Q: Engine misfires when weather is humid. on 2002 Mercury Villager

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The engine misfires in rain or just when is very humid. Runs great when dry. Changed the plug wires, roter and dist cap and still have same problem.
(1) Answer
Make sure the wires you purchased are high quality or even better yet - original equipment wires. I still think you may be experiencing some arcing from the wires. The key thought here is the moisture content in the air when the problem occurs.

To confirm, get a spray bottle with water and while the engine runs, spray around the plug wires. See if the car stumbles.

Beyond that, you could have an injector or harness issue.