White smoke would be very worrisome, but I can't imagine it being caused by bad fuel. The result of bad fuel would be plugged filters sometimes even to a point that the engine no longer runs.

It's time to investigate. Start by checking the radiator for traces of oil and the dipstick for traces of coolant. Smell the exhaust to check for an antifreeze smell. Also inspect for leaks. It might be time for a compression check as well.

Hope this helps,