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Help, my truck won't run when it's raining/wet outside

Hi guys, I've been having a problem with my truck not wanting to start/run in the rain. Sometimes I can get it started with difficulty, but the last time I did that and tried to drive it, it shut off and let me sit. It works perfectly fine in dry weather so I am a little confused. I thought it was just the ignition wires getting wet, but now I'm not so sure, they don't really get that wet and I did spray them with coating. Any ideas what could cause this? The only thing I changed on the truck was I removed those pcv valve things on the water inlet and put plugs in because they didn't have any hoses going to them anymore, and they were snapped off where the hoses used to go. I figured those were some emissions thing I didn't need. Was it a mistake to eliminate them?

1977 f-150 400
the problem could be anywhere in the ignition system. i would start at the dis and work my way back. doing 3 things at each connection. 1) pull test on the electrical connector to make sure it locked together. 2) take apart the connector and look for signs of corrosion and water. 3) on each side of the connector give hold the body and give the wires a firm but light rug (about a 10 pound pull). 4 look for any signs that the terminals in the plugs are spread or dont contact each other well.

i would also look for signs if water getting into the dis cap, loose conection of the coil, and inspect the wiring , including grounds (dont for get the engine block ground strap) for loose connections, broken/ punctured insulation, etc.

if you cant find the problem that way. carry a test light or cheap volt meter ($10-$15 will get you a very useable one) with you and check for a dim light or low voltage at the coil, and power feed for the ignitor mod.

with a volt meter you cant also test each connector you open up for resistance to ground, you should get some form of open, O.L or 1m ohm plus of Resistance from any non ground wire to ground. i say 1m ohm because some things like have mild electronics in them and give very high ohm reading when the working right .1- 5 ohms is acceptable for any wire from point to point/ connector to connector or ground wire to frame. i do like to see less the 2.5 ohms on all non ground wires.
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i should also say good luck. you have the toughest type of electrical problem, but systematic troubleshooting with a volt/ ohm meter and a good eye will solve 9 out of 10 electrical problems.
Thanks, I will get on that. Looks like I've got a lot of diagnostic work ahead of me. I should also mention that I have a Mallory unilite distributor, so my ignition system is far from stock.
Yep, it's like figure 1. Looking at that now, I think I may actually have a wire out of place. Unfortunately I have to work until 11 tonight, so I won't be able to get to the truck until tomorrow. Thanks for all your help, I will definitely put it to use.
Thanks for posting that diagram! I know I looked at it before, but for some reason I had the red wire on the wrong side of the resistor. I checked all the connections and they seem fine, I drove it in the rain today, and after switching that wire it's running great!

Big Jim F150

73-79 Ford Trucks Rock
I've made a mistake like that before, but with wiring going to the starter solenoid, so we both made mistakes. Glad to hear that your truck is running great, and to hear that your problem was nothing more than a simple wire being hooked up wrong.smiliegitrdonesmilieFordlogo
that good stuff. i love the simple fixes. smiliepeelout

i do this for a living and still make mistakes, its just got to run right be for it leaves.