Anyone swapped a 4.3 TBI to 5.7 TBI please read

Has anyone done this swap without switching to the 5.7 wiring harness?I did this swap in my S10 last summer retaining my 4.3 harness and just using the ECM from the 5.7. The wiring harnesses appear to be identical on paper besides for the A/C wiring. With the 5.7 ECM it runs rough- either to rich or lean when the engine is cold (open loop) but runs fine warm. If I plug in my 4.3 ECM it runs fine cold and warm- but I know I shouldn't used this computer. All the sensors between the two engines are identical. Somebody else has to have done this swap in a full size- anybody keep the 4.3 harness and have problems?

the coolant temp sensor is different on each engine. try getting one for the bigger engine.

I'm guessing that the same goes for a 305? I bought a "warmed up" 305 out of a 92 camaro to install in my '88 1500. It came with all the wiring harness and 'puter. Do I need to strip out the old harness and then install the one from the camaro, or just plug the 'puter into the old harness?

you need to look at the wiring diagram as you did and compare. if they are the same you should be ok as far as the wiring goes. but as far as the sensors go you have to use the proper sensor for the engine you are using. if the numbers on the parts match up your ok. if not then you cannot use sensors for a 4.3 on a bigger engine. you just can't fool the computer when you use more air and gas. it will run too rich or lean or both, or have the wrong fuel parameters or spark advance.

Make sure you are using a throttle body off a 5.7. The 4.3 and 5.0 have different rated injectors

I do have the throttle body off the 350. I'm convinced its something with the ECM but I can't figure it out.

Did you change the whole computer or just the chip? I'm not sure but I think the 350 computer has different part #. Also make sure the chip is correct for the transmission ( auto or standard). I had a friend who just switched to the 350 in a full size truck a few months ago and when he went to the chevy dealer they gave him the wrong chip and it wouldn't run right.

I am using the whole ECM and it was used with a 700R4- which I have. I have checked every sensor, all the wiring, fuel pressure, timing, compression, injectors, and pretty much everything I can think of.
Also I have switched electronic spark timing modules to the one from the 350.
Everything is identical to what was connected to the 350 wiring. The only thing different is the harness itself- it's for a 4.3. It does run pretty decent when in closed loop (warm) and I have no check engine codes coming up. Anybody have anymore ideas?