misfiring stilll...

Discussion in 'General Chevy & GM Tech Questions' started by reggiecab2000, Aug 27, 2012.

  1. reggiecab2000

    reggiecab2000 Epic Member 5+ Years 1000 Posts

    hey guys,
    ive been through this issue time and time again...
    i get a rough idle and misfiring in my truck everytime after its been sitting overnight, or sitting for a long period of time... but only on cold starts like mentioned...
    once its warmed up it runs fine with no missing at all and no hesitation...
    injectors possibly? anything else?
    just a great annoying issue on my truck LOL
  2. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Hmmm, could be a number of things, I think I would change the front O2 sensor (there will be two, if you have two cats). It may not be warming up (they have an internal heater to speed warming) on it's own, and waiting until the exhaust has warmer it sufficient to turn on.
  3. Skippy

    Skippy Member 2 Years 100 Posts

    This is an unlikely event, since the O2 Sensors are not in play during the open loop "cold" cycle (your computer is controlling things without the sensors at this point). The O2 sensor readings come into play once your engine is warm. Basically, the O2 sensor is ignored until warm-up occurs. (see the link and info below)

    I'd first take a look at your Idle air control valve. The second option I've listed below is a total crap-shoot, but worth looking into. Why? I had this failure once, it was a BUTT of a problem until diagnosed! Now I just look for it as a matter of course. It's an easy look, and if it's actually the case you'll save nearly a decade of time looking elsewhere. LOL

    1) Idle Air control valve. (IACV) This unit allows air to bypass the throttle plate into the intake. Lack of air into the intake will cause misfires for sure. Stumbling, rough idle, the works. I've had some success cleaning these, but never long term. Replacement usually fixes the cold start problem. This can also cause intermittent stalling (even when warm), and idle fluctuations as it opens/closes. Rarely do I see all three at once. Usually just one of them. For the 2000 4.3L, this part is about $58, and not too difficult an install.

    2) Does the idle/misfire occur at idle after warm-up at any point? Sometimes I'll see idle misfires when the plug wires are arcing to the engine block... (You can basically do a visual check on this in darkness.. Open the hood have someone start the engine, and look for blue coronas at the plug wires. It has to be REALLY dark though (Closed garages are great for about 60 seconds, and then you choke on exhaust.) Try for ventilation first, of course, and above all, be safe!
    If you're unsure and would like to test your wire integrity a second way, just wrap the wire with quality electrical tape (overlapping bands) and reinstall. If you have electrical bleed, this will stop it to some degree. While I don't think this is a strong likelihood, expansion of materials may actually close the electrical bleed somewhat when warm. Primarily I see this failure at idle though, since the arcing to the engine will have a greater effect (stumbling) at low RPM.

    As I mentioned #2 is a LONG shot for your symptoms, but it's certainly worth looking into given the fact you've mentioned you've tried a bunch of other stuff.


    [Edit] Here's a link and description of the O2 Sensor at cold:


    "When no signal is received from the O2 sensor, as is the case when a cold engine is first started (or the 02 sensor fails), the computer orders a fixed (unchanging) rich fuel mixture. This is referred to as "open loop" operation because no input is used from the O2 sensor to regulate the fuel mixture. If the engine fails to go into closed loop when the O2 sensor reaches operating temperature, or drops out of closed loop because the O2 sensor's signal is lost, the engine will run too rich causing an increase in fuel consumption and emissions. A bad coolant sensor can also prevent the system from going into closed loop because the computer also considers engine coolant temperature when deciding whether or not to go into closed loop."
    Last edited: Aug 27, 2012
  4. reggiecab2000

    reggiecab2000 Epic Member 5+ Years 1000 Posts

    ill definitely look into those, but i might like to add my truck is sitting around 160,000 miles right now, and im definitely hoping for it to get me through college and then some, ive been reading alot about people saying injectors should be replaced and the apparent fact that my current spider injector set up has actually been updated by GM, not too sure as far as a performance update, but i think just more reliability instead, but not too easy about fronting 350.00 for the part, then the amazing labor charge that thing would cost to get installed :/, so i would hope its not the injectors but at the same time it points me to make me think that they might need replaced anyway,
    and i know for sure its not arcing to the block ive checked that before
  5. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    Skippy, I can not disagree with your O2 info, when cold, the PCM ignores the O2 sensor. And, a faulty O2 sensor (after the engine is hot) should throw a code.

