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Hello Everyone!

I have an 04 Spec V with a rebuilt Jasper engine. I put Megan Racing headers(eliminated cat) on and an N1 cat back exhaust. I replaced the plugs with NGK plugs. I also replaced the mass air flow sensor and both oxygen sensors. I also replaced the coil packs. I have a Nismo cold air intake and a weapon R engine damper. I also replaced the engine mounts. I have a mini cat spacer for the second o2 sensor. Unfortunately I am getting a random multiple cylinder misfire. WHAT DO I DO? I ALSO TRIED 2 DIFFERENT O2 SIMULATORS.
1 put a high flow cat in?
2 relocate the second o2 sensor behind the second cat?
3 ANY OTHER SUGGESTIONS WOULD BE GREAT!!
 

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I could be wrong, but I don't believe any of those solutions you suggested there would help random multiple cylinder misfires. All those would be solutions to P0420, the pre cat code.
 

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maybe ECU crapping out? maybe some wires got messed up when u replaced the engine?
 

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i had the same problem ion my 03 spec...it would idle real bad and lost all its power...turned out one of my spark plug coils busted...once i replaced it, the car ran fine...good luck hope this helps
 

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i had same problem. cam sensor was the culprit.
 

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May be due to low-idling caused by the engine build components. Cams are known to cause P0300 on the QR. I had the P0300 (multiple cylinder misfire) with my Nismo Cams, but bought the Cipher cable and raised my idle by 50RPM and the problem is completely gone. Nissan can also raise your RPMs, or anyone with a consult II for that matter-- most folks say you can get it done for $20-$40, but my local nissan dealership quoted me 1 hour of labor -- $149! I just bought the cable for $200.

Update:
I just read it was a "rebuilt" engine/not a "built" engine, so hmm...but maybe the idle raise might still work; I'm assuming they used stock internals? At a minimum, I would hook it up to a code reader and see what it's idling at, if it's low ~700-740RPM, I would have it raised to at least 750RPM (although mine is currently at 800)
 

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Did you find out the problem? I have a 2004 spec all stock doing the same thing. It just started the other day while driving. Started running rough then threw the code P0300.
 

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You have described many potential variables for an issue to turn up. You will have to eliminate installation error and faulty component for everything you described besides the post cat O2 sensor. ( I'll get to that later)

I'm assuming you had the rebuilt engine in the car running fine before you put any of the bolt on / replacement parts listed above on. If not you will have to return the engine to as close to factory as possible, and go from there. An engine rebuild leaves many potential areas for failure, even for an experienced engine re-builder. If the engine ran fine after the rebuild, read on.

Since it's a random multiple cylinder misfire, it is not likely that it's an engine mechanical issue. Mechanical issues should be constant with the only variable being heat, and compression increasing when the engine warms up.

You are left with the possibility of it being fuel, spark, or a vacuum leak. If the vacuum leak isn't obvious and you hear a hissing that's easily located is to use carb cleaner to locate the leak. Have the engine running, and spray all vacuum hoses, intake ducts, intake gaskets, etc.. with carb cleaner. (A propane torch with the gas on but not ignited will also work.) The engine should start to stumble when you spray it at the point of the leak. Since you just put an intake on, verify that there is not a leak there. Maybe reinstall the factory airbox to see if that fixes your problem.

If it's not a vacuum leak, than you are left with a fuel or spark issue. This can be caused by any number of sensor failing, or issues with the fuel and ignition systems themselves. A post cat 02 sensor will not cause a misfire however. The pre cat O2 sensor is used to adjust fuel injector pulse width. The post cat sensor is used to verify proper operation of the cat. If it doesn't see enough oxygen, it will set a CEL. It will not affect driveability. You said you replaced the MAF. Did you use an OE Nissan part. MAFs are on of the more precise sensors in the system, and if the values aren't exactly what the ECM expects, you may have very poor issues. This is one part I would not be cheap with. I have seen many cars come in running poorly with aftermarket MAFs. Install an OE unit and they run great. Did you have any issues with the ignition coils before you replaced them? If not, you could try to put those back and see if works.
 
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