Wiring harness differences?

[Vortec350S10]

I recently swapped a '90 350 TBI engine into my '89 S-truck. I had a 4.3 TBI to begin with and when I made the swap I kept the 4.3 wiring harness and plugged it all into the 350.
All of the sensors for both engines are all identical part numbers:
MAP sensor
02 sensor
TPS
IAC
Knock sensor
EGR-(not used)
Coolant temp sensor
VSS

The ECM is from the 350 as are the throttle body, distributor, Electronic Spark Control Module (ESC).

I have the wiring diagrams for both ECM's and they seem to be identical on paper. The engine runs OK except for when it is cold- when the ECM is in open loop. It requires a lot more throttle than when in closed loop.
It idles fine but if you are idling and then give it a quarter throttle and hold it there it will quickly rev up to about 2000 rpm and then start running rough and slow down to about 1400 rpm.
I have checked the timing advance and that doesn't change when it runs rough so I know it has to be the fuel mixture. When driving you can feel it switch to closed loop like a light switch and you have to let off the gas about a half inch to keep it going a steady speed.
I know this is a very complicated problem that not many people have faced so if anyone even knows of a site to find wiring diagrams or any more help I would really appreciate it. Thanks for reading.

[Jeremy-WI]

You could try autozone.com and enter the vehicle information, then click repair info, vehicle repair guides, then choose what you want. I think the info is same as Chilton manuals. You might want to check fuel pressure

[Ali Croff]

You need to check your fuel pressure at the intake when the truck starts to run rough. You might not be getting enough pressure, in that case, you need the fuel pump from the 350. I had a similar problem in my Toyota.

[Vortec350S10]

I checked fuel pressure and it's right at 10psi. Specs say between 9-13. If I run it on my 4.3 computer it runs fine in open loop and I can't even tell when it changes to closed loop. I know it does cause I had access to a scantool at one time. It has to be something with the 350 computer. I got another computer for a 350 from the junkyard and it does the same thing- regardless if I put in my aftermarket PROM or stock PROM. Any more ideas?

[Ali Croff]

Try the mass air flow sensor off the 350, not the 4.3

[Vortec350S10]

neither have a mass air flow sensor- that's only used in multiport fuel injection or tuned port injection

[DrKiss_7]

How does it start when cold? Does it fire up on command? Is the idle rough? Like it is slightly flooded?

[Vortec350S10]

it starts up just fine and idles at about 1200 for about 20 secs then idles down just like it should- runs nice and smooth