I have a 90 xj 4.0 ltr. Itstarts and runs, with a skip in the idle randomly. In the winter it starts fine, and in the summer as well. Button very humid mornings it will crank untill the battey is dead and wont fire up. After it sits and the weather dries I can start it right up. It has 240,000 miles on it and I dont know if everything under the hood is original. I have cleaned the power terminal on the battery and the main power distribution point, I have also checked under the distributor cap for moisture and found none. I dont know where to look, I thought about changing the coil but I wastold that the distributor could also be the problem.
Anybody got some suggestions?

 

you need to go through and replace the ignition components one by one starting with the oldest and you will fix it. the humid mornings are letting the spark jump somewhere and its not making it where it needs to go. I would start with the coil, coil wire, and distributor and my guess would be that it would fix it.

 

I forgot to say that I have replaced the distributor cap, rotor, plugs and wires.

 

The porcelain resistor on your front left fender by your air box is notorious for cracking and causing this. Also the Crank position sensor could be going out, but i'm not sure if the moisture would affect it that way if it was.