2.3L ('02-'11) PO300 code diagnosis

[DaveH]

2.5 engine block 2.3 head with OEM ECU for the 2.3. I built this engine about 3 months ago and have about 3K on it. When first built it ran with multiple misfire codes and replaced plugs, wires and OEM coil. It cleared the code problem for about 2500 miles. The PO300 code is back this morning. There is no noticeable change in engine and runs smooth. So after a few pages of reading I am thinking, 1 fuel injector problem 2 Catalytic converter 3 O2 sensors, vacuum leak? Ideas on diagnosing this misfire issue before I throw more parts at it??

 

[franklin2]

Check the pieces you replaced to repair it the last time. The parts you buy now are terrible as far as quality. I would ohm out the sparkplug wires you installed while wiggling them around. Don't get too rough with them, they seem to break too easily inside. If you have the old coil you could try putting it back in place.

 

[DaveH]

2.5 engine block 2.3 head with OEM ECU for the 2.3. I built this engine about 3 months ago and have about 3K on it. When first built it ran with multiple misfire codes and replaced plugs, wires and OEM coil. It cleared the code problem for about 2500 miles. The PO300 code is back this morning. There is no noticeable change in engine and runs smooth. So after a few pages of reading I am thinking, 1 fuel injector problem 2 Catalytic converter 3 O2 sensors, vacuum leak? Ideas on diagnosing this misfire issue before I throw more parts at it??

I pulled all plugs , looked perfect. Checked fuel pressure, 65 psi. Held fine after shut down. Did see the coil screws were not tight. Cleared the code and runs smooth as before. Will run to see if the code pops again.