Here goes, I own an 89 rs with a 350 (5.7) TBI when I bought the car it ran fine I drove it about a year I got home from work one night and popped the hood for an unknown reason. I saw the maniflod was leaking so that weekend me and my dad replaced the gaskets and it was still running fine. well a few months went by and I parked it for about two days. (running fine when I parked it) got in it to go to work and it would start fine idel fine but when I would start off it would do fine until I started to get in to the throttle about 1/4 of the way. So me and me dad noticed that the temperature was a little higher than normal and thought it had a cracked head. Since I wanted to rebuilt the engine in the first place, I did. here I am a year later finally have it finished and back in the car. I still have the same problem. I have changed the fuel filter, map sen, distributer, and frankly I'm totally mind boggled. Think you can Help??? Thanks in advance, -Jeremy

 

Sounds like someone swapped a 350 under the stock 305 TBI setup, without changing the injectors, chip, and knock sensor.

Start here, read through this thread. It will awaken you to the problem.

https://www.thirdgen.org/forums/tbi/...-350-swap.html