Misfire Issue when it rains

StephenCasiday · Dec 25, 2023

Hi everyone, I have this issue where when it rains fairly heavy my truck starts freaking out and starts misfiring very bad on Cylinder 6, i’ve checked the plug it’s solid, the wire is on as tight as it can be and the coil pack wires are binded or showing copper anywhere. Where should i start looking to try and figure this out? I haven’t seen anything about it online yet, but something is getting shorted and i have no clue where to start. Thanks. -Stephen
2005 Tahoe LT3 4x4 5.3 4L60e

OR VietVet · Dec 25, 2023

It sure sounds like you need plug wires. If they are original, replace them with Genuine GM wires. For giggles, use a spray bottle under the hood at night and spritz the wires with water and see if you see a light show where the spark jumps out. It should not.

lyncht1967 · Dec 25, 2023

swap the 6 coil and wire with cyl 4. if problem moves you have a coil or wire issue. if so, switch wire back to 6..if it returns back to 6 its the wire. if it stays on 4 its the coil.

Scottydoggs · Dec 25, 2023

at night in the good and dark, use a old spray bottle on mist, with water in it, and spray the coils wires and down by the plugs. 90% of the time you'll see sparks jumping from them metal plug boots. why i toss them right in the trash if they are still on a car i buy.

if that dont yield anything. before dumping cash into it, swap the coil with the one next to it. see if the miss comes back or moves. if no change swap the plug wire and lastly pull the plug out and check it. if its not clearly fouled out, look for a black line in the ceramic part of the plug. they can crack and misfire then.
coil packs also crack, so inspect it when its removed for cracks. as well as for bare wire off the plug for it.

StephenCasiday · Jan 24, 2024

Scottydoggs said:
at night in the good and dark, use a old spray bottle on mist, with water in it, and spray the coils wires and down by the plugs. 90% of the time you'll see sparks jumping from them metal plug boots. why i toss them right in the trash if they are still on a car i buy.

if that dont yield anything. before dumping cash into it, swap the coil with the one next to it. see if the miss comes back or moves. if no change swap the plug wire and lastly pull the plug out and check it. if its not clearly fouled out, look for a black line in the ceramic part of the plug. they can crack and misfire then.
coil packs also crack, so inspect it when its removed for cracks. as well as for bare wire off the plug for it.

I did this, but nothing happened, i figured a faulty knock sensor was shorting and nope, i replaced them both and it’s still doing it, and only when it rains

Scottydoggs · Jan 24, 2024

you swap the coil plug and its wire with the cylinder next to it one by one too?

StephenCasiday · Jan 24, 2024

i am today i’ll get back to you when i do

Scottydoggs said:
you swap the coil plug and its wire with the cylinder next to it one by one too?

Doubeleive · Jan 24, 2024

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?

StephenCasiday · Jan 24, 2024

Doubeleive said:
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?

Could you elaborate a little more on where specifically to look? and it’s only Cylinder 6 that misfires when it happens, and it bogs the truck down horribly, and the splash guards are the wheel well plastics? if so they’re all there, everything is on the truck besides the factory steps, but I don’t know where to look for the ecm or bcm, and the harness is still wrapped up pretty well from what i can see, a lot of the wires around the throttle body are exposed though, the plastic grommets are mostly gone up there

Doubeleive · Jan 24, 2024

StephenCasiday said:
Could you elaborate a little more on where specifically to look? and it’s only Cylinder 6 that misfires when it happens, and it bogs the truck down horribly, and the splash guards are the wheel well plastics? if so they’re all there, everything is on the truck besides the factory steps, but I don’t know where to look for the ecm or bcm, and the harness is still wrapped up pretty well from what i can see, a lot of the wires around the throttle body are exposed though, the plastic grommets are mostly gone up there

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

StephenCasiday · Jan 24, 2024

Doubeleive said:
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?

Doubeleive said:
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 · Jan 25, 2024

Doubeleive said:
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 · Jan 25, 2024

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 · Jan 25, 2024

lyncht1967 said:
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 · Jan 25, 2024

lyncht1967 said:
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 · Jan 25, 2024

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 · Jan 25, 2024

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 · Jan 25, 2024

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 · Jan 25, 2024

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 · Jan 25, 2024

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

Misfire Issue when it rains

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