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Thread: Weird plug fouling

  1. #1
    Tuner in Training
    Join Date
    Nov 2016
    Location
    Middleburg fl
    Posts
    24

    Weird plug fouling

    99 5.3 swapped into a 69 nova, stock motor rebuilt and 4l60e. So the base tune is good but RPMs dropped heavily and has a high/low surge when the A/C came on so I increased the idle speed upon A/C request by 200 RPM, this helped some but after about 10 minutes of idle the motor started smoking real bad (rich condition) I quit for the day and downloaded 2 similar files to compare and found +- 10 points variance in only a few tables, this morning I pulled all spark plugs and only 2 plugs were fouled ( the 2 rear plugs on opposing sides ) strange! I ran a compression test (130 +- 5 across all cylinders) did a leak down check on cooling system, it held -15 psi for 20 minutes, swapped injectors on passenger side and did a positive pressure smoke check on intake plenum with 8psi and found no leaks. Data log did show a strange increase in total advance when A/C engaged increased number to 46 degrees advance timing. Loaded my second tune back in the car and she runs good again, no smoke, plugs look good now but still have the surge and now I ask....

    1. Why did it only foul the 2 rear plugs on opposite sides?
    2. Why was the computer commanding 46degrees advance timing
    3. what do I need to tune to stop the "surge" when A/C is engaged

  2. #2
    Advanced Tuner
    Join Date
    Jun 2008
    Posts
    539
    Quote Originally Posted by Paintbytony View Post
    99 5.3 swapped into a 69 nova, stock motor rebuilt and 4l60e. So the base tune is good but RPMs dropped heavily and has a high/low surge when the A/C came on so I increased the idle speed upon A/C request by 200 RPM, this helped some but after about 10 minutes of idle the motor started smoking real bad (rich condition) I quit for the day and downloaded 2 similar files to compare and found +- 10 points variance in only a few tables, this morning I pulled all spark plugs and only 2 plugs were fouled ( the 2 rear plugs on opposing sides ) strange! I ran a compression test (130 +- 5 across all cylinders) did a leak down check on cooling system, it held -15 psi for 20 minutes, swapped injectors on passenger side and did a positive pressure smoke check on intake plenum with 8psi and found no leaks. Data log did show a strange increase in total advance when A/C engaged increased number to 46 degrees advance timing. Loaded my second tune back in the car and she runs good again, no smoke, plugs look good now but still have the surge and now I ask....

    1. Why did it only foul the 2 rear plugs on opposite sides?
    2. Why was the computer commanding 46degrees advance timing
    3. what do I need to tune to stop the "surge" when A/C is engaged
    Do some tests that will find a rich condition. Like fuel pressure, does the tune file match the injectors, fuel pressure, and regulator type....
    Not really sure how a cooling system test is going to find something making it run rich.

  3. #3
    Tuner in Training
    Join Date
    Nov 2016
    Location
    Middleburg fl
    Posts
    24
    Was grasping at straws, it's a fresh rebuild so I checked everything I could, fuel pressure is 55 and matching, after loading my second tune everything is back to normal. I did a third time today increasing A/C RPM and now the surging is gone as well