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Thread: Engine Surging at Constant Throttle, No load

  1. #1
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    Engine Surging at Constant Throttle, No load

    Help me solve this problem. Scan file and tune file attached. I have replaced every sensor, tried 2 different (0411) PCM's, two sets of injectors and two different wiring harnesses and the problem persists. Engine is a 468 cid LSX tall deck, E85 fuel, Mast split manifold with Wilson throttle body and huge cam on an SD tune installed in a race boat. The problem can be seen in my scan file. Engine held constant at 3200 RPM and it sags below 1000 rpm every 2 to 3 seconds. Everything is properly set up and has been running for more than 3 years with this configuration. I even ran the engine with a Dominator carb using the PCM to run the sparks and it ran fine. This verified the PCM, reluctor wheel, and crank position sensor. I can only figure that something in the tune is causing the problem. This has been killing me for months and I hate failure.
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  2. #2
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    That datalog just shows the engine not running.

  3. #3
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    Thanks for pointing that out, I picked the wrong file. I have a better one now which I will attach. Please examine from 10.0033 to 10.0039 just after start up there is 7sec of stable operation at around 1900 RPM. All parameters look good to me. Fast forward to 10.00.50 to 10.01.04 to see the oscillation. I think this is driven by a bad MAP signal, especially since this is a speed density tune. Your opinion would be greatly appreciated.
    This is a boat on a trailer running at no load.
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    Last edited by Asteroid; 02-28-2020 at 09:35 AM.

  4. #4
    Senior Tuner Frost's Avatar
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    You are not logging much that is helpful, unfortunately. Can't see the position of the throttle in the log. We can't see the IAC. We can't see any airflow PIDs. "From here" it just looks like a cold engine with not enough airflow through the IAC and around the throttle blade (probably the latter). What are your IAC counts at hot idle? What is your current closed-throttle TPS voltage?
    Steve Williams
    TunedbyFrost.com


  5. #5
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    Quote Originally Posted by Frost View Post
    You are not logging much that is helpful, unfortunately. Can't see the position of the throttle in the log. We can't see the IAC. We can't see any airflow PIDs. "From here" it just looks like a cold engine with not enough airflow through the IAC and around the throttle blade (probably the latter). What are your IAC counts at hot idle? What is your current closed-throttle TPS voltage?
    Thanks for your observations. The data log shows data in strip chart form and digital form on the left. TPS voltage is in the digital list along with other data. This was not an idling run and the throttle was not closed. This is a race boat and idle control and drivability are not something I care about. There is no IAC motor, screw adjustment is all I need. This is a speed density tune with no MAF so no airflow data is available. The SD tune is driven by RPM and manifold pressure, all other parameters follow. The difference between the early stable 7 sec run and the later 14 sec of surging is the instability of the MAP from one event to the other. With a constant throttle blade position the later engine data should have resulted in a constant 4000 RPM condition, not the surging shown. The MAP swings caused the timing, PW and the resulting RPM to oscillate. There was no physical condition that would have caused this MAP instability. So I believe that the MAP is faulty. I have verified the MAP pin outs so I am ruling out a wiring or PCM issue. I will change out the MAP to verify my theory. If there is something I am missing, please tell me.

  6. #6
    Senior Tuner mbray01's Avatar
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    Even in sd, there most certainly is airflow pids, and you should be logging them. You need actual airflow to determine fueling, as well as cylinder airflow, which determines timing. You are confusing speed density with alpha-n, which is simply throttle positon vs rpm
    Michael Bray
    Rusty Knuckle Garage
    Slidell, Louisiana
    20yr Master Tech.
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  7. #7
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    Quote Originally Posted by mbray01 View Post
    Even in sd, there most certainly is airflow pids, and you should be logging them. You need actual airflow to determine fueling, as well as cylinder airflow, which determines timing. You are confusing speed density with alpha-n, which is simply throttle positon vs rpm
    Thanks for pointing out my inexperience. I will try to find the airflow pid's. If you can suggest the one's I need I would greatly appreciate that. On a good note, I replaced my MAP and logged a new scan. The result was solid MAP data and no surging over a 31sec run. RPM was stable up to 3000 rpm and idle was also good considering the cam. I am attaching the most recent data log for anyone interested.
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  8. #8
    Senior Tuner mbray01's Avatar
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    WHEN TUNING, I LOG THREE AIRFLOW PIDS ON MOST SETUPS
    VOLUMETRIC EFFICIENCY AIRFLOW, MASS AIRFLOW, AND DYNAMIC AIRFLOW. DYNAMIC IS WHAT THE ECM IS ACTUALLY CALCULATING AS THE AIRFLOW. iF ITS NOT AGREE WITH YOUR AIRFLOW PIDS, THERE IS SOMETHING WRONG. ALSO CYLINDER AIRMASS, THIS IS WHERE TIMING IS CALCULATED
    Michael Bray
    Rusty Knuckle Garage
    Slidell, Louisiana
    20yr Master Tech.
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    Custom Car Fabrication, Customization, High Performance.
    GM World Class Technician
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