91 ST185 ECU

I bought the car used, previous owner said a shop told him it needed an injector for cylinder #4.

I did a full tune up, had the injectors cleaned, and still running on 3 cylinders. We traced the issue back to the ECU, which isn't sending out a signal for the injector to pulse.

The current ECU in the car has an ATS rom tune, according to the previous owner. I have not removed the ECU yet to check.

Could the rom tune cause this issue?
If I get another 91 USDM ECU, can I swap the ROM chip to the new ECU to carry the tune over?

Find the bad transitor aka injector driver

Pull the ECU and inspect the solder joints. There may just be a cracked one that would explain the problem.

If the actual injector driver transistor is bad, it's not too hard to replace them. The hard part is finding a comparable transistor without knowing exactly what Toyota (Denso) used.

Also make sure you have 12V on one wire on the injector with the key in the on position. check continuity on the other wire to the ECU. Dont do surgery to your ECU if your wiring is bad