 0000"

After reset the code has not come back, and there are no other symptoms at the moment. The only abnormality I can recall is that back in February upon leaving the underground garage to og home after a full day's work, I got the "check your fuel cap" message and icon in the instrument panel. In both cases it happened at exactly the same point in my route home, approx. 5 minutes after leaving the garage, which is at a pretty constant 78 F, into outside temperature in the lower thirties. In both cases, the message and icon disappeared after rebooting (i.e. turning off engine, pulling key out of ignition, then turning on again).

My guess is that the EVAP fault code is somehow related, and may be triggered by a sharp change in the fuel tank's internal pressure due to the sudden significant change in outside temperature. Still, I wodner why it has never happened before in almost seven years of ownership, and only started this past winter, which was quite mild by local standards. Thoughts?
Stefano

[invisiblewave]

Mine's regularly, though infrequently, popped up that message ever since I've had the car. Did you put different gas in it? I'm 99% sure mine's related to the anti-knock rating of the gas I put in. After someone posted a link to that gas quality website, I started filling up at the nearest place on the list and didn't get the message again until I put in the half tank of 89 before I put in the 93 (they don't sell 91, so I mix 93 with 89). So long as I put the 93 in first it's fine. I think people have posted before just how sensitive the V8 is to what it's fed from the petrol tank. I'm reluctant to run it just on 93, partly because of cost, but mostly because of the hotter combustion and the potential effect it might have on the valves.

[Motorista]

Thank you - I hadn't thought of changing gas station, but indeed I have done so this winter. I will watch it and report if the probme happens again.
Stefano

[PanEuropean]

My guess is that the EVAP fault code ... may be triggered by a sharp change in the fuel tank's internal pressure due to the sudden significant change in outside temperature.

Hi Stefano:

That is exactly what is causing the fault (and the check fuel cap) message to appear. It is a known issue that affects all models of cars under the circumstances that you described.

The fact that the fault code you observed has the word 'intermittent' after it indicates that the fault was present at some time in the past, but was not present when you did the scan. Hence, it's not something that you need to get service attention for.

Michael

[Motorista]

Thank you very much Michael: one fewer code to worry about! I'm now focusing on the TPMS fault codes (see my other thread), am still puzzled that I'm getting a general error even thouhg 4 out of 5 sensors are still transmitting fine (according to VCDS). In any case, I've ordered a set of 5 from the Tirerack, we'll see if the fault codes go away.
Stefano