P0300 Random Misfire Intermittent - PCV Valve

[Wiseguy11]

Wanted to share my P0300 Random Misfire story:

I bought a 2002 GMC Sierra 2500HD 6.0L 4x4 267k miles for $2k with Meyers plow. Truck was in great shape with some surface rust on the undercarriage. Bought it from a tow truck driver that plowed in the winter, but needed the cash. Started up and showed a CEL light and rough idle. There was a fuel line leak noticeably under the driver side door which severely rusted lines which the previous owner stated was why the CEL was flashing because he ran it and it was a misfire on cylinder 1. I drove it around for about 5 minutes and checked 4x4 which worked great on Low and High. The brakes were very squishy though  having to push to the floor to stop completely. Seller said the brake lines were just replaced as well as a newer transmission. It shifted better then my existing vehicle so gave the benefit of the doubt. The plow worked great without skipping a beat. However, the truck died after running for about 30 minutes and the battery icon showing. Battery was brand new (date sticker showed it) so i expect the alternator. The seller wanted over $3k for the truck but got him down to $2k with plow and to tow it to my house. I brought a friend whom stated i was taking too much a gamble, but i was optimistic. 

 

Anyway, got it back to my house and got a Duralast 130amp alternator for $120 at Advanced Auto. Installed it same day and got the truck up and running and the CEL cleared! I bought complete set of Nylon Inline Tubing fuel lines (pressure,return,evap) for $157 and replaced the old steel lines with Nylon making sure i ran it in line with the old lines and removed the steel lines. I checked the brake lines which looked newer. I checked the master cylinder for the brakes which reservoir was filled past max which led me to believe that the seller never bled the brake system. I tried to bleed the system, but the bleeders were stripped.  I bought 4 reman calipers with new break pads (rotors were fine) for $175 total excluding core cost on Rockauto and swapped them out and bled the system and the braking increased significantly. I was so excited to get the fuel line, alternator, and calipers fixed so the truck was 100% operational, OR SO I THOUGHT!

 

THE MYSTERIOUS ISSUE of P0300-

Turned over the engine and everything was okay for the first 5 minutes, then the CEL came back on for about 30 seconds and then turned off for a few minutes and then came back on. I plugged in my OBDII reader which gave me code P0300  Random Misfires AND THATS IT! The cabin was shaking a lot while the CEL was on, but i could still accelerate without any operational issues and cabin shaking would stop. I saw that there were tons of reasons for a P0300 and hoped to get a specific cylinder misfiring. I thought it might have been a fuel pressure issue since i just replaced the lines and my buddy mechanic thought fuel pump or fuel injectors got rust in them from the bad lines, or an ignition coil or spark plugs or wires went bad. I gave it a few days to see if anything changed. After a few days, i turned over the engine and saw smoke coming from the engine bay (but no CEL light)! i checked and saw that it was something burning on the exhaust headers on my drivers side. I checked online on a bunch of forums where it got me to find it was the valve cover that was leaking on my drivers side only. i ordered a new valve cover gasket and spark plugs since they were inexpensive on Rockauto for $45 total since if the oil from the valve cover was getting on the plugs it could've also caused a misfire. 

 

FIXING THE P0300- (PCV valve)

I replaced the valve cover gasket and replaced all 8 spark plugs and turned over the engine to see that there was no more leak! I was really excited until 5 minutes into the engine being on the dreaded P0300 came back with a fury! This time it came on and never cleared. The engine was idling very rough and when i tried to accelerate it bogged bad and i wouldn't dare try to drive it anywhere under that condition. My friends told me it was a blown head gasket, but there wasn't any smoke from the exhaust outside a normal vehicle. I was stumped! Then, i started thinking why the CEL light came back and why it couldn't clear as well as why my valve cover started leaked on only one side and not the other. Then, i realized there was a little part on a hose connected to the valve cover and i looked it to find it was a PCV valve. I took it off again and cleaned it and could hear a rattle when i moved it, but the issue didnt stop. I looked back on forums which said my symptoms matched the PCV vale being bad. I ordered a new PCV valve for $6 at Advanced Auto. I turned over the engine and waited 15 minutes for the engine to warm up to operating temperatures and NO CEL! I drive the truck down the street and no issues and no codes! I was stoked because i took a gamble on the truck and it paid off!

 

Conclusion-

If you get a P0300 random misfire, remember this is due to either an fuel, spark, or air (PCV valve) issue. I was about to replace the fuel pump, spark plugs cables, and test fuel injectors, cylinder compression, and coil packs, but before going into those routes just think what parts are affected because it could be as simple as a $6 PCV valve. All in all, i spent around $500 in parts for a working 2500HD that KBB for over $3,000 with current miles so im happy i could at least make my money back and more now. 

[David89GMC]

Thank you for sharing your experience with us. I think you got a great deal on that truck, about a $2K below my '03.

I also had a P0300 code when I first got the truck. It went away after I cleaned contacts, replaced corroded fuses and cleaned the throttle actuator and sensor.

Thank God these trucks don't have any more electronics than they do.