I have bought my '01 Blazer off of FB Marketplace with 209k miles., and I have done nothing but dump money into it with the amount of problems its had. Finally I've reached the end of the tunnel, and just recently put in brand new Standard Motor Products FJ503 fuel injectors since the old ones were at the end of their lives, and caused misfiring and all of that. The first week after installing the injectors, it got great mileage, around 17 MPG. The next week, it got 15.9 MPG. The next week, 13 MPG. Now this week its getting 11.8 MPG. I use the trip button on the dash to record how many miles I drove to do calculations.

I'm at a brick wall with this thing, its not smelling like its burning gas, or running rich. Its not throwing codes for anything else, I just replaced a downstream O2 sensor since that was throwing codes on and off, but that was a few days ago and it still didn't increase mileage in the slightest. I've been driving the same route between work and home, and its fairly hilly, but I haven't given it more gas up the hills than I usually do, I don't think. We replaced the sparkplugs, wires, and distributor. It has new fuel lines, fuel tank, and a fuel pump. We replaced the MAF sensor, and some other sensors on the motor. We tried to run cleaner in it with the previous injectors, and I don't think it'd make a difference with the new one in. I don't know if older cars like this need their computers to get used to running a new type of injector, since it is a MFI conversion from the old spider injector. Any suggestions?

 

Don't get caught up in the details of thinking injectors, you may have second problem like seized brake caliper, it ran well initially.
Check simple things first like vacum leaks, PCV, air tube between intake and left valve cover, no leaks betweem MAF and intake, plugged air filter, EGR (if you have one) and etc.
Put vacum gage on manifold. Its possible intake gasket slipped out of place. Spray carb cleaner around gaskets while engine running to check.
Can your scanner read fuel trims and O2 sensors?
The right and left trims should be fairly close.
same for right and left O2 sensor ratios.
This should help you narrow down which bank is problomatic. Lots of youtube videos on reading fuel trims and O2 sensors.
​​​​​ECM learns fuel trims almost instantly after changing injectors, this isnt an issue.
ECM records short and long term fuel trims. If your having an injector issue, the trims will show it.
The 3rd or after cat O2 sensor has no impact on fuel strategy. Its only job is to notify that catalytic converter is not working and sets check light.
All 3 O2 sensors are the same. You can move them from one location to another to test them or confirm diagnosis.
Assume you have converted to EFI injectors?
In worse case senario switch back to old SFI injection system.