UPDATE!!Changed the MAP sensor and no constant misfire on cylinder 4 any more. Now it's down to a P0300. Plugs and coil packs changed 12k miles ago. Any suggestions?
GM had a bulletin on the rain water getting into the spark plug wells years ago. The fix was to put a length of weatherstripping under the seal at the back of the hood. I bought the "kit" from a dealer but there was not much to it.Had the same issue a few years ago on my 4.2 with #1 spark plug. Wife was parking nose down in the rain and water was accumulating on #1, coming off the windshield, shorting out the system. Fixed it with a new spark plug, lots of dielectric grease and getting her to park nose up.