What causes shaking, gas smell, white puffs of exhaust?

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My Chevy Express van 2000, was running very well, has good engine 350 v8 After taking out the cluster instrument for testing and replacing it, drove away and then the van began to shake - worse when at stop light, thot it was out of fuel, put 60 Litres in, shaking worse, white puffs out of tailpipe, gas smell inside coming from vents. It was a hot day, lots of stop and go, AC on max. Drove 1 hr that day, and the next still all these symptoms, no AC on. Mechanic did some checking around, didn't find much except 1 cylinder running slower. 1 week later started it up and no shaking, drove around the block still no shaking or gas smell. (tho he didn't fix anything!) Any ideas what was wrong? and if it will happen again? Thanks! M What seems to make the problem better or worse? worse when hot? full tank? How long have you had this problem? few days??