I didn't want to spam the board with a new message, but it looks like I've narrowed down the problem. I did some more reading and ended up pulling the 4wd fuse on a whim, and the blazer did drove normally. I stopped and popped it back in and it went to driving like crap.

So I'm figuring it's something on that fuse (PCM, Transfer Case Shift Control Module, Transfer Case Shift Control Switch, Front Axle Vacuum Solenoid, Front Axle Indicator Switch.

1. I put the fuse in and pulled out the 4x4 push button selector and it drove like crap. I put the selector back in and unplugged the orange plug on the TCCM and it ran fine (service 4wd lit up).

My next move is to replace the TCCM this weekend. I think the TCCM is "working" because I can hear the clicks and clunks of shifting into 4hi, 4low and 2hi. Could it be the Solenoid or Indicator Switch? Or possibly something unrelated-but-related? I don't know how the PCM is effected by pulling that fuse, so that's the bottom of the list right now.

Thanks.


The short of it:
My 2000 4.3 Blazer with 103K miles will not shift while under load in overdrive--in fact it's pulling timing and dropping rpm/power. If i start in first, I can run up the gears just fine with proper acceleration and no issues. If I need to downshift I can do it manually and it is fine. There are no codes pending or stored, no SES light at all during any of this.

Work done:
Plugs/Wires/Cap/Rotor
PCV
TPS
Transmission Filter
Fuel Filter

The long of it:
I'm having an issues with my 2000 Blazer. I had an issue with it sometimes pulling timing and not running correctly but it was not a constant thing, and it would go days/weeks running fine and then start acting up again. I've done the basic stuff of plugs/wires/cap/rotor, pcv, fuel filter, tps, and trans filter. I took it to a shop thinking the knock sensor might be bad (ran fine for a week on 91 octane), but he ran it for codes and said the only thing was a stuck open egr valve. He cleaned it and it seem to run fine for a while, but would bog down when it was warmed up. I could put my foot in it and it would not gain any rpm. I put in some Lucas fuel system cleaner and it seemed to run fine until I got down to about a 1/4 of a tank 87 octane gas and it's back to the original issue. I've tried different gas stations and there seems to be no constant. I got a fuel pressure tool and it ran 54psi at the test port with the ignition in the Run position and that's as high as I would get it. I have not had a chance to check the pressure at the filter. I do not have a "hiss" of pressure release when I take off the gas cap. I've looked at some of the fuses mentioned in other threads and they appear fine. It has also become more stubborn to start-up, requiring a second or two to catch vs immediately in the past.

My issue now is, it will not shift while under load in overdrive--in fact it's pulling timing and dropping rpm/power. If i start in first, I can run up the gears with proper acceleration and no issues. If I need to downshift I can do it manually and it is fine. There are no codes pending or stored, no SES light at all during any of this. I have not taken it to a transmission repair shop as the closest one is a trip down the highway.

I've read a lot of the threads dealing with transmission issues and the common 4L60e problems. I think I'm down to a bad fuel pump or leak, wiring issue, PCM (#22 common problem), or ??? I know I don't know a lot about this stuff, and hoped someone with more experience and knowledge would be willing to help with my specific issue and point me in the right direction. At this point I feel like I've thrown a bunch of stuff at this, and I'm missing what is actually causing the issue.

Thanks.

Edit:
And as the norm, I got home today, took it for a drive, and everything was fine. Yesterday, it ran like crap as stated above, and today it ran fine...Lets see what tomorrow brings.