Sunfire stalls

2001 Pontiac Sunfire
Engine started to lose power a few days ago and is now very hard to start hot, but will start cold and run rough and then stall out.
Code P0300 Random engine misfire code.
Parts replaced; new fuel filter, new spark plugs, good used o-sensor at exhaust manifold, good used crank sensor, good used coil/module/wire assembly , good used control module.
I have not been able to check the fuel pressure but the fuel pump does cycle on and pump fuel (checked at the fuel filter inlet) until the control module turns the pump off.

One thing you might want to check is the fuel pressure regulator. Check to see if the vacuum hose attached to it is drawing fuel. That could explain why the emgine chokes out when it's hot. It'd be good if you could hook a scanner up to it and see that the coolant temperature and intake air temperature sensors are reading right. Also check and clean your throttle body and where the Idle air control port feeds in, and also check for vaccuum leaks and that the pcv isn't clogged.

Check the fuel pressure with a gauge. Pumps will sometimes get weaker as they heat up. Also check the injector resistance cold and then again as it heats up. The resistance should be close, within a few ohms. Heat expands the coils and can partially short them if the insulation is damaged. Also check the strength of the spark once it gets hot and dies. Could even be a corroded wire getting hot and causing high resistance. CKP sensors can short out too, check for it's input to the PCM. Ideally you should look at the waveform on an oscilliscope, but looking at the RPM readout gives you good indication, although it can be wrong if the waveform's bad (and causing your problems)

I just wanted to say thanks to manicmechanix and J-Ri for all your help. I did find the problem. The Catalytic converter was the problem, i changed it yesterday and now everything is back to normal, again thanks for all your help.

I just wanted to say thanks to manicmechanix and J-Ri for all your help. I did find the problem. The Catalytic converter was the problem, i changed it yesterday and now everything is back to normal, again thanks for all your help.

Your welcome. Your hard starting and stalling symptoms didn't sound like a typical clogged converter. Usually they will start and idle OK, but will have no power or accelerate. Yours must've been really bad. Good to hear about the fix. If you don't mind me asking how did you discover it was the converter? One easy way to check for an exhaust restriction is with a vacuum guage (or maybe reading the live map pressure with a scanner), if the vaccuum isn't near 18-20 in (higher pressure) and doesn't breifly shoot even lower, maybe 20+ in, after opening then closing the throttle (pressure high) that's a sign the engine isn't breathing right.

hi manicmechanix,
I pulled the o-sensor out at the exhaust manifold and installed a pressure gauge, I was told that if you have more the 3" at the manifold you have a restriction.
My pressure gauge goes up 15", so when i started the engine it went all the way round , in fact i thought it was reading zero untill i noticed that the needle was on the other side of the pin at 0 psi, on cranking it went to 10 psi.
Thanks again.