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  • decel fuel cut

    I have an issue that is confusing me re decel fuel cut - SM4.

    No matter what I do, the cut does not seem to work completely. I have played around with many settings, and at the moment I have overrun COM at >1200 and overrun REC at <1000. (yes, the engine catches fine with COM at 1000)

    On MegaquirtII, this worked exactly as expected - flawless. On SM4, it isn't. What I get is that above about 2000rpm, the fuel cut seems to work. Then as I am coasting down, fuel comes on again at around 1900 and commences with enormous cracks and bangs out the exhaust. It stops banging and appears to fuel cut at about 1400rpm until you COM at 1000.

    Now, here's another weird thing, when coasting down a hill in 5th and the exhaust is banging away, if I clutch in, let it idle for 5 seconds, then clutch out again, it will fuel cut where it wasn't before!

    I'm struggling to to work out how, but I feel this has something to do with the idle CLC.

    Thanks for help

    Pete
    Last edited by Nutzy; February 13th, 2010, 03:04 PM.

  • #2
    Well it works like expected....I just did a run in my emulator. As soon as you are above the com rpm and tps & load is under set values in Revlim/Overrun setup fuel is cut to zero..

    But you cannot be in warm-up or if the TPS follover is active.

    A log and a cal file would help us to solve your problem.
    Robert Jansson
    MRM-Racing

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    • #3
      Try turning your TPS follower off. It may be the manifold pressure is rising due to the IAC valve opening as the engine gets close to idle and that could cause the manifold pressure to rise above the overrun load limit.

      The overrun load limit can be set in M5/revlim/overrun setup. You should datalog the overrun to see what the load is when the fuel re-commences again.

      The popping and banging is usually due to overfuelling and retarded ignition on overrun. Try increasing your overrun ignition and reducing the fuel in the overrun cells in the base fuel table.

      Comment


      • #4
        Dudes, you rock! I set TPS follower to zero....and....ahhh, the sounds of silence!

        Seriously, thanks for that. It has been bugging me for months and I kept forgetting to post the question.

        Cheers,

        Pete

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        • #5
          That just tells you that the decel load limit is set too low 'or' your TPS follower is too aggresive. You can set your TPS follower first for best drivability and then log the load as you decelerate to see what the decel load limit should be set at to stop it interfereing with the TPS function.

          Comment


          • #6
            TPS follower gain was set at 50.

            I'll have to check the load setting, but it idles at around 18 and decels at around 21 inhg.
            Last edited by Nutzy; February 15th, 2010, 09:04 AM.

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            • #7
              OK, decel load limit was set at 60. Decel load is around 27, idle load is around 45.

              Tweaking the TPS follower up to even 5, commenced popping again.

              Comment


              • #8
                In the revlim/overrun setup it states under decel load limit that "overrun shutdown is locked out until idle follower has finished its decay."

                Obviously then when you switch on your TPS follower action it must be the decay of the TPS follower that is stopping the fuel cut.
                Do you have a speed sensor connected?
                Turn the TPS follower on and play with the TPS decay values to see if it makes a difference.

                Comment


                • #9
                  Originally posted by Nutzy View Post
                  OK, decel load limit was set at 60. Decel load is around 27, idle load is around 45.

                  Tweaking the TPS follower up to even 5, commenced popping again.
                  Did you solve your problem?

                  In reading the manual it implies that a higher TPS Follower Gain will speed up the action of the TPS follower. Have you tried a large number such as 200?

                  I have a similar problem and am still struggling to get fuel cut on overrun. I will be cranking up the decay values next to see if that helps.

                  Comment


                  • #10
                    Hi.

                    I'm not using the fuel cut at the moment but I am using TPS follower because I need it to catch the return to idle. The overrun fuel cell values seem to be high enough not to pop and bang. I'm still playing with these settings. Possible sticky AIV because I've never had an issue with return to idle before now.

                    Comment


                    • #11
                      Thanks. I solved the 'pop and bang' issues with more ignition advance. See here: http://www.mrm-racing.se/forum/forum...4141#post44141


                      Fortunately my car is able to 'catch' the idle with TPS Follower gain set to zero, possibly because of the idle ignition setup.

                      Originally posted by gsportcars View Post
                      That just tells you that the decel load limit is set too low 'or' your TPS follower is too aggresive. You can set your TPS follower first for best drivability and then log the load as you decelerate to see what the decel load limit should be set at to stop it interfereing with the TPS function.
                      What is 'too aggressive'? A big TPS follower gain number or a small one?

                      I have driven up the decay rpm values with no joy.

                      There are quite a few numbers to fiddle with, so it would help if I could understand what set of values drives a very rapid TPS decay so that overrun fuel cut works. The recommend default values don't seem to do the job.

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