Hi, I am new to this sight and have searched everywhere to find an
answer to my problems.
Here is my problem. When it rains or is very humid my truck runs terrible,
hesitates, surges, misfires... and gas milage drops 30 percent.
My engine light also comes on.
WHen everything dries up, it runs excellent and my check engine light
goes off.

I have had numerous scans at several different places, performed. They all come back with misfire codes, egr codes, plug/wire/cap/rotor codes.

I have replaced the plugs, cap, rotor and wires (2) times, no help.
I have repleaced the EGR sensor, no help
I have installed a new fuel filter, no help

What can be the problem? Can anyone help me?

Phil

What year? engine?

Here is the missing information:
It is a 1999 with the 350 vortec engine

check for a leaky intake manifold gasket or anything of that nature. Maybe It's sucking up unfiltered wet air. Just a suggestion since these things have a history of leaky plastic intake manifolds. Hope I helped.

might want to have the ignition module checked