P0300 - after car wash

DeanHensler · January 25, 2022

I've got a 2003 2500HD with the 8.1L. I am the original owner and have taken exceptional care of this truck. Lately, I have been getting some extremely random misfire conditions. I changed the plugs and wires at 100k (18 months ago), the truck now has 123k on it. Within the past month it has done this three times. The first time it was just a cold morning and I started the truck and immediately it had a rough idle and I could tell while driving it, it was down on power. This lasted about 5 miles and once the truck got warm it went away. The second time it happened I had already been driving the truck for about 30 miles, including towing a camper and I stopped at a car wash after dropping the camper. Immediately after the car wash (this was a handheld power washer) it started misfiring again. I hooked up my ODBII scanner and it pulled a P0300 code. Since then I've made a 1200 mile round trip drive into the Rocky Mountains and had zero issues. Today the same thing happened. I was towing a trailer about 20 miles, dropped the trailer off and went through a drive through car wash. After the car wash it immediately started the misfire problem the rest of the drive home. After sitting for an hour I went out and started the truck and it ran fine.

I'm uploading some video's and will post them as replies when youtube finishes.

Any thoughts what might be causing this? I opened the air filter box and while the air filter seems dry, there was a little bit of water in the bottom of the air filter box. The top of the engine was completely dry.

DeanHensler · January 25, 2022

CamGTP · January 26, 2022

If the problem happens after things get wet, it's obviously an issue with water getting in or on something.

P0300 means random cylinder misfire, so it's pretty generic as far as the description goes. A handheld scanner or nice scan tool that can show each cylinder would be wanted to see what exact cylinder(s) are the cause of the code. This would make it easy to figure out.

Could be as simple as a bad plug wire that is arcing out because of the water, could be a coil pack doing the same thing as well. I like to test some of these things at night with a spray bottle full on water and the nozzle on a mist. I spray the wires and coils and look for a spark, makes it pretty easy to tell what the problem is.

vucelick · January 26, 2022

14 hours ago, DeanHensler said:

I've got a 2003 2500HD with the 8.1L. I am the original owner and have taken exceptional care of this truck. Lately, I have been getting some extremely random misfire conditions. I changed the plugs and wires at 100k (18 months ago), the truck now has 123k on it. Within the past month it has done this three times. The first time it was just a cold morning and I started the truck and immediately it had a rough idle and I could tell while driving it, it was down on power. This lasted about 5 miles and once the truck got warm it went away. The second time it happened I had already been driving the truck for about 30 miles, including towing a camper and I stopped at a car wash after dropping the camper. Immediately after the car wash (this was a handheld power washer) it started misfiring again. I hooked up my ODBII scanner and it pulled a P0300 code. Since then I've made a 1200 mile round trip drive into the Rocky Mountains and had zero issues. Today the same thing happened. I was towing a trailer about 20 miles, dropped the trailer off and went through a drive through car wash. After the car wash it immediately started the misfire problem the rest of the drive home. After sitting for an hour I went out and started the truck and it ran fine.

I'm uploading some video's and will post them as replies when youtube finishes.

Any thoughts what might be causing this? I opened the air filter box and while the air filter seems dry, there was a little bit of water in the bottom of the air filter box. The top of the engine was completely dry.

Did you use OEM spark plugs? Is the 02 sensor the original one? If you used OEM parts and the 02 sensor is new, re-check the previous work the make sure that is not the problem. My 8.1 was doing something similar and new OEM plugs and a new 02 sensor fixed it.