1998 GMC K1500 5.7 vortec 4 speed auto 4x4
Howdy, I was taking the truck out for a day trip into the mountains the other day, and on my way back, the service engine soon light came on. Came on while I was driving in a normal fashion, nothings special occurred that I know of. When I got home, I plugged in my harbor freight scanner and it says I have a p0300 code, which from what i see is a random misfire code. Correct? I started trying to look into it today, but I am a bit confused because the truck seems to be acting just as it has for the past number of months. I don't personally notice any misfire to write home about, but maybe it is just so slight I don't notice? The truck has nearly 220k on it now so its no spring chicken. I checked the things I could think of, but I have not found anything obviously wrong. I started the engine and used one of those laser thermometers to check for even heat on the exhaust manifold. From what I could see, everything was even. Cylinder 3/5, and 4/6 do share a runner in the manifold, but they matched side to side. I pulled all the plugs one at a time (they are not very old either, I don't think they even have 1000 miles on them yet, and I am 99.9 % sure they are the correct plugs, I was very carful to get the right ones) and to my eye they looked very good. Nothing to write home about. I also checked for fuel pressure, and I get 55 psi. This truck has made 55 psi for a long time. I know because I have had to check it multiple times over the years doing lots of other repairs. Unfortunately, this truck seems to break a lot. I don't think the previous owner was very good to it. But hey, what do I expect. It was my first truck, I have had it for 6 years now and probably put over 35,000 miles on it.
My scan tool does not really give me much more help than the p0300 code. I can not find a way to see if the computer is seeing a specific cylinder causing the problem or not, which isn't helpful. It will show me the freeze frame data though, but at least to my novice eye, I don't see anything.
The only other "clues" I have that I can think of is when I go into the "OEM enhanced" scan for GM on this scan tool, it lists a p0605, U1026, U1042, U1064 & U1193. I am not sure if this is just gibberish, but it seems the computer thinks it has lost communication with some other stuff.
Other than the light coming on, nothing new has come up on the truck.
It has had a bit of an intermittent long crank problem for a couple of years now, but for the life of me I have not been able to track it down, and trust me, I have tried! I am not sure if that is, or is not related just because it has been an ongoing issue for a long time now.
I hope that is enough detail to get started with. Here is hoping it is something simple! I am getting really tired of working on this truck all the time.