1991 F150 won't start when it has been raining...

Edit: I have found the last couple of long hard rains here my '91 F150 Lariat won't start, it turns but will not fire up. The battery gauge shows below 12vdc but I know the battery is good. The truck starts up any time it has not been pouring rain. It does't happen on a light rain, it has to be an all day rain.
The plugs and air filter are new 3 months ago, I am not sure where to start....Someone mentioned it could be cap and rotor b/c that is what happens with their Jeep.

Thanks in advance for any help!

Scott

Voltage on battery shows 12.4vdc when off, 14.0vdc when started so the alternator and battery should be fine.

i had the same problem with a 92 ford f-150. I found that the ign coil was the cause. When the weather is clear start the truck and, and using a spray bottle spray on the coil. If the truck dies, then start there. I hope this helps, If any questions feel free to write back.

Thanks for the tip. I replaced the cap and rotor and it is running real nice now, it rained yesterday and it still started so I am feeling pretty good that it is fixed...if not I will check the coil next.

Have a good one!

Melbs