Question about 1999 Pontiac Grand Prix

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I AUTO SCANNED MY CARE AND GOT CODES PO300 A RANDOM MULTI MISSFIRE, I CHANGED THE SPARK PLUGS AND WIRES. I ALSO HAD TO CHANGE THE FUEL PUMP THE GAS COMPRESSION WAS AT 18 AND THAT WAS AFTER TURNING IT ON 15 TIMES, SO IT IS STILL MISSFIREING AND NOW IT EVEN BACKFIRES AND SHIFTS AT ABOUT 4 AND A HALF RPM NO POWER,COULD IT BE A COIL OR THE MASS AIRFLOW, HOW CAN I TELL?

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  • Jonathan S
    Jonathan S May 30, 2010

    Which engine is it? Your description indicates that the problem actually became worse after changing plugs and wires. I would start back at the plugs and wires just to make sure the wires are on the corresponding cylinders with the coils. The 3.1 and 3.4 are commonly known to have fuel injector issues as well as intake gasket leaks. The 3.8L is a pretty solid engine but does have a few quirks to it as well. Any additional information would be quite useful in helping you find a solution. Is the scanner your using just a code reader or a scanner this would also be helpful.

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  • Pontiac Master
  • 3,640 Answers

Go to any auto parts store and buy a can of maff sensor spray, and clean maf sensor. it can get dirt in it and make it mess up. follow directions on can. also make sure air filter is clean and free of dust and dirt. to remove it is in air flow from air cleaner box to throttle body . remove the hose then the wiring connector, then remove maf sensor for cleaning. let dry then reinstall. first dis connect maf sensor and if it is better then the maf is your problem. there are many things that cause backfiring, you may need proper equiptment to diagnose problem. you can try the coils they could be bad.

Posted on Jun 23, 2010

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