P0300, P0303 and P0304
 

Hi all, 04 LX 5 SPD 203k
I put new spark plugs, oil pan, valve cover gasket and a new oil pan gasket, 4 to 1 headers and 3 in exhaust. Cleaned the EGR also.The car ran fine during idle before replacing the header and exhaust. I painted the header and exhaust and started it up today to bake the paint. I noticed the engine running a bit rough. I pulled out my code reader and it shows no codes but, I checked the stored codes and it shows P0300, P0303 and P0304, yet, no check engine light. I had to jump the car because it has been sitting for a bit due to replacing the exhaust and headers. So, no check engine light, engine running a bit rough during idle and stored codes.
Any idea what might be going on? Would the new header and exhaust cause the engine to run rough?
Are the stored codes old and just wasn't cleared?
The car ran fine before removing the header and exhaust.
I let it idle for about 20 minutes and roughness was still present. I figured since it sat for a bit maybe it's just the gas but, after 20 minutes it would have dissipated.
Could it be weak battery? again, I let it idle for about 20 minutes so the battery can charge.

Thanks in advance..

Re: P0300, P0303 and P0304
 

Just started the car again and now the check engine light came on with those codes. I'm trying to figure out what could have happened. I had the car on and off, on and off, on and off and never misfired until I swapped the headers and exhaust.

Re: P0300, P0303 and P0304
 

What plugs did you put in? Were they gapped properly?

Re: P0300, P0303 and P0304
 

I honestly can't remember which brand but, they were gapped correctly. The car ran fine with the new plugs prior to replacing the headers and exhaust. The first thing I replaced was the VCG, then the plugs. Started the car several times to verify any leaks and/or misfires due to the new plugs. Car ran great. Then I replaced the cam seal next to the EGR. Then cleaned the EGR and added a gasket since the PO removed the EGR gasket. Changed the oil and oil filter. Started the car several times. No issues. Then replaced the oil pan and oil pan gasket, along with the header and exhaust. I added a 90 degree elbow for the downstream O2 since it's an after market header with no cat. I checked under the car to see if there might be air seeping through the new oil pan gasket but, no oil leaks.
Does the ECU need to be reprogrammed because of the additional exhaust flow? The car was on E so I added about a gallon of gas. I completed the exhaust install yesterday and started the car and let it run for about ten minutes. It ran great but, had to turn it off because I was performing the baking process and couldn't turn it back on because the battery wasn't strong enough to restart the car so I waited until today to start the car to finish the baking process. Started up fine then after about 5 minutes I noticed the idle bogging a bit. Checked for a CEL, no CEL. I decided to check for codes anyway and it said no codes. Then I decided to check stored codes and then I saw the P0300, 303 and 404 codes. Turned off the car, went inside to research what the codes meant. Came back out, started the car and the CEL came on.
Check the codes again and the same three codes are present.
I have not replaced the coil packs yet but, I doubt the coil packs would fail on three cylinders and P0300 means random misfires, not related to a particular cylinder like P0303 and P0304. No codes on cylinder 1 and 2, just 3 and 4.