    A mis-fire should also throw a code, this would rule out coils, wires and distributor.

    The OP did not suggest the CEL was on, so we can assume, no codes.

    It's probably not a mis-fire problem, it only feels like one, in other words, a rough idle when cold. The IACV works like the choke on the older carb-ed engines, you could be on the right track.

    I could be wrong, but I think a faulty IACV might also throw a code.
    Last edited: Aug 27, 2012
  6. dpeter

    dpeter Epic Member 5+ Years 500 Posts

    Has it had a recent tune-up? Cold rich mixture harder to light off with marginal plugs maybe.
  7. reggiecab2000

    reggiecab2000 Epic Member 5+ Years 1000 Posts

    i had a misfire problem earlier in the year but discovered it was due to a piece of a small screw being flung around inside the distributor housing. and when it was misfiring it would only throw a code when it got REAL bad i mean, my truck had to start shuttering and bogging down quite noticeably before the SES light would actually come on and throw a P0300 code,
    but yeah its got all new cap,rotor,plugs,wires, etc in the last 10,000, did ALOT of work trying to find the cause for the last misfire lol so all common problems were inspected and either replaced or were fine
  8. dpeter

    dpeter Epic Member 5+ Years 500 Posts

    That had to be a real bugger to find, what lead you to the discovery of a screw in the distributer?
  9. reggiecab2000

    reggiecab2000 Epic Member 5+ Years 1000 Posts

    funny story actually....well dumb kinda LOL
    a while back i had to replace my Crankshaft position sensor, threw the code blah blah blah, a friend was helpin me out who has some noteable knowledge about vehicles(obviously not) and we set out to find where the heck this sensor was on my truck to replace it, he had this bright idea that it had to be somewhere near my distributor. so we took apart the cap and rotor and nonetheless it wasnt there, (this was one of the dumber parts of my life) i figured he was on the right track or else i couldve googled it in about 10 seconds LOL but when he put my rotor back on, he over tightened one of the 2 screws holding it to the distributor and the screw broke, the screws are TINY! he said not to worry about it that piece doesnt move or anything and i said uh im pretty sure that thing spins super fast inside the cap, he assured me nope, i was just following what i thought was his "expertise". so we put the cap back on and put everything back together, i googled the CPS found where it was and replaced it in 5 minutes. LOL
    sooo.....like 4 months later im driving on the beach and im parked playing some music (truck on) i decide to leave so i put my truck in reverse, boom it dies.... never started again that night had to get towed home by my friend. went through tons of things finally decided to check cap and rotor and boom what do you know! the rotor busted loose from the distributor inside the cap! from that mistake 4 months prior... well when i replaced the cap and rotor i had a running truck but it was still misfiring and that went on for months and months... needless to say after many nights scratching my head to sleep, i finally decided wtf why dont i just replace the distributor. went and bought one, put it on myself (first try :)) and when i pulled the old distributor out a rusty piece of screw fell out...(the piece that broke off uhhh... 8 months prior!!!) misfiring was officially solved...
    hope that was short enough
    but of course dealing with misfiring problems your always chasing your own damn tail:grrrrrr:
  10. RayVoy

    RayVoy Epic Member 5+ Years 5000 Posts

    I've been thinking about this a bit more. I've had problems, in the past, that have gone away when the engine gets hot, why? The heat has caused something to expand and seal the problem.

    Ok, so with that thought, maybe a loose intake manifold, letting un-metered air into a couple of cylinders.

    Thinking about un-metered air, maybe there is a leak after the MAF sensor around the throttle body, maybe the tb gasket.

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