how to clean out sugar in a fuel tank
Remove, drain, clean, and replace. By a professional, preferably.
How do you know that there is sugar in it?
What’d you do? Who did you anger? 
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Removing your fuel tank is a pain. The good news is that if the 2006 model is like the previous generations of 3-series BMWs and it is not an ultra-low emissions version, you can pop up the back seat and remove two large caps that will give you plenty of access to the two lobes of the fuel tank through which you can clean it out. This should be done by a professional with spark-free tools because it is rather dangerous. I would also replace the fuel filter which is probably located beneath the car, under the driver seat,
The good news is that sugar is not the instant death sentence to an engine that it is reputed to be. It is not good, and I would clean it out, but it is not as bad as most folks think.
Everything I have read about sugar from a credible source says it doesn’t dissolve in gasolin and jusl lays inert in the bottom. Has the car stopped running?
I believe it dissolves in ethanol however so in that case, some percentage of it will be consumed if left in the tank…
And as a hydrocarbon, the sugar will burn in the engine. I wonder if using E15 for a couple of tanks would accelerate removal?
Tester