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Major Misfire While Driving - Help!!

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  • Major Misfire While Driving - Help!!

    I've got an all stock '93 Z-28 5.7L automatic with 133K mi. Idle is fine (smooth) and gentle cruising regardless of speed (0 - 70mph) where very light pedal pressure is held steady (smooth too). But, upon any harder acceleration, I'm getting the most God-Awful sputtering misfire I've ever encountered! I swear it's like somebody yanked four friggin plug wires off until I let off the pedal! And it rocks the car to the core!

    I know the motor is getting adequate spark and fuel (idle and light cruise, runs almost perfect). Chevy installed all new plugs and wires and although before the visit I found a very slight bit of oil that has leaked on bottom part of the distributor "opti(?)" all else seems fine. Can someone please share what the heck might be going on here? Why would it run so smooth during idle, neutral revving and light cruise but, then suck like hell while in gear under a load?

    The Chevy dealer ruled out the distributor (a month ago) when it was misfiring even worse (this was due to a couple of bad wires, tho and ran great for 3 weeks after that fix). NOW, it's bucking and kicking like crazy until I let off the accelerator some (and it's getting progressively worse everyday it seems). Recently, Chevy said they need to replace the distributor AND WATER PUMP because they saw some oil drippage (only on) the base of the distributor unit. They said: "It's notorious for these cars" (and I believe that part) BUT THERE WERE NO LEAKS AROUND THE WATER PUMP AT ALL! I checked before and after that visit.

    FYI, the SES light has been on with no performance problems since I've owned the car (over a year now) and ran beautifully. The previous owner said the MAP sensor was removed when installing the K&N intake system). But, I don't know how to check for "codes." So, I feel stuck and vulnerable to dishonest techs. Hey, sorry for the article here. But, any ideas why I'm getting this horrible misfiring? Any help is appreciated.

    Hawk

  • #2
    I don't think I've met anyone else with a problem like this besides myself, my opti was blow to high hell and back in the cap and rotor section. Most likely you are not getting spark at a certain degree of timing (ex, when you load the motor by stomping your foot on the gas).
    I suggest you replace at least the cap and rotor. Also, blow into the vent tube to the opti, if you hear rattling at the cap, your problem is confirmed.

    EDIT: The dealership probably just ran a tech tool to check the optical position sensor in the opti which is most likely fine unless the car wont start. I had actually ripped off a rivet on the rotor and a full prong off the cap, without it, any really advanced timing wasnt even making it to the plugs, but anything midrange ran smooth(er).
    -Alex
    1995 LT1 ECU (GREAT for flashing!)
    ZO6 wheels (clones)
    LED exterior and interior lighting
    With questionable guts:
    Forged bottom end
    free flowing 3 1/2" exhaust w/
    pacesetter longtubes
    T56 with a 6 puck ceramic copper heavy duty clutch
    Built T56, 3.5" 4130 driveshaft w/spicer HD's
    K&N RAM air from 96 ws6
    96? ws6 hood
    96? ws6 spoiler
    full emissions delete
    polished heads with oversize valve job
    Edelbrock IAS shocks
    Full tubular Chassis minus k member
    Daily Driver and love it that way
    Motor is not what you'd think.

    Comment


    • #3
      You need to find out why the SES light is on. Didn't the dealer do that? Why not? Shoiuld have been the first thing he checked. In fact you can check for codes easilly, by shorting two pins in the ALDL connector and flashing the codes on the SES light. 93 is the only year F-Body LT1 you can do this on.

      Courtesy of Shoebox:

      http://shbox.com/1/dlc2.jpg
      http://shbox.com/1/Dtcs.htm

      The "MAP" sensor does not get removed when you install a K&N intake. The MAP sensor reads the manifold absolute pressure, and is the black, domino size piece at the front/top/passenger side of the intake manifold. A 93 will not run correctly without the MAP sensor, because a 93 LT1 is run in speed-density mode, and the MAP reading is critical to calculating the air flow into the engine.

      Could he possible have told you the "MAF" sensor was removed? That would be false also, since the 93 is the only year that does not have an MAF sensor. In the 94-97 LT1's the MAF sensor would connect to the end of the K&N kit. But even on a 94-97 it wouldn't be removed. Your 93 never had one.

      It is VERY normal for an engine to misfire more when it is under load than when it is idling or under very light light load. The ignition requires a lot more voltage to fire the plugs when the engine is under heavy load, and a faulty wire, a bad plug, a loose wire, corroded wire terminals, etc would show up EXACTLY then.... under heavy throttle.

      You say you have enough fuel because it idles well and runs well under light load. Again, does not prove it has adequate fuel. Fuel demand increases dramatically with engine load. You have to measure the fuel pressure at wide open throttle/max RPM/max engine load to determine if the fuel system is capable of supplying adequate fuel.

      If the dealer didn't check the code [it would take him about 1 minute with the dealer's Tech 1 (or Tech 2) scanner], he didn't do a very good job, and I would serioulsy question his competence. Maybe he installed a plug wire against the exhaust manifold, and its burning through the insulation, and the spark is arcing to the manifold.

      You need to scan it for codes. You need to check the fuel pressure. You need to find a new mechanic.
      Fred

      381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC engine management - 8 LS1 coils - 58mm TB - 78# injectors - 300-shot dry nitrous - TH400 - Gear Vendor O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

      Comment


      • #4
        Originally posted by Injuneer
        You need to scan it for codes. You need to check the fuel pressure. You need to find a new mechanic.
        +1

        Sound to me like the opti is toast.

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