[itititit]

Is it specified somewhere it's for whole weekend? And anyway, if the finnish news article is correct, the problem wasn't so much stopping after the lap, but having so little fuel that you couldn't drive to pits and have enough for sample.
So they probably have estimated how much 1 lap takes fuel, and if the car has that + 1l for sample, they let it go

Rather than have all this speculation, why not just go to the actual regulations?
[quote]

6.6 Fuel draining and sampling :
6.6.1 Competitors must provide a means of removing all fuel from the car.
6.6.2 Competitors must ensure that a one litre sample of fuel may be taken from the car at any time during the Event.
Except in cases of force majeure (accepted as such by the stewards of the meeting), if a sample of fuel is required after a practice session the car concerned must have first been driven back to the pits under its own power.
6.6.3 All cars must be fitted with a –2 'Symetrics' male fitting in order to facilitate fuel sampling. If an electric pump on board the car cannot be used to remove the fuel an externally connected one may be used provided it is evident that a representative fuel sample is being taken. If an external pump is used it must be possible to connect the FIA sampling hose to it and any hose between the car and pump must be -3 in diameter and not exceed 2m in length. Details of the fuel sampling hose may be found in the Appendix to these regulations.
6.6.4 The sampling procedure must not necessitate starting the engine or the removal of bodywork (other than the cover over any refuelling connector).
[\quote]

source - http://argent.fia.com/web/fia-public.nsf/13C06BF289E0E36FC12579C9003CB5B9/$FILE/1-2012%20TECHNICAL%20REGULATIONS%2009-03-2012.pdf

The appendix referred to says
Article 19.8.1
Fuel samples will be taken and handled in the following way :
At any time during an Event a fuel sample can be taken from a car, a fuel drum or from a fuel rig by
the FIA. A representative of the team should be present during sampling.
SAMPLING
When sampling from a car or a fuel rig the fuel is sampled with the fuel sampling hose mentioned in
Article 6.6.3 of the 2009 FIA Formula One Technical Regulations. This comprises a tube approximately
2000 mm long x 4.6 mm internal diameter with a “‐2” female snap fit connector at one end and a
valve at the other end attached to ca. 200 mm of 4.6 mm internal diameter stainless steel tubing.
Fuel drums are sampled by siphoning using approximately 1500 mm of rubber fuel piping.
Before sampling the sampling hose is flushed with the fuel to be sampled using a volume of fuel
approximately equal to three times the volume of the hose. The fuel is then sampled sequentially
into three tinplate cans. Therefore at least one litre of fuel has to be on board a car at any time
during the Event. The tinplate cans should be filled to at least 85 % of their total volume with fuel.
During the sampling the fuel temperature may be measured. A fire extinguisher should be present.
LABELLING
After sampling all three cans are labelled and sealed with a numbered seal. On the label the
following is recorded: the Event, date, team, car number, seal number, team representative including
signature and FIA delegate including signature.
SAMPLES
One sample remains with the team, two samples remain with the FIA. One of these two FIA samples
is used for the “Track‐side” analysis. The other is kept refrigerated and used in the case of a dispute. source - http://argent.fia.com/web/fia-public.nsf/BED822D8DE2A9311C125797B0035300E/$FILE/1-2012%20APPENDIX%2007-12-2011.pdf

I may have missed some other regulation but, from that, I take it that it's OK to stop except for practice when something actually prevents the vehicle returning to the pits, as there is no mention about the car having to return to the pit under it's own power in qualifying, or the race, before providing the sample?
I think McLaren were screwed, partly their own fault if they protested using the wrong, IMO, part of the regulations!
If that's correct, Lewis could legally stop on track but would still need to have the 1 litre of fuel available for the sample - anyone got the specific details of the penalty? Even if he didn't have the fuel, I would have expected him to be put to the end of that qualifying segment but it it's deemed that it is the full qualifying session that is in question, failure to provide the required sample would mean it was null and void and he'd be put to the back and, just to make it interesting, if the qualifying wasn't regarded as being legally completed, he may need the steward's clearance to start?

F1.com has
"Note - Hamilton originally qualified on pole, but was excluded for failing to return to the pits under his own power and supply a fuel sample. Karthikeyan failed to meet the 107% requirement so races at the stewards' discretion."
It also has
"McLaren’s Lewis Hamilton has lost his pole position for Sunday’s Spanish race after the stewards found his car did not have sufficient fuel onboard to comply with the rules on its return to parc ferme.

As a result of the decision, Williams’ Pastor Maldonado inherits pole for the Barcelona round, whilst Hamilton - excluded from the qualifying results - will start from the back of the grid.

The Briton stopped out on track at the end of Q3, after being told over his radio to pull over at the side of the circuit. There wasn’t enough fuel in his MP4-27 to enable it to return under its own power to the pits and for the team to provide a one-litre fuel sample in post-qualifying scrutineering, as required by the sport’s technical regulations.

After a lengthy discussion, the FIA stewards rejected McLaren’s case - put forward by team representative Sam Michael - that the car had stopped out on track due to reasons of ‘force majeure’.

"A team member had put an insufficient quantity of fuel into the car, thereby resulting in the car having to be stopped on the circuit in order to be able to provide the required amount for sampling purposes,” explained the FIA in a statement. “As the amount of fuel put into the car is under the complete control of the competitor, the stewards cannot accept this as a case of force majeure.

"The Stewards determine that this is a breach of Article 6.6.2 of the FIA Formula One Technical Regulations and the Competitor is accordingly excluded from the results of the Qualifying Session. The Competitor is however allowed to start the race from the back of the grid."

I would say the Stewards got it wrong for the initial disqualification but called it correctly regarding the 'force majeure' protest - bit of a SNAFU all round, I'd say