Jeffrey majeske

I am having a similar issue with my 08 Sierra 5.3. started with p0307, misfire in 7th cylinder. did all the normal things still misfired. Took it to dealer, they said they think valves on 7 aren't seated well, needs to be machined but they can't do it. Took heads to machine shop, good guy, told me there's not enough material to machine, I needed new heads. Heads were $1600 but i didn't want to go through the trouble to have DOD problems down the line so I bought a remand motor which came with heads for $3200. Installed the motor, new knock sensors, crank position sensor, crank shaft sensor, already replaced all injectors, plugs, MAP and MAS sensors. After all that, I am having the EXACT same code p0307, now with a p0300 (random misfire) in-between. Its driving me crazy!!

Thank you for stopping to read my post and potentially add vital information, I am so grateful for any information I can get on the matter. So I own an 2008 GMC Sierra 1500 2 wheel drive. Issues started mid way through 2020. I was driving home one night and I started to feel the truck misfire. Shortly after that, the check engine light started flashing confirming what I was feeling. I got home pulled the codes and had a P307 code. Misfire in the 7th cylinder. At this point I started with the basics. I switched coil packs to see if the problem followed the pack but it did not. I then replaced the injector with a brand new ACDelco injector. No change. So then I replaced all of the spark plugs/wires, no change. At this point I decided to take the truck to the local GM dealer to have them look at it. They had the truck about a week and told me they did all the same things I did and that and concluded that they believe the heads needed to be machined. They told me that they believe cylinder 7 valve might not be seated properly and that I should do a valve job. They said they could replace the motor for 7K or I could take it and have the heads machined because they couldn't do it there. Heres where things get interesting. When I went to pick the truck up it ran worse than when I brought it in, I couldn't even cross the street without the truck stalling. I had it towed back to my house and went to work myself. After a little research I found that a faulty MAP sensor could cause the issues I now seem to be having so I changed the MAP and MAS sensors with a new ones and it immediately helped with how the truck ran. It started running much better but I was stilling having a misfire issue. I then removed the heads and took them to a machine shop where I was informed that there was not enough material to have them machined. The truck has 100K miles but the shop said they looked like they had 200-300K on the heads. At this point I needed to buy new heads. Heads were around $1600 so I decided to get a remand motor for $3200 from rock auto which came with new heads. I replaced the old motor with the rebuilt, new water pump, harmonic balancer, all new injectors, plugs and wires. after I got everything buttoned up and fired her up I had THE SAME ISSUE. P300 (random misfire) at first but then it went back to P307 (7th cylinder misfire). So at this point I replaced both Knock sensors. I also have a Diablo computer that can show 7th cylinder misfire counter and it continues to count misfires every time I run the truck. So your all caught up. I have a new, remand motor, new knock sensors, new harmonic balancer, new plugs, wires, injectors, MAP sensor, MAF sensor and I am still having a 7th cylinder misfire which also disables stabilatrack. I would LOVE and insight I might be over looking or suggestions of things I can try next. Im thinking crankshaft sensor or PCM (Computer) next. HAS anyone reading this ever had a problem like mine? and Info? PLEASE I NEED YOU and YOUR KNOWLEDGE