I have a 2000 cavalier 2.2l 4 cylinder with a trouble code P0171 system lean. Replaced MAP sensor spark plugs are new but they are now fouled. There are no air leaks i replaced the fuel filter and it still pops that code up. Idles rough low power when accelerating only when under 3000 rpms. After over 3000 rpms it accelerates like normal. 02 sensor number 1 is pretty new. only had it for about a year. Aftermarket headers and intake system. Brand new head, springs, and valves. Cleaned out the throttle body. Still no change

SOURCE: PO171 OBD-II Trouble Code Engine Rough Idle When Under Load

Check EGR valve. Also check all injector pulses with NOID light especially when vehicle is stumbling. It may be computer related. Get back to me if you need more help. Good Luck

SOURCE: PO171 OBD-II Trouble Code " System Lean To Bank 1

Check PCV valve and be sure it is not stuck open.  Check for vacuum leak at intake manifold.  

SOURCE: 92 Honda Accord boggs down when throttle is pressed at idle speed

try unplugging the egr vacuum source and plug the line and drive. If no difference you ruled out the egr as being the issue.
Please do not rate this solution since by not seeing the car and doing my own test there is no way to tell if maybe you just hooked up something wrong.

SOURCE: I have a 1997 Mitsubishi Galant with rough idle

I am having the same issue on Mitsubishi Galant 02. Clattering on acceleration, herky jerky idle. Pulled a code multiple cylinder misfire. My mechanic's diagnosis is bad valve cover gasket causing oil to seap into the spark plugs and bad timing belt, which I have not replaced at 150k miles. I do however know my fuel injectors do need cleaning, he hasn't ruled that out either. Have you pulled any code on it? I have read on engine knock sensor going out on some of these also. Hope this helps some.

SOURCE: 1990 Buick Regal 3.1 has rough idle, hesitates and dies when accelerating.

A lot of new parts! Have a compression test done to gauge the engine's internal condition, and go from there. If compression is good on all 6 cylinders, scan for trouble codes. The compression test may find your problem, something like an exhaust valve burned or not seating properly.