Edmunds Answers

Answers

  • zaken1 02/28/12 11:39 pm PST

    Manifold absolute pressure is the inverse of manifold vacuum, so the absolute pressure would be highest at the heaviest loads; widest throttle position, and greatest air flow; and it is. Those of us, like myself, who are used to thinking in terms of manifold vacuum would do a double take at those numbers; but they are correct.

    The fuel trim numbers look less consistent than I'd expect; but I'm not familiar enough with this parameter to be certain of that. My approach to this situation would be to first replace the fuel filter, and thoroughly clean the mass airflow sensor and throttle body. I would also carefully check the motor for disconnected or damaged vacuum hoses or manifold vacuum leaks.

    I also need to point out that the original equipment spark plugs on this motor are Iridium, and the stock plug gap is .040" (which is considerably smaller than the plug gap on previous GM engines.) If the replacement plugs were not Iridium; or the plug gaps were not confirmed or adjusted to the correct setting; the new plugs would not perform satisfactorally.

    If that fails to improve the situation; I would conclude that the ignition coils are failing; which is a common issue on this motor. If your Denali has the 6.0 liter motor, or the 4.8 or 5.3: I would recommend replacing all 8 coils with a better quallity part.

    There are two different coils that were used on this model. One is a square coil made by Mitsubishi and is stamped 12558693. MSD makes an improved replacement coil for this application; under their part #MSD-82468. This part number is for a set of 8 coils. Summit Racing sells them for $604.95 per set plus shipping.

    The other coil used in this application is a round coil made by Delphi; and is stamped 19005218. Accel makes an improved coil specifically for this application, under their part #ACC-140040. Summit Racing sells these coils for $53.95 each plus shipping.

    Please note that all trouble codes must be cleared from the computer with a scanner after repairs are done.

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