I have a 1991 firebird 5.7 My scanner showing I need a coolant temperature sensor. I've replaced it 3 times with no luck. What should I do next

What operational symptoms are failing? Overheating? Temp gauge readings? low coolant level? If you have replaced the sensor 3 times, it is getting a false trigger from somewhere else I would think. By the way, who did your conversion to a Vert? You could not buy a formula, or for that matter, any model convertible with a 5.7. Was it a customer order direct to ASC or another company? Just curious. My '91, 5.0L, TPI, 5 spd was all I could get when I bought it. still runs great at 162K miles.

It's a 350 out of another car that's been bored .060 ,cam TBI. It runs great until it reaches 165 degrees, then it starts running real bad and dies. I replaced the sensor connector also. All sensors are new, along with a new coil , plug wires, plugs, distributor, I'm thinking I might need a new ECM computer.

Maybe your engine uses a different sensor than the original engine and it is not compatible with the current ECM and electronics in the new one.