Hello there. New to the forums. Hoping to find some direction. I have a 1987 firebird with a 305 TPI automatic. Came in for a hard starting issue. Did a tune-up, pplugs, wires cap, rotor, pcv valve, air filter. Tested fuel pressure and found a low reading. Replaced fuel pump and now have 50psi at key on and 43 when at idle. But still have a hard start issue. Car will start but then stumble and stall. Will restart but will do the same thing. sometimes it will idle, but suddenly run rough and stall. Did check for codes. None. Check engine light does not come on at all. Did test the TPS. set to .54 and also set base timing to 6 degrees per spec. Ohmed all injectors. All in spec. Did try running car without MAF connected and car ran very similar. Verified Cat is not plugged. Replaced o2 sensor. Car will eventually idle but then have a bad hesitation when accelerating or revving engine. Has no bottom end power and sometimes feels like it shudders. Also noticed that the cooling fan would come on when the key was turned on, but the engine was cold, like sitting overnight. Did check datalogging and did not see anything out of range. I am stumped. Here is the kicker. Swapped out the ecm with a scrap yard one. was told it was an aftermarket ecm and the prom would have to be swapped. So the prom was swapped and installed ECM back in car. Car started and idled beautiful. Throttle response was perfect. Took it for a test drive to get the idle learned and ran great. Still a lack of power on bottom end, but was an improvement from original test drive. went back to shop for a bit, and went to go for another test drive and as I was backing out, the car started acting up just like earlier described. Not sure what else to do. Please help!

 

If changing the computer changed anything, I'd look into bad connections to it, grounds, even that second ECM being problematic. That's weighing heavily on changing that ECM actually being the reason it worked. You may also have more than one problem.

 

Your fuel pressure is high for a stock engine, it should be 33-35 @ idle and 43-45 @ WOT.
When you did the datalog, was the car running poorly?
I hate telling people to just throw parts at it but since you have already I suggest a new ICM. I have seen these go bad in different ways with no consistency.

 

I had a similar problem with mine. Pulled all the wire loom off which looked new and found all kind of pinched and burned wires. Fixed all the wiring issues and still had the same problem. The shorts must have killed the ecm because when I replaced it the car ran great afterwards.