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2005 Spyder GT
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New (and final) update. I decided to go ahead and change all of my intake gaskets, empty the fuel rail, and disconnect all of the O2 sensors. I don't know which one of them did it but she's running like she's brand new now. Thank everyone for pointing me in the right direction and sticking with me from the beginning!
Disconnected all O2 sensors? Do you mean replaced?
Sounds to me like it was probably a leak in one of the gaskets.
 

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2005 Spyder GT
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No I unhooked all of them and the idle leveled out. It wasn't really what I expected but it runs perfect now so I'm going to get the three I didn't replace soon.
On the 3.0l, I believe only the two upstream O2 sensors affect fuel trim by sending info back to the ECU, which then adjusts it. The downstream O2 sensors report emissions only. Not sure how the gutted cats would affect the downstream O2 sensors if you aren't using O2 "spacers"?
Theoretically, the downstream O2 sensors should throw a code with gutted cats and NO spacers...
 

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btw, by spacers I mean an adapter that goes between the O2 sensor and the pipe which makes the O2 sensor sit further out, thereby fooling the ECU into not throwing a code. Guys who do cat deletes use spacers to avoid throwing codes...
 

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2005 Spyder GT
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My experience with O2 sensors with Mitsubishi's [I have owned 3. 1987,1997,2001]. Be careful using cheap, knock-off O2 sensors with these cars. I have had bad experiences TWICE that way. Make SURE what you buy is SPECIFICALLY for this car. You obviously cleared some issue by disconnecting everything. Did you disconnect the battery in the process ? Let's see if the issue comes back. ...J.D.
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.
 

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2005 Spyder GT
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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.
 
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