02 Blazer runs rough/misses when it rains

[Bigfoot]

Its a 2002 Blazer 4.3. It has new distributor, dist. cap, dist. rotor, plugs, wires, and a known good coil off of another truck, and fuel pump. the problem is it will run very rough almost to the point of losing speed unless WOT, it will do it when it hasn't been running for about an hour when its wet outside or if i even park on top of a puddle. It will run rough until you go WOT then it will clear out and wont do it anymore until you let it sit again. it throws a p0300 code, multiple misfire.

I can have the truck running and spray the whole motor down with water, the top of the gas tank and the whole underside of the truck, and it WILL NOT miss or run rough. Only when it rains.

I am very confused on this

[CRed05]

MAFS?

[Bigfoot]

I cleaned it and it had no effect and when hooked up to a scanner it seems to be operating correctly

[mjrsplat]

i had this issue in a blazer i used to own. check positive wires to the starter.

[Bigfoot]

i had this issue in a blazer i used to own. check positive wires to the starter.

 

Were they loose or were they corroded? Cause I know that mine are not corroded. I will have to go out when I get off work to look though

[Nickey4271647545519]

i had this issue in a blazer i used to own. check positive wires to the starter.

 

wtf? how would that cause a miss?

[Bigfoot]

wtf? how would that cause a miss?

 

That's what I was thinking but at this point im up for any possibility

[mjrsplat]

wtf? how would that cause a miss?

 

The casing on the constant positive wire for the starter was old and had hairline cracks in it.When it got moisture in it, the moisture caused it to ground out, causing low voltage spikes resulting in a random misfire. It only happened when it rained, or when the humidity was high.

 

FWIW it was the strangest issue I had on that vehicle, and it took forever to diagnose. The diagnosis came from an old hillbilly that owned over a dozen blazers that were like mine.