When we get a bad storm with a lot of rain My 98 5 liter sierra misfires like hell
also when I put on the air it runs reallly rough. I changed the cap,rotor and wires and still no change.I was wondering wht next, coil,plugs or what have you any suggestions would be greatly appreciated thanks Ralph-o.

 

Did you change the coil wire as well? That seems to be an issue on these along with the 4.3l trucks. Also I would change plugs as well as one may have the porcelain cracked. Also, even after changing the cap and rotor, take it off again and check for condensation or signs of crossfiring. There's a TSB out on the distributors where the vent holes on the bottom of the distributor get plugged with **** and then the inside of the distributor can't breathe. You get moisture built up inside and then things begin to fail. What you're supposed to do is locate the 2 holes in the base of the distributor and drill them out to clear any garbage also it'll knock out the screens at the base. Once you do that you shouldn't have the problem again.

 

I know on my camaro, my o2 sensors would cause my car to missfire when wet.