I have a 1997 Sierra with the 4.3L Vortech engine. A couple days ago my FIL went out and the truck wouldn't start. The next day i tried it and it acted like it wanted to start but then died immediately (it did backfire a couple of times). Today my FIL goes out and it starts but is running really rough. He drove it and he said that it was missing and "loping" at idle but seemed like it was doing fine at speed. After driving it for about 15 miles the Check Engine light came on and it seems to be running fine now.

The fuel pump was replaced about 10 months ago so I don't think that's the problem. It had an almost full tank of gas.

Any thoughts on where I should be looking? It has been extremely rainy and wet around here lately, is it possible something got wet and once it dried out things are fine now? Do these engines have a "cap and rotor"?

I have a 98 Sierra with a 4.3. I was having the same problems and i had to replace the rotor cap and button. Dont buy an aftermarket one thought, it wont last. I put on an after market one and it lasted 6 months. Then i replace it with an Ac Delco part and it worked great.