hard cold start

[jowiliam]

Problem solved.
Answers here:
The hesitation at wot is NOT in relationship with the cold start issue.
I have fixed the cold start issue.

There was water in fuel.
This water was caught in the fuel pump basket and froze sometimes, making difficult or impossible to suck gas and deliver proper flow.

Flooding condition turned out to be an illusion. Spark plugs were wet but lack of fuel couldn’t make the engine running. That (no flood) condition was hard to diagnose because of wet plugs.

I had to start over my investigation from the beginning as I probably miss something on my way…

Before that, I tested several sensors and parts (coils, ECM, Mass air flow …) by putting them in the freezer to see if they would show any sign of failure while installed in the warm car sitting in the garage. I did not find anything there.

Returned to the basics:
Hooked up a fuel pressure gauge (again) and this time, it was colder outside (-12). The no start condition was steady. The engine fired up and died immediately. Fuel pressure dropped to zero for a few seconds and turned back to normal after a while. I was so happy to see that after all!

Took rear fuel pressure (bypassing the fuel pump relay under the hood), same condition. That told me it was the fuel pump.

When I pulled out the pump, its strainer was stuck in ice/mud/slush. I dismantled and clean everything in the pump housing and reinstalled it. Put a new fuel filter. Fuel pressure was then back to normal and the engine started perfectly.

I kept my fuel pressure gauge hooked up for a road test and found that fuel pressure was dropping around 29-30 psi at WOT.
Changed the fuel pump. Fuel pressure at WOT is now 55 ish PSI. But the hesitation is still there with DAS light flashing. I’ll probably start another thread for this issue.

I hope this can help someone. Thanks to jimmy57 and alschnertz for your input.

cheers!