This is on my mom's 1998 chevy Z71 4x4, with a 5.8 liter Vortec. Problem is, when driving for a while, and when you put your foot into it, say on a hill, the check engine light comes on, and the thing just dies out, little power, if any. Let it sit about 15 minutes, and it starts, drives fine. If you drive it and park it overnight, the C.E.L will go out in a day, until it wants to cough on another hill/load situation.
Fuel filter/air is good/new. Fuel pump makes little noise, if any, but that was my first suspect. Mechanic is saying it's getting an air lock. How and what to do to solve the issue, or what else could it be? It's a total new engine last year, maybe 8,000 miles on it now.

 

This sounds like a fuel flow problem. . . I had a problem with my 97 SFI Vortec a few years back. Apparently the over use of fuel additives had dissolved my fuel regulator causing a rough idle that sometimes resulted in the engine dying. This can cause engine misfiring which can be detected specifically by a diagnostics computer from your local auto store. You should also check your induction system to make sure you're getting good airflow (poor airflow causes the engine to run rich and burn excess fuel)

 

Check your fuel pressure,it should be 50 to 60 psi.The injection system used on the vortecs have the electric injector that opens and closes but the difference from other systems is they have a poppet type spray nozzle (like a diesel injecter that pops off at a certain pressure).If your fuel pump is geeting tired, when you put the extra demand for fuel on the system the pump can't keep up.the poppet nozzles will not work under 46 psi. Chevy's weak link is there fuel pumps, they fail frequently and the vortec pumps are very expensive.Also they had alot of problems with the poppet nozzles failing, you might try a quality fuel inj. cleaner to help prevent any problems as the injector system is also a pretty penny. hope this helps