I have a 1999 Blazer with the 4.3 W engine. This all started this last winter when there was an ice storm. The next day it wouldn't start up, instead would crank and crank, sometimes backfiring, acting like it wanted to start but would only violently fire once, even ether didn't do anything. I unbolted the Ignition control module, and had it tested. By the time it got tested it was warm, and tested good, so I bolted it back on and after some long cranking it fired up. I ran it fine, but ever since it idles at 550 rpms and kinda rough. Also the transmission, on even small overpasses on an express way, will kick out of TCC and sometimes won't even hold in 4th gear for longer than 5 seconds and immediately downshift to 3rd (I upgraded the 4th gear servo, no help). It also has been getting horrible fuel milage, close to 10 mpg on the highway, I was getting 22. I have replaced the ignition module, coil, and TPS. I did replace the poppet spider injectors with the upgraded style as well, both thinking it would help and because the old poppets were leaking. I've pretty much ran out of ideas without just throwing parts at it and hoping it works. I have thought of flashing the computer, or it either being a weak fuel pump or a bad crankshaft position sensor. Once again, I just don't want to be throwing these parts around and hoping for the best. Any help would be appreciated.

 

Has it thrown any codes? If so, those will help with diagnosis.
If not, a shop with a scanner like the one in the video below will be able to diagnose the rough idle/mileage issues.
Chev / GMC Misfire, poppet injector diagnosis, troubleshooting and replacement - YouTube

My gut says that if the fuel system checks out, vacuum might be causing the rough idle/stumbling. It has happened to me before.

Just my $0.02.

 

Well honestly I'm only 15 and know little about cars, but my friends trailblazer was doing the EXACT same thing... Low rps at idle, it would run at a low gear and burn up the gas, also his sounded weak too.. they got it checked and the catalyctic converter was clogged/bad so that was creating back pressure on the engine which gave you the low idle rpms.. so once they got it fixed it worked fine and dandy again so might wanna check the catalyctic converter..

 

I would also say check the catalyctic converter.

 

It sounds like your engine is running in "safe" mode. The electronics aren't sending the PCM good readings (or any readings) so the PCM is dumping fuel into the engine and pulling a ton of timing causing rough idle and poor mpg. Any check engine lights? Could be a bad knock sensor or O2 sensor. The transmission is downshifting because you're engine is not making any power due to the excess gas and loss of timing.