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need help

This is a discussion on need help within the Computer & Tuning forums, part of the LSx Technical Help Section category; I have a 99 c5 vette and is coding p0300 and p0420. it is showing misfire on 1,3,5,7. with low ...

  1. #1
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    red
    1999 corvette

    need help

    I have a 99 c5 vette and is coding p0300 and p0420. it is showing misfire on 1,3,5,7. with low efficiency bank 1 on the cat. the o2's are reading so I am assuming they are ok. It will misfire for some time and then will smooth out but return a short time later. It sometimes will not do this again for weeks. Now it is happening with more frequency. I pulled the plugs after all the misfiring and they did not show any fouling problems. Would the misfire be caused by the bad cat or is the cat failing because of the misfire? I would love to drive this car but my first six months of ownership has not been a pleasant experience. Had to replace engine after the first 30 days and it just seems to continue.AAA is going to start sending me a membership check just not to call them.It is a nice garage ornament though.

  2. #2
    Senior Member ninobrn99's Avatar
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    Pewter
    all bore 370 Trans Am

    thats the whole left side of the motor. Id look at the wiring harness and the coil packs.

  3. #3
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    red
    1999 corvette

    thanks I am trying to see the wiring behind the left rear side of the head. I believe that there is a clip that can cause chafing on the harness supplying power to the fuel injectors.This might explain the clean plugs with a misfire as no fuel would be introduced to the cylinders but would misfire just the same. Spark with no fuel would equal misfire I would think. The only thing I don't understand is that it doesnt show any injector codes or harness voltage problems.This one could take awhile.Thanks for replying I have this listed on 2 forums and no one has responded until yours.I will post results when I get it all sorted out.

  4. #4
    SUPREME member-oderator oneBADDz's Avatar
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    Custom Slate Blue
    99 B4C

    It can actually be caused by the O2 and/or the cat also. check the wires to the O2 and maybe switch it to the other side and see if the codes move with it

  5. #5
    We'll be back... GatorSS's Avatar
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    Light Pewter Metallic
    2000 Camaro SS M6

    Quote Originally Posted by neverlate View Post
    ... Had to replace engine after the first 30 days and it just seems to continue...
    Ouch! Previous owner(s) must have really ragged it out. Who replaced it?

  6. #6
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    red
    1999 corvette

    I replaced the engine with a brand new ls1 direct from GM It came with everything new right down to the spark plugs. The engine install went well but now I have developed multiple random misfires and now one whole bank is showing misfire. What is strange is that it will smooth out and run fine for some time then when you least expect it flat on its face it goes.This did happen when i first bought it with the old engine so I would think it is more of an electrical problem. I pulled the Gm history on this car for warranty claims and it has had the pcm replaced 3 times and the bcm 1time. I have the car at Revxtreme in Tampa Fl. as of this writing.I don't know much about them but they were listed on the Corvette forum.I hope they are good at this because it is wearing me out.

  7. #7
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    Definitely sounds like a wiring/connection issue.

  8. #8
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    Silver/Black
    2002 Camaro SS Cnvt

    I'd look to the driver's front O2 sensor first, and see if
    you can log its voltage (or even just read it with a DMM)
    when it is misbehaving. But this seems like an unlikely
    thing to go bad, so early on a fresh motor. Unless the
    sensor was reused and a connector pin was for sh!t, or
    something.

    You might want someone to inspect the PCM and see
    it there is any sign or prior meddling, codes turned off
    that might have been helpful in diagnosing etc.

  9. #9
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    red
    1999 corvette

    Unhappy

    I had the car to REVXTREME in Tampa FL. and the cats were bad and replaced, the o2 sensors are reading within specs. The intake was pulled to check the harness to the rear of the engine and that shows ok. I picked up the car and drove it for a day and it started all over with a terrible miss. I was by a chevy dealer and they cleared all the codes so I could get it home.
    It showed the following: 10pcm po300

    40bcm u1016

    60ipc u1016

    ao-ldcm u1064 , u1255 , u1016, u1096
    a1-rdcm same as above

    28tcs u1016 ,c1276
    etc etc
    what is baffling everyone is that it will run just fine and then it goes into limp home mode.It is showing a lot of loss of communications with pcm and serial data line malfunction codes. The guys at the shop have spent alot of time on it and it will run fine while there It just seems to be a hit and miss thing but it is doing it more frequently now. I have used up all my AAA tows and the shop is 50 miles from home and it is tough walking or waiting in the heat.

    Any thoughts would be appreciated .

  10. #10
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    Talk about problems! Hi guys im from perth Weatern Australia,ive got a VX SS not sure if you guys know what that is but its like your GTO from OZ but with 4 doors.lol. Ive got a different problem with my gen 3.From idel to a bout 2500rpm it misses like crazy,GM told me for some reason its poring fuel in both banks but there not sure why.I replaced MAP sensors an all sorts of things but cant find the problem, and according to the ECU everything is running just fine!dont think so. Im using about a 20 Ltrs to do 35 mile hows that for good fuel usage. Any one ever heard of this sort of problem any help would be so helpful???

  11. #11
    Junior Member m.j.lecki's Avatar
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    red
    2002 ws6 trans am

    good morning just read your situation..sound like the same prob ihave now..
    set of headers and some bolt ins and...supersmog the air rich situation..
    no codes no help from gm.did u everfix yours..if so how...thanks

  12. #12
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    Silver/Black
    2002 Camaro SS Cnvt

    The 'vettes (and other drive by wire) have a whole
    separate throttle angle controller deal and if you
    have an unreliable bus that's probably going to throw
    a fit and put you to a "safe" mode. Chasing that kind
    of thing down, ought to be fun. With so much messing-
    around been done in that engine bay, there's a good
    chance of electrical mishap. You just have to inspect
    for physical damage, especially around the obvious
    areas that people would have been working (headers,
    whatever). Also check that the ground strap to the
    rear of the head is intact and tight, as well as any
    other sheet metal - engine block grounds. Of course
    you can't very well check something that went and
    got lost. Might put the voltmeter from block to sheet
    metal, verify that there is no difference (try a few
    combinations of chassis and engine accessory loads,
    A/C, lights, boom-boom etc. just for yucks). Even a
    few hundred mV can jack some of the sensors around
    that don't have their reference ground sourced by
    wire straight from the PCM. Of course the fun thing
    about intermittents is that they only misbehave when
    you take the test gear away....

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