I have a 01 blazer ls 4x4 4.3 and it has a rough idle.
It only runs rough at idle. Ive cleaned maf sensor, throttle body, iac valve, checked for vacuum leaks, changed spark plugs and wires, rotor and cap, air filter, and fuel filter. There are no engine codes. It is quit at idle no valve train noise or leaky exhaust no smoke. It just seems to vibrate vehicle at idle. I checked fuel pressure and it is 52 psi with key on ignition off and when i turned key off it didnt lose pressure so im stumped.

 

Check motor mounts

 

mine does this when any 02 sensor is going out