P0172 Code and idling roughly

Hello All,
I have a 1997 Plymouth Neon SOHC 2.0 with 155,745 miles.
Was running fine until wife got home last night. As she pulled in and idled the car before turning it off it started idling very rough. Today when I started it, it was still idling rough and when I gave it some gas is smoothed out but there was still some chugging for lack of a better description. When I checked it I got a code P0172. Plugs, wires are fine. No vacuum leaks I could tell (without a tester). I listened for fuel pump after key on and I can hear it start up every time. Fuel pressure at key on engine off is 45 and when idling jumps between 45 -50 as the car shudders trying to stay on. Any suggestions? Fuel pressure regulator? I appreciate all help.

does the fuel pressure bleed off? Check the egr operation and the 02 !

Pressure did not bleed off with key on engine off. I did not check for bleed off after running it. How do you check EGR and o2? I will do those checks in the AM and report back.

HI ¡

Are you checking the fuel pressure in the fuel injector´s rail ? are you checking it after the output of the fuel pressure regulaor ? In which part are you checking the fuel pressure ?
Is your neon having the fuelpressure regulator in the engine or in the fuel tank??
If it is in your engine, these 45-50 pressure is incorrect and you must have a fuel pressure regulator failure.
Also check the vacuum hose that is attached in the top of the fuel pressure regulator, check if it has vacuum, check if these hose is not loosen or clugged.

I am checking pressure at the fuel rail and the regulator is in the fuel tank.

Wow a doctor. Are you a real doctor?

Nah....Sherlock Holmes fan. Just trying to figure out the car. Any ideas?

Well, it's perceiving a rich running condition my dear watson. It could be an actual rich condition or a stuck o2 sensor giving a false rich condition. IT'll take a bit of elbow grease to check. Id start with o2(monitor with a scan tool) see if it is switching between .1 and.9 volts. IF so, it's okay.
Watch the fuel trims (STFT) and see where theya re. Post back results

I checked the o2 before the cat with ohm tester. 7.5 ohms with key off. o2 after cat 6.9 ohms. What kind of scan tool do I need and what is proceedure?
Thank you for your time and assistance.

An ohms test on an 02 sensor is pretty sketchy!

Just going by the Haynes manual and trying to figure out what to do next.

I'd try a new sensor from the dealer at this point unless you have access to a scan tool that you can watch the o2's voltages on! WAs the fuel pressure within spec and not bleeding off? is the car misfiring at all?

Fuel pressure was 45 at KOEO and dropped to 41 after 5 minutes. Jumped between 45 and 50 at idle as car stumbled (misfired?). Stumbles (misfires) at idle that mostly goes away (only a slight stumble) as I increase rpms. I don't have a scan tool for the voltages.

OK heres an update. I cleared the code and took it for a drive and it seems to drive fine. RPMs steady, does not seem to stumble or misfire. However it idles like crap to the point of stalling or almost stalling. Anyone have any ideas before I break down and take it to the shop?

Without a scan tool to watch the fuel strategy, there's no way of knowing what's going on inside the engine. With such a general problem and a rich code, it's either running rich or the PCM THINKS it's running rich. Stalling at idle would indicate a rich problem, but no stall at speed could support the rich condition, because at speed the vehicle would use more fuel and a rich mixture would be harder to detect.