FAA advised revised fuel sampling procedure before preflight.
With the airplane in the normal ground attitude and starting at the highest drain location, check all drain locations for contaminants before every flight, whether or not refueling has occurred. Have fuel sample disposal provisions, proper lighting, and a small ladder at your disposal to properly check for fuel tank system contamination.
• Drain at least one cup of fuel (using a clear sampler cup) from each drain location.
• Drain the fuel strainer as required to completely flush its contents in each of the fuel selector positions.
• Check for water, clarity, cloudiness, haze, proper fuel type/grade (i.e.; 100LL is light blue in tint, jet fuel is clear or yellowish), odor, or other contaminants.
• If any contamination is detected in the fuel tank system, thoroughly drain all drain locations again.
• If contamination is observed, take further samples until the fuel appears clear, and gently rock the wings and lower the tail to the ground (or raise the tail and let back down on tail draggers) to move any additional contaminants to the drain points.
• Take repeated samples from all drain locations until all contamination has been removed. If contaminants are still present, do not fly the airplane.
• Have qualified maintenance personnel drain and purge the fuel tank system. Remove all evidence of contamination prior to further flight.