I have a 1999 Chevy Silverado V6 pickup truck. It starts and runs very well, except when the weather turns wet. After a hard night of rain it refuses to start. When I finally do get it to start, it runs rough for the first 5-10 minutes, then everything is perfectly OK and it runs normally. The problem started about 1-2 months ago. I've been told it's likely the distributor cap or something in the electrical system, but the cap was replaced along with the wiring about a year ago. It's a pain to get to, so I haven't removed it yet, but from what I can see it appears fine.

Any ideas?

Thanks

Clean the spark plugs so they are free from grease/dirt on the outside. Occasionally the dirt is enough to allow the spark to track to chassis. Occasionally happens in older cars, maybe that's the prob. Worth a look

Thanks for the response-I'll check it out, but the plugs were also replaced within the last year, and I'm not sure how that would relate to problems only in wet/damp conditions.

Keep spark plug wires clean and spray them with silicone spray. It helps prevent the electrical system from grounding out, due to moisture. Electricity finds the shortest path to ground and moisture can allow secondary voltage to discharge somewhere besides the plugs. It's also good technique to use silicone dielectric grease on spark plug terminals and other key electrical connectors; e.g. ignition control modules. Just don't use it on any sensor connections, especially oxygen sensors.

Distributor caps are not airtight sealed so it can be somewhat common the symptoms you are having in the wet weather.
You likely have the electronic HEI {high energy ignition} distributor. It may not be easy
To get at but periodically you need to remove the cap and check the condition of the
Brush,rotor, tower terminals,connections at the ICM {ignition control module} {igniter}
And where wires enter the cap.
It only takes a little moisture or corrosion at these areas to cause problems.
Also as mentioned above use dielectric grease at electrical connections and plug terminals.