Misfire Issue when it rains

[StephenCasiday]

if it only happens when it rains and not ANY other time cold/hot/dry
then I would be looking less at a plug/coil and more at perhaps a scuffed wire that shorts when wet, ecm/pcm harness, under fuse box, etc
look around and feel around the wires and see if one has been rubbing against something and has exposed bare wire. get under the vehicle and look up for the same with a flashlight.
are all of the splash guards in place?

THE PCM (COMPUTER) is mounted in the left front of the engine compartment, has 2 big multi wire harness's connected to it. It is metal and clips into a big plastic holder
View attachment 419722

i’ll check this and do my coil swap

 

[StephenCasiday]

THE PCM (COMPUTER) is mounted in the left front of the engine compartment, has 2 big multi wire harness's connected to it. It is metal and clips into a big plastic holder
View attachment 419722

i checked the pcm, no broken wires, just dirty, and i swapped the coil and wire and it still misses on 6

 

[lyncht1967]

hmmm...at this point i would check the integrity of cyl 6 coil connector plug and the 4 wires i believe it has feeding into it. you may have to take some tape off and see it there is a chafe...perhaps seal up where wires enter connector with some sealant..from your symptoms....fine when dry...misfire when wet...you must rule out moisture getting into connector and coil signal wires or an exposed wire in the wrap that may gain the tendency to short when moisture is introduced...

 

[StephenCasiday]

hmmm...at this point i would check the integrity of cyl 6 coil connector plug and the 4 wires i believe it has feeding into it. you may have to take some tape off and see it there is a chafe...perhaps seal up where wires enter connector with some sealant..from your symptoms....fine when dry...misfire when wet...you must rule out moisture getting into connector and coil signal wires or an exposed wire in the wrap that may gain the tendency to short when moisture is introduced...

i will try and look, i did a spray test, and it’s also raining, but i sprayed water all around the pcm, and all around where 6’s coil is and the plug and wire and even back behind the intake where the lines run and nothing

 

[StephenCasiday]

hmmm...at this point i would check the integrity of cyl 6 coil connector plug and the 4 wires i believe it has feeding into it. you may have to take some tape off and see it there is a chafe...perhaps seal up where wires enter connector with some sealant..from your symptoms....fine when dry...misfire when wet...you must rule out moisture getting into connector and coil signal wires or an exposed wire in the wrap that may gain the tendency to short when moisture is introduced...

also, i’ve noticed when it starts misfiring so badly, it will throw P0300, and say random misfires detected, i’m wondering if there’s a short in one of the harnesses, the truck almost stalled yesterday from how badly it was misfiring

 

[Doubeleive]

Use a meter or lamp to check if there is voltage to the coil pack/injector on #6 if not then there is break in the harness somewhere

 

[Scottydoggs]

pull #6 plug out and check it over. might have a crack in it. pull the one next to it too. compare their condition. could be a bad valve.

 

[strutaeng]

I'll echo what everyone else has said. It's looking like a wiring harness issue, perhaps.

I would get myself a wiring diagram and install temporary wires for the coil and injector for #6 all the way back to the fuseblock/ECU by back probing. Do the ground as well. Doesn't have to be pretty, just make sure connections are good. Then drive it like that until it rains. May require trial and error to find the offending wire.

Wires in the rust belt are known to be a real pain in the neck with all the corrosion. Often, they can still conduct 12V but not much amperage across the corroded area so a simple 12v check is not enough and can fool you. Maybe your issue IS as the connections?

You could also just tear into the harness for inspection. Just get new loom and some electrical tape to redo it.

 

[cjmcglaughlin]

Have you tried going to a car wash to see what happens? My truck will miss on cylinder 8 when going over the underbody sprayer, and will do so until it dries out. I replaced the plug wire but haven’t went back to the wash to test if it worked yet.

 

[lyncht1967]

yeah, the spray test is just to show if spark is jumping out...wont show problem with wires to coil connector..except maybe to cause a misfire...need to check coil signal wires in harnes and i believe each bank has common ground...you can try splicing into one ground wire with a piece of wire taken to a good block ground (anywhere on block, this will effectively give that whole bank a new ground) to rule in or out faulty ground..this has been common issue on these models before