Hey my 89 camaro 305 tpi is running rough( it shakes at around 1500-2000 rpm) and it feels really weak(no torque). The ECM is throwing no codes but the standard code 12 but clearly something isn't right. What kind of things will the ECM not pick up that can cause these symptoms?

 

Best advice to give without the car throwing codes is hit all the basics, ive had alot of my TPI cars stumble over the smallest things. Plugs, wires, cap and rotor, Ignition module/connector, fuel pressure, ohm the injectors, inspect your wiring at the MAF sensor, inspect the burn-off relay and wiring for possible corrosion or the like.

This doesnt necessarily mean start throwing parts and money at it,
a thorough inspection of all the main ingredients is always the best way
to start a diagnosis..

I hunted a misfire/stumble for 3 weeks, kept telling myself the plugs
and wires are good to go. Finally, I upped and replaced them and the
car never ran better, go figure.

 

If the ECM isn't throwing a code, then it isn't detecting any faults with the sensors it's reading. That doesn't mean that something that can't affect the sensors isn't causing this. I would say do a basic tune up and go from there.