I have a 2002 neon se. The car was running fine one night, and then the next daythe enginestarted up horribly. The engineran very rough and threw OBD code P 0508 (idle air control). The plugs looked dark and sooty a little wet withgasand the exhaust smells a bit rich. I cleaned the throttle body and replaced the idle air control with a new oneand it still runs very rough. It barely idles very roughly and will not increase rpms smoothly. I also replaced the spark plugs with new NGK copper plugsand plug wires. The OBD doesn’t show any codes now, but it still runs very rough.

I did some testing. I pulled the plug wires off one cylinder at a time and started the engine to test that each cylinder was operating. Pulling the plug wire from cylinder 2 (#2 from the front of the engine to the back) didn’t seem to change the way the engine runs so while it was running with the plug wire off cylinder 2, I pushed the plug wire back on. The wire sparkedon the plug sleeve of the valve cover as I pushed the wire back onto cylinder 2 while it was running so I know its getting spark, but didn’t change the way the engine was running after I plugged it back in. It really does seem to be running on 3 cylinders.

The only possibilities I know about are:

The fuel injector on cylinder 2 is clogged?
The PCM is bad? https://dodgeforum.com/m_551846/tm.htm(No OBD codes showing though)
O2 sensors? just a wild guess...

What could it be?

If it isthe PCM, could I just disconnect the neg on the battery, remove the old PCM, and replace with a pre-programmed one like this one? http://cgi.ebay.com/ebaymotors/96-04-Dodge-Neon-ENGINE-COMPUTER-ECU-PCM-ECM_W0QQitemZ320232809655QQcmdZViewItem?hash=item3 20232809655

Any help would be greatly appreciated.