My new to me 96 Tahoe 5.7 vortec is having serious running issues. Runs rough, backfires (popping sound) coming from the exhaust, the exhaust smells heavily of fuel and is white and thick. Oil smells like fuel. Sometimes it will idle normally and sound fine, no weird noises, but if you give it any fuel it will start popping and running rough again. And don't even think about turning on the a/c, the vehicle immediatly cuts off as soon as the compressor tries to kick in. The exhaust and cat is new. Fuel pressure regulator is new. Cleaned the maf and the egr. New plugs and wires. I realize there are several things that could be wrong or a combination of things but I am hoping someone can give me some good ideas or a starting point as I don't want to chase this problem. This vehicle sat for 6 months before the problems started so we added a can of seafoam and some fresh gas thinking it may have some bad gas but no change. Help if possible please, thanks.