2000 Jimmy - Miss at idle when hot.

Discussion in 'The BS Topic' started by Twisted_Metal, Jul 6, 2018.

  1. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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    4.3L Vortec V6.
    No "Service Engine Soon" light or OBDC codes.

    Starts (hot or cold) idles and accelerates normally.
    But after it's been driven on the highway for several miles on a hot day.... It will have a sporadic miss at idle. (Every second or two)
    You can feel the miss and see the tach needle dip.
    I haven't noticed any overly rich smell form the exhaust but I will check that the next time it happens.
    MPG seems to have dropped a little too.

    It doesn't stall, even with the A/C on and there is no hesitation during acceleration.
    The hotter it is outside... The worse the idle miss becomes.
    Like it's possibly related to the under hood temp.

    The coolant temp gauge behaves normally.
    (Does the cooleant temp sensor provide computer input for fuel management?)

    Rotor, cap, plugs and plug wires were are replaced in January.
    TPS was replaced last Summer.
    Without any OBDC codes to go on... I'm guessing.

    I've heard the coil can act like this when it begins to go bad.
    Could it be something as simple as the Intake Air Temp sensor?

    Any other suggestions from the professional mechanics here?
     
  2. Knuckle Dragger

    Knuckle Dragger Mayor of Simpleton Staff Member Lifetime Gold Member

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    Can you get a hold of a scanner with a missfire counter? The first thing I try to do is isolate what cylinder, then check fuel trims.

    Those had a lot of issues withe the CPI injector poppet valves but I don't have enough hands on to give anymore then what I've heard.
     
  3. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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    How much Lucas Injector cleaner will it take to fix that? ;)

    No access to a scanner without taking it in to a shop. :(

    The fuel spider was replaced about 40K miles ago.
    Seems odd it wouldn't show up at cooler temps if it was a fuel delivery issue.
    I'm thinking it's either an ignition issue or sensor input for fuel management.

    But... I don't fix cars for a living either.
     
  4. Knuckle Dragger

    Knuckle Dragger Mayor of Simpleton Staff Member Lifetime Gold Member

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    I doubt it's a sensor. Generally speaking when they fail they give overall poor performance not an intermittent miss. Fuel is unlikely too but I like to see the fuel trims to give me a picture of what's going on. I kind of discount the coil because you would think that would be worse with load, but it might still missfire at load you just don't feel it as an intermittent. If I had to make an off the cuff guess I'd be looking at the ignition module given everything is new and it's coming with heat. But really there are a whole bunch of things it "could be". Defective ignition wire, cheap or not well made ignition wire inductively firing to anything metal, another wire, carbon tracking in the cap, cracked plug etc. I know that's not much help but I find most issues through a good visual inspection. Being intermittent at idle it should be fun to track down.
     
  5. Twisted_Metal

    Twisted_Metal Administrator Staff Member Lifetime Gold Member

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    I was going to check it when my GF got home last night (to look for arcing) but I forgot. :(
    She said it wasn't nearly as bad with the cooler temps we had yesterday.

    I considered several of the things you listed (distributor cap, wires, ignition module) so they are on my checklist of things to look at more closely.

    Thanks for the input, Dave!
     
  6. Todd80Z28

    Todd80Z28 Moderator Staff Member Lifetime Gold Member

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    I think the Torque app for Android can do that... do you have an OBDII WiFI or Bluetooth dongle?
     
  7. ronzz572

    ronzz572 Veteran Member Lifetime Gold Member

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    I've seen a lot of secondary ignition problems with the blazers of that era. Coils, coil wires, distributor caps. All could cause your issue. Not sure if a scanner will give misfire counts on a 2000. It's been a while since I scanned one. A older shop with a engine analyzer that can check the secondary ignition patterns should clear up any questions regarding the ignition
     
  8. Mike N

    Mike N Administrator Staff Member Lifetime Gold Member

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    I'm pretty sure there's a separate sensor for the dash gauge, versus the sensor the PCM uses to monitor coolant temp.

    With that being said, I've seen numerous coolant temp sensor failures over the years. For $12 there's nothing wrong with tossing one at it.
     
  9. Camaro75LT

    Camaro75LT Masshole Staff Member Lifetime Gold Member

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    When my tbi blazer needed a coolant sensor, I could make the idle change by putting a screw driver on the side of it and grounding it to the intake manifold.

    Do you have access to a smoke machine. A vacuum leak isn't probable but it's not hard to check for.
     
  10. Mike N

    Mike N Administrator Staff Member Lifetime Gold Member

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    Most common vacuum leak on those is the lower intake manifold gasket.
     

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