Re: closed vs open loop
 

Take a plastic hammer (or small ball peen) and tap lightly but firmly on components such as ECM, MAF sensor, coils, module(s), relays, etc.

Check grounds and harness connections everywhere.

Also tap on and otherwise shake or vibrate the battery.

Re: closed vs open loop
 

GM recomends the "slap test", at least on my last trip to the GM Training center. May recomend mallets now, but I would be surprised.

Re: closed vs open loop
 

The slap test works okay, on the ECM. The right hammer tap (and no, I'm not advocating beating it to death) will often expose a lot of components that have an internal fault, due to cracked boards, bad solder joints, loose pin fit, and various other "intermittents", especially those that show up on rough road, or under various sharp maneuvers.

Re: closed vs open loop
 

I was working on a Ford a few years ago which was acting the same, the DPFE valve would short and loose the 5V reference to the other sensors, causing it to stall. No codes either. Unplug it and try.