[Donnysim]

Plug your O2s back in.... Your issue has nothing to do with the EGR.

Have you inspected your distributor cap and rotor?
Have you swapped plug wires/injectors around to isolate the issue?
What's the weather like when the misfire occurs?

New cap, rotor, plugs, and wires. The injectors were fine. The only serious issue i found before the issue came up was that the cats were melted inside which is probably what messed the O2 sensors up. The misfire happens in any weather but only in closed loop but I'm going to get those sensors replaced and use some spacers to kind see if it completely gets rid of the issue but the only time it's ran right in months is when I disconnected the O2 sensors. I plugged them back in to make sure that was the issue and the misfire immediately came back

 

[WhiteSpyder]

I use either Denso or Bosch and haven't had a problem. Both come with exact length harnesses, so no splicing needed. The difference in part numbers appears to correspond with the custom length of the harnesses.

Yes the different length harness has a different part number. I have been using Bosch here because that is what is available. There ARE O2 sensors out there though that are sold as unbranded, generic [translate fits anything from Yugo to GM]. I found on my 2nd. gen Eclipse, when the ECU asked for richer using some O2sensors, you got leaner and visa versa. Does not pay to go cheap here. Also do not understand the remove the O2 sensor mentality. If the ECU was not setup to make use of the downstream O2, I am sure Mitsubishi would have not spent the money to put one in. Some think they know more than the engineers. Guess when you do not know --what you do not know-- anything is OK. I would rather benefit from thier expertise.
😁😆🤣. ...J.D.

 

[Sisyphus58]

New cap, rotor, plugs, and wires. The injectors were fine. The only serious issue i found before the issue came up was that the cats were melted inside which is probably what messed the O2 sensors up. The misfire happens in any weather but only in closed loop but I'm going to get those sensors replaced and use some spacers to kind see if it completely gets rid of the issue but the only time it's ran right in months is when I disconnected the O2 sensors. I plugged them back in to make sure that was the issue and the misfire immediately came back

Are you using low octane gas by chance?
Premium is recommended...

Yes the different length harness has a different part number. I have been using Bosch here because that is what is available. There ARE O2 sensors out there though that are sold as unbranded, generic [translate fits anything from Yugo to GM]. I found on my 2nd. gen Eclipse, when the ECU asked for richer using some O2sensors, you got leaner and visa versa. Does not pay to go cheap here. Also do not understand the remove the O2 sensor mentality. If the ECU was not setup to make use of the downstream O2, I am sure Mitsubishi would have not spent the money to put one in. Some think they know more than the engineers. Guess when you do not know --what you do not know-- anything is OK. I would rather benefit from thier expertise.
😁😆🤣. ...J.D.

I don't think he was intending to run without sensors, but rather was just trouble-shooting to see if it made a difference. Also don't know if he unplugged all four sensors, just the downstream, or just the upstream. The results under all three scenarios might be useful in pinpointing the problem, right?
I was suggesting that the downstream sensors probably were not contributing to his misfire problem because it is my understanding that they DO NO affect fuel trim like the upstream ones do.

 

[WhiteSpyder]

Yeh, I know. That was a little unclear. I sometimes mention these things to add a bit of narrative for others that are reading the thread. Bare with me. ...J.D.

 

[Donnysim]

Are you using low octane gas by chance?
Premium is recommended...

I don't think he was intending to run without sensors, but rather was just trouble-shooting to see if it made a difference. Also don't know if he unplugged all four sensors, just the downstream, or just the upstream. The results under all three scenarios might be useful in pinpointing the problem, right?
I was suggesting that the downstream sensors probably were not contributing to his misfire problem because it is my understanding that they DO NO affect fuel trim like the upstream ones do.

I've been using 93 octane since I got the car. But yea the O2 sensors were disconnected just to try to see if it would help anything. Sorry for not clarifying

 

[WhiteSpyder]

I have been known to unplug O2 sensors, one at a time to track down the offending part, but deleting any O2 sensor on an ECU controlled engine is complete ignorance, unless you are throwing the whole system in the trash and going to something else. ...J.D.

 

[Donnysim]

I have been known to unplug O2 sensors, one at a time to track down the offending part, but deleting any O2 sensor on an ECU controlled engine is complete ignorance, unless you are throwing the whole system in the trash and going to something else. ...J.D.

That's how I did it. I got it down to the upstream on bank two. I'm gonna order the part from Napa sometime next week

 

[Donnysim]

I think when the headers over heated it fried the sensors

 

[Sisyphus58]

I think when the headers over heated it fried the sensors

Oh geez, so NOW you tell us the headers overheated!!!
😖😣🙄

 

[man300mmello]

So I've been trying to find the cause of my misfire and have absolutely no clue where to start. I've check everything and I'm to a point where it isn't showing any codes so I'm at a loss, but I pulled the freezeframe data and wanted to see if anyone could help me interpret it and find a solution. So far everyone has been helpful but I'm at a loss.

DTCFRZF: P0300
FUELSYS1: CL
FUELSYS2: CL
LOADPCT: 29.4%
SHRTFT1: -4.7%
LONGFT1: 0%
SHRTFT2: -6.3%
LONGFT2: 0%
SPARKADV: 36°
MAF: 1.464 lb/m
TP: 4.3%

Like I said I don't know what to do with the information. Some of it makes sense but google isn't really helping me piece anything together and I know some of you guys know a lot more about it than I do. Some more information that might help is that it only does it when it's up to temp and the throttle basically dies when I press the gas. It runs fine above 30 but God forbid I have to drive through town. As of right now I have it parked for safety's sake but I need to get it back on the road ASAP.

Clean the EGR valve and replace the vacuum control solenoid controlling the EGR, I had the same issue and had to repair the ECU four times to fix the PO300 problem. IAC or MAF sensors should be ckecked.