Been having issues for a while now without any codes. Symptoms include misfire under load and thats about it. Occasionally the SES light would flash while misfiring going up a hill on the freeway. Hills in general is the only time I would have the misfire, or getting on the freeway.

Anyway, did a light tow of a small trailer and it got really bad. SES flashed but when it was done it did not stay on. For sharts and giggles I decided to do a scan and codes P0171 and P0174 popped up, remember the SES Light DOES NOT stay on while running. Like I said I've been having this problem for the past year and a half with no avail in fixing it. Since it started I have done:
Plugs, wires, cap, rotor, air filter, fuel injector, fuel filter, fuel pump, intake gaskets. I've also swapped multiple sensors from another blazer to test if I had bad ones (I didn't).

I've looked up some probable causes and solutions to these codes, most I have done (replace parts, sensors). I tried testing for a vacuum leak but couldn't find anything. I also replaced O2 sensors 2 years ago (before the problem arose).

Hoping someone can maybe give me a few more ideas more direct to what I have going on, thanks.

 

Can you please give us also the year, engine, transmission of the blazer. From the codes it is an OBDII but then all are after 1995.

 

Scan for pending codes in same key cycle as the misfire. Bet you will have a pending misfire code. That might help give additional info. Also check cam sensor retard with a capable scanner and make sure is zero ±2° - this being off can cause those pesky misfires..

 

Its an 04. 4.3 with 4l60e

 

Have you read these two threads:

P0171 Explained

P0174 Explained