Chevy Silverado wont start

I have a 2001 Chevy silverado 1500 4x2 with a 4.3l. it had been sitting all winter and ran in the fall i had tried starting it but it just does a little diesel type action once i let off the ignition. I checked for fuel and spark and im getting both. the truck has 69k miles on it. i also dumped 10 more gallons of fresh gas in the tank but that diddnt help any, could it be a weak coil or the timing belt had slipped a notch? any help would be apreciated