[Team ZR-1]

You sure this is not a low fuel pressure on cold start ?
Have you measured and seen if the PSI is around 45 PSI ?

Have you done a leak down test by once warmed up measure fuel pressure then with hood up shut down
and then record how long before pressure drops and what that pressure is ?
How old is the fuel filter, possible clogged reducing PSI or volume ?

I assume the GM ECM calibration is within a E-prom and if so Have you switched around those
in your different ECMs and seen if one ECM functions better with a certain one of those E-proms

Have you looked closely for any type of air or exhaust leak ?

What does this engine have, multi fuel injectors or some MFI type carb ?
Measure that air temp heater is getting hot enough ?

 

[Team ZR-1]

38-40 PSI fuel rail pressure seems low and at what temp or RPM is that ?

Is there a fuel regulator, and is it vacuum controlled, if so check that hose of leak ?
Is regulator adjustable, and if so what happens if railing fuel rail PSI to more like 45 PSI ?

When starting cold and idle is high about 1,200 RPMs something is driving ECM as to timing or
what it command to the fuel injector pulse width ON times

Check vacuum, what is reading when cold started
Do leakdown of fuel pressure, with 8 injectors must have a fuel rail and it should have a valve to
screw on a fuel pressure tester to assure it is not what the issue is and also assure the regulator is
working correctly or adjusted for proper PSI

If there is some type of intake leak which will effect vacuum then ECM may be fooled as to what engine mode the ECM is

If fuel system and injectors not a issue then how about getting or making a smoke machine and looking for small leaks

 

[Team ZR-1]

Looking at the website you point to it shows what looks like max fuel rail pressure is
you could do the test they stated :

7. Since diagnosis this far has failed to find a reason why the fuel pressure is low, begin to suspect that the pump or the regulator may be the problem.
With the engine at idle, pinch off the fuel return hose. This action is how the regulator raises the rail pressure.
If the pressure does not increase to at least 46PSIG, a defective chassis pump is indicated.

Also I see this has a MAP sensor, check for a hose leak/vacuum or test to see if MAP is faulty when cold
No way of knowing with those ECMs if they in fact were setup for your engine, model or year

Get tests out of the way if it is a fuel, MAP or intake leak, using the diag flow of that website you use

MAP voltage values should pin down if there is some vacuum leak (inverse of MAP) or again use a smoker

 

[Team ZR-1]

What you need to find out is what MAP is reporting and follow the diag process has for MAP testing on that website
to determine what MAP reports on cold idle and if fuel system is functioning correctly

 

[Team ZR-1]

I am thinking that it is possible the fuel pressure was normal after warming up, but maybe when it hasn't run for a while the pressure is low.
I haven't had it checked under those conditions.

I read the in tank pump has a filter. And, since the in tank pump hasn't been pulled out, I assume the old filter may be clogged.

I think that is the next thing to go after.

When the car was at the dealer last summer they drained and filled the tank which they said smelled like turpentine.

The technician wrote on the receipt, I have to drive it more.

I suggest then your next task is to do what is needed for the in-tank pump and filter and then test your fuel rail pressure
during cold start