Help 2004 Chevy Silverado 2500 HD-P0300

I have Silverado HD with a 6.0 in and it is reading code P0300 and is backfiring and misfiring.We have replaced the crank shaft sensor and had it recalibrated , both knock sensors, spark plugs and wires, cleaned the injectors, replaced the intake manifold gasket and still it is misfiring!! We took it to GM to have a diagnostic test done on it and they were just guessing and of course were WRONG, infact the truck ran worse after we picked it up than it did when we dropped it off, it never backfired until they touched it. Needless to say I'm tired of wasting money. ANY SUGGESTIONS?? PLEASE HELP!!

What did they do it? All the things you listed? Do you have anyway of seeing what cylinders are doing the misfires? P0300 is really no good to go on.

No thats all it reads is P0300 and the dealer wasn't supposed to do anything but a diagnostic test on it and I did all the other repairs!! If there was water in my gas tank would it come out on its own or would I need to add a treatment? I've gone through at least 2 tanks since the code came up!

I would think two tanks of gas would do it. You can take a fuel sample and see what it looks like. Another thing to consider is the ethonal content of the gas. I dont know if this is flex fuel or not(I dont have then VIN number) if there is too much ethonal it would run lean and misfire and so on. It would be better if you had a scan tool to see what cylinders are doing the misfiring the lost. There are a couple more things it could be but need more info.

I took it back to the dealership again and they claimed that they looked at it but everytime I drove by it was still parked in the same place!! They said they think it's something internal but won't look without charging me more. At night time we can see sparks coming off all of the coils which to my knowledge is not normal, I know they spark but this is not normal sparking. What else do you need to know and what do you think it could be??? Thanks for your help!!

No you should not see spark from the coils at night! Do you see it at the coil at the valve cover or at the wire?

It's at the valve cover! What could cause this?

Hi:
My story is about parallel to your P0300 code too. I have a 2001 Silverado 2500HD with the 6.0L V8
I brought my truck into a service center last fall, and two fault codes, one being a vacuum leak and the other a misfire (assuming this particular code). They suggested a fuel system service and an intake manifold gasket, much like your circumstance. Well, thought the $550 was well spent and went about my business. The engine service soon came on again pretty much after this service was done. Was kind of pissed at first, thinking $550 "solved" the problem, but realizing it could be something else. So instead of returning to the service station, I bought my own OBD code reader. And sure enough the P0300 code is there. I replaced the plugs, and still got the code error. I just ordered high quality plug wires hoping that this will solve the problem. If not, I'll probably suspect the quality of the gasoline. In metro milwaukee, wi the government mandates what is known as "reformulated gasoline". And many people have suggested that this type of gas is harder on gasoline engines than "non-reformulated" gas. When this gas first became available here, I made the effort to drive 20 minutes out of my way to get the non reformulated gas across a county line. But over time, I became less motivated and "lazier" and succumbed to the reformulated version that is so convenient to obtain nearby. If I discover its the quality of this reformulated gas that poses the problem, I'm going to beleive what everyone else has advised me on. I will likely invest in returning to my old ways of driving out of my way for non reformulated gas. In the meantime, at 101,000 miles I'm likely due for new plugs and wires anyway, so the investment is worth it for the immediate diagnosis. I just hope I haven't burned an exhaust valve by prolonging needed service.

No visual sparks are not normal! IS the truck equipped with coil on plugs or does it stuill have a dist cap and wires! If the wires are original then change them out for a good quality set with the correct plugs.

This problem was that the chevy dealer cracked two of my plugs in the process of doing the diagnostic testing. Replaced them a third time and this did the trick!

I have the same story and symptoms with my 2003 Silverado 2500 HD 6.0 with 89,000 miles. It returned the P0300, random misfire code. It started to flash the code during 55-60 mph. It recently started to get worse and happens above 35. Temperature outside or of the engine does not matter. The interesting thing, on another thread a user mentioned, his battery died around the same time. Mine also died around the same time. I have short somewhere, since I measured .5 mill amps draining somewhere. Fuses are ok, I was more focused on the misfire, maybe I will focus on the short. I have replaced the plugs and wires with ACDelco. Note, I replaced the plugs 1 year before this started to misfire, during my regular maintenance.
Last night, I replaced the intake gasket per an ALL DATA service bulletin. Referring to the original ‘orange’ gaskets have had issues, and to replace with the newer ‘green’ gaskets. This would make sense for misfires if there would be an air leak. Sure enough same issue, however, it only misfires again at 55-60, like it got ‘a little’ better. The fuel pressure is good and holding (60psi), fuel pressure value works. The O2 sensors seem to fine, the numbers are not off. I am unable to graph the curves, so it is hard to be certain. It starts immediately when you turn the keys, so I doubt it is the coil. I guess my next step will be to locate the short, maybe take some of the plugs out (inspect) and re-seat all the plugs. I will also try running the engine in the dark and see if I see sparks. Each plug has its own module.

Thanks,
Sebastian