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I have a 1997 Chevy s-10 blazer, 2 door LS, 4wd v-6, vortec, 4L60 auto trans, 216,000 miles, not a oil burner or smoker, Whenever I go to climb a steep grade it cannot maintain a steady speed and starts to slow down and shakes then check engine light comes on or when I try to accelerate on a level grade, it hits around 2500 to 3000 RPM and then it starts to shake a little and check engine light comes on flashing for a long time and then finally staying on. It shows (P300) random misfire. Other than that, it starts up and idles fine except shakes a tiny bit because of the 90 degree v-6. I have put many new parts on it to try and nail down the problem. Here is a list.

  1. New muffler and catalytic converter
  2. (4) new Bosch oxygen sensors
  3. New Delphi egr
  4. New Delphi IAC valve
  5. New distributor, cap, rotor, wires, ign module and coil.
  6. New Throttle position sensor.
  7. New Delphi fuel pump and filter.

All of this was done by a mechanic I know at reputable shop in my area and I'm out of money and options. They have double checked everything from fuel pressure to all of the parts they put on it and even they are stumped as to why this keeps happening. Could this be something as simple as a spark plug because they are the only thing that have not been changed and the set in the truck have approx. 20,000 miles on them. They are Autolites. Could this problem be something very unique to this engine and vehicle model. I've looked for recalls on something like this but no luck. Any Ideas???

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With only 20k miles on the plugs, I wouldn't think that would be the issue, but it looks as though the only thing which is left to replace. As RPM's and cylinder pressure increases, the spark plugs have a harder time covering the gap. You may pull out a plug and read it to see how it looks and if they are functioning correctly. Best guess I have for you. –  Paulster2 Jun 5 at 15:03

1 Answer 1

When it comes to misfires at high loads, this indicates a lean AFR (too much air/not enough fuel). Of many possibilities, the following reasons are the most likely contenders:

  • underestimation of the air mass flow

    This could be due to an under-reading MAF (mass air flow) sensor. These sensors tend to foul over time, which makes the fuel management pump in less fuel than what is actually needed.

    This may also happen due to unmetered air finding its way into the combustion chambers (thanks Paulster2 for the reminder!)

  • insufficient fuel delivery

    A lack of fuel flow could be down to a failing fuel pump, clogged fuel filter or clogged fuel injectors.


Given your list of already-replaced parts, I believe the MAF sensor is the most probable culprit.

Of course, post-MAF-sensor air leaks could also be to blame. A thorough inspection of the air intake should be performed before sinking money into a new MAF sensor.

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With your underestimation of the MAF, this could also be caused by air coming into the air tract after the MAF (leaking air intake system before the throttle body). I've seen it where you get a hole on the bottom side of the air intake where you cannot see it and never know it. It really isn't a vacuum leak, just unmetered air being consumed. –  Paulster2 Jun 5 at 18:58
    
@Paulster2 : yup, that could definitely be the case as well. Best check that first, at least with a visual inspection –  Zaid Jun 5 at 19:50

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