I just bought another truck after crashing my previous '94 Sierra. It's a 2007 Sierra with only 108K miles on a 4.3L engine. It's a completely different truck from what you have but the symptoms are amazing close to what you are reporting.
My '94 had an OBD I system so reading the codes didn't do much good. I managed to drive it for 257K miles without using a code reader and my '07 wasn't throwing any codes anyway so I just started up the parts canon. I replaced the IAC (had a lot of carbon on it), the CST (also had a lot of carbon) (the CTS by the thermostat housing but on the '07 there's only 1 on the driver's side of the head), and since it seemed to be carbon related I went ahead and put 20 ozs of Seafoam in the gas. That cleared up most of the problems.
I checked the fuel pressure and it was right at 60PSI, maybe 59. I also cleaned the MAF sensor by soaking it overnight in 90% alcohol and then letting it air dry. Interestingly, once I got it back together it started throwing a MAF sensor code. After I drove it for a while that cleared up on its own.
I was looking for the EGR valve but didn't find one. I'm not sure whether of not it has one. Where it supposedly goes there is just a straight pipe that comes out and runs somewhere down the driver's side of the engine. If you have any info on the EGR on an 2007 GMC Sierra 4.3L Classic I would surely appreciate getting a copy from you.
Since it's running OK I haven't done much to it lately.
I did have one bizzaro thing happen. I came out one morning and it wouldn't start - dead and I mean DEAD battery. I put on my thinking cap on and started to believe it was something to do with the engine computer. I put the battery charger leads on the correct posts (red to positive and black to negative) and DID NOT plug it into the wall outlet. I also did not disconnect the truck's battery cables from the battery. I left it like that for about 2 or 3 minutes, disconnected the charger leads, and tried it. It fired right up! I went to the auto parts store and got a new battery and have not had any problems since.
My '94 had an OBD I system so reading the codes didn't do much good. I managed to drive it for 257K miles without using a code reader and my '07 wasn't throwing any codes anyway so I just started up the parts canon. I replaced the IAC (had a lot of carbon on it), the CST (also had a lot of carbon) (the CTS by the thermostat housing but on the '07 there's only 1 on the driver's side of the head), and since it seemed to be carbon related I went ahead and put 20 ozs of Seafoam in the gas. That cleared up most of the problems.
I checked the fuel pressure and it was right at 60PSI, maybe 59. I also cleaned the MAF sensor by soaking it overnight in 90% alcohol and then letting it air dry. Interestingly, once I got it back together it started throwing a MAF sensor code. After I drove it for a while that cleared up on its own.
I was looking for the EGR valve but didn't find one. I'm not sure whether of not it has one. Where it supposedly goes there is just a straight pipe that comes out and runs somewhere down the driver's side of the engine. If you have any info on the EGR on an 2007 GMC Sierra 4.3L Classic I would surely appreciate getting a copy from you.
Since it's running OK I haven't done much to it lately.
I did have one bizzaro thing happen. I came out one morning and it wouldn't start - dead and I mean DEAD battery. I put on my thinking cap on and started to believe it was something to do with the engine computer. I put the battery charger leads on the correct posts (red to positive and black to negative) and DID NOT plug it into the wall outlet. I also did not disconnect the truck's battery cables from the battery. I left it like that for about 2 or 3 minutes, disconnected the charger leads, and tried it. It fired right up! I went to the auto parts store and got a new battery and have not had any problems since.