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Discussion Starter · #1 ·
So my 2003 GTS has been having a really bad misfire on bank one. Like, all three cylinders on bank one are misfiring and running really rich. It's fine until the engine warms up but it's been getting progressively worse. I'm thinking it has something to do with the butterfly valve leaking, but wouldn't that cause it to misfire on all cylinders? I can't really find much information on it. It seems like an issue specific to a mistake I made somewhere along the lines. I'm just wondering what you guy's opinion on the matter would be. If it is because of the plenum does anyone know a good place to purchase a new plenum or a way to repair the butterfly valve? It still opens but the o ring in it basically because brittle plastic and fell apart when I tried looking at it. I know that's part of the issue but could anything else cause it?
 

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2001 Spyder GT
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You don't mention whether you have any SES codes for your bank 1 position 1 oxygen sensor, but that's the first thing I'd check. If that's good, I'd look for an exhaust leak on bank 1.

It's unlikely that it's the plenum or any of the other common causes for misfires and excessive rich AFR, because that would probably affect all cylinders, not just bank 1.
 

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2001 Spyder GT. Koni adjustable shocks. cold air intake, ST sway bar.17X8.5 rims. LED headlights.
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Do you have a check engine light on ? If so what code is set ? I would think you have a faulty short term O2 sensor. That would cause both the misfire and the rich mixture. Also might consider what shape your distributor cap and rotor are in. I would not run it too long in this condition for fear of ruining a cat. THAT would be expensive. Check the codes and get back to us. ...J.D.
 

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Discussion Starter · #4 ·
Do you have a check engine light on ? If so what code is set ? I would think you have a faulty short term O2 sensor. That would cause both the misfire and the rich mixture. Also might consider what shape your distributor cap and rotor are in. I would not run it too long in this condition for fear of ruining a cat. THAT would be expensive. Check the codes and get back to us. ...J.D.
Hey sorry it took a while. It's running a PO135, PO403, and a PO300. Out of all of them the O2 sensor seems like the best bet to cause it. I wasn't sure because I've gotten a lot of mixed opinions on the O2 sensor causing that misfire but it's cheaper than continuing to throw parts at it. Thank you two for the help I'll let you know how it goes
 

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Discussion Starter · #6 ·
Check compression. My GTS did the same thing before I discovered bank one pre cat was clogged and eventually, all of bank 1 lost compression.
Hey thanks for the advice but I'm a step ahead on that one. The cats were all gutted well before the issue came up and I just did a compression test the other week and every cylinder was exactly at 240 psi.
 

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Hey thanks for the advice but I'm a step ahead on that one. The cats were all gutted well before the issue came up and I just did a compression test the other week and every cylinder was exactly at 240 psi.
240 psi? That kinda high but, I don't know why. The factory manual says ea. cylinder should be around 212 psi.
 

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Hey sorry it took a while. It's running a PO135, PO403, and a PO300. Out of all of them the O2 sensor seems like the best bet to cause it. I wasn't sure because I've gotten a lot of mixed opinions on the O2 sensor causing that misfire but it's cheaper than continuing to throw parts at it. Thank you two for the help I'll let you know how it goes
Check or just replace the EGR vaccum control solenoid which will cause the EGR to stay open causing the PO300 and PO135 codes. After replacing the Egr vacuum control with a OEM replacement then drive over 35 MPH and let coast down to see if the Egr is closing correctly. You can do all of this by resettting the ECM monitors and scanning the vehicle at warm up and after driving at low speed for two to three miles and then 35 or 40 MPH for another 4 to 5 miles. A vacuum leak or valve cover gasket leak will cause these types of code also' Take the upstream and downstream O2 sensor out and clean or replace to rule out incorrect readings.
 

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Discussion Starter · #10 ·
Update. Turns out the erg code was from some mixed up vacuum hoses but it still has the same issue. I ordered my oxygen sensor and it should be here tomorrow so we'll see how that effects it.
 

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Discussion Starter · #11 ·
New update. The O2 sensor helped some but less than I was expecting. It isnt completely dying on take off now. The sputter is still there and the p0403 came back so the vacuum lines weren't hurting as much as I expected. Next I'm going to check the EGR solenoid and try to see if there's a vacuum leak other than the intake runners. Also if anyone knows what sizes o ring I could use to fix the leak in the runner it would be greatly appreciated. I can't find another GTS at a junk yard that isn't two or three hours out of my way so I'm gonna have to make due with what I have until I stumble across another one or find a diamonte
 

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2001 Spyder GT
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When you say "intake runners", are you talking about the passenger side of the intake plenum, where the servo arm for the variable intake valve penetrates the plenum? That's a common vacuum leak point and there's no replacement seal.

P0403 is the EGR solenoid or it's wiring.
 

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Discussion Starter · #13 ·
When you say "intake runners", are you talking about the passenger side of the intake plenum, where the servo arm for the variable intake valve penetrates the plenum? That's a common vacuum leak point and there's no replacement seal.

P0403 is the EGR solenoid or it's wiring.
Yea the butterfly valve thing. Could it be the cause of the misfire? I've pretty much been throwing parts at this thing but I'm not ready to give up yet. If it's not that leak could the EGR solenoid be causing it? I've already ran all new vacuum lines so it can't be a leak in the vacuum
 

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I'd fix the EGR problem first. Normally I recommend troubleshooting before replacing parts. In your case, I think you're at the point where you replace the EGR solenoid.

Once you fix the EGR problem, see how the car runs. If you still have problems, check how bad your vacuum leak is at the MVIM shaft. You can also just disconnect the electrical connector and seal the leak with silicone as a temporary troubleshooting step. If the silicone makes the car a lot better, you know you need to remove the silicone and address the problem.
 

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Discussion Starter · #15 ·
I'd fix the EGR problem first. Normally I recommend troubleshooting before replacing parts. In your case, I think you're at the point where you replace the EGR solenoid.

Once you fix the EGR problem, see how the car runs. If you still have problems, check how bad your vacuum leak is at the MVIM shaft. You can also just disconnect the electrical connector and seal the leak with silicone as a temporary troubleshooting step. If the silicone makes the car a lot better, you know you need to remove the silicone and address the problem.
I'm gonna give it a shot. Believe it or not I started with like 16 codes so I was just working my way down the list 😂
 

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Discussion Starter · #16 ·
New update. Every code is gone. I sealed the butterfly valve up and there's no leak there anymore. It's running a little better (i.e. not completely dying at stop). It's still sputtering really bad when it gets up to temp though. It's almost like a machine gun sound. It only does it when it's at a lower rpm but once it gets up to speed it's completely fine. The idle isn't perfectly consistent either.
 
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