Pontiac Repair: Car Stumbles and shuts off, water pump replacement, o2 sensors

Question
QUESTION: I have a 1997 Sunfire w/ the 2.4L engine and recently it will stumble and shut off after 20 to 30 seconds of run time.  it will start right back up like nothing happened.  just spent over 200 dollars replacing the O2 Sensors and cat because of the ceck engone light on and the code the computer spit out.  Had autozone reset the computer.  went back to autozone a couple of days later and there were no codes in the computer nor does the check engine light come on.  They were hinting at a new PCM.  It does idle rough when placed in gear and will smooth out a bit when placed in park or neutral.  it idles about 800 to 900 rpms.  I have replaced all ignition parts i could think of (new coils, plugs, wires etc..).  Compression is fine (just replaced the head and some other internal engine parts about 2 weeks ago).  i have noticed that the roughness and the engine shutting off got worse after 1/2 tank of fuel was used.  before that it would only idle a little rough.  once the rpms are above 1100 it smoothes out and seems to be fine.  Could this be some thing as simple as a fuel filter or is it another issue with the fuel system or could it be a sensor malfunction?

ANSWER: Hi Jeff:

What internal engine parts were replaced? anything to do with the timing chain? if so I would look there first. these engines if the timing is off will have a bad erratic idle and lack power. VERY common after water pump replacement. What was the service engine light on for before? What code was stored? I don't know if the low on fule has anything to do with it or not. You may want to do a fuel pressure test to see how much pressure the pump is putting out.. It's a place to start.. Good luck :)


---------- FOLLOW-UP ----------

QUESTION: Yes i replaced the timing chain,  timed it and then turned the engine over 3+ complete revolutions and checked the timing again. it was dead on.  the other parts replaced were the pistons, rods, rings, replaced the head and head gasket set,  new intake and exhaust gasket.  He is something else i noticed, if i remove the vacume line off the pressure regulator nothing changes in the way the engine is running; is this normal?.  the code that was stored before was a p1133, and a couple others indicating the O2 sensors.  I do not have access to a fuel pressure testor nor is there any test valves on this system.

Answer
Well.... first off taking the vacuum line off the regulator you may not see any difference in the way it's running. I would still look long and hard at the fuel pressure. I would look at the vacuum line going to the MAP sensor it could be cracked or kinked. If you have o2 sesnor codes then you could have a fuel trim issue which is the computers control of the fuel delivery. I would look and make sure all the ground wires are sight and clean. give the wires a little tug on the engine to transmission studs and see if they are loose or break off those are for the computer and the 02 sensors. with out the use of a scan tool It's really hard to tell you what is wrong...