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How to change in realtime the value of MAP/MAF sensor by OBD port?

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  • How to change in realtime the value of MAP/MAF sensor by OBD port?

    Hi guys, I'm new here and I'm looking for a small help..
    I have to develope a new OBD hardware embedded tool for trimmer/Save fuel under the HHO CELL generator.
    The hardware prototype is ready, but now after some research seem that is not full possible rewritten a new MAF/MAP value by OBD.
    Click image for larger version

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    It seem for me strange that is not possible rewritten new value in realtime, because the Volo Performance chip already make this job under OBD:

    Instead to request the diagnostic PID's from OBD, we can also read in realtime the flow data:

    all of this data are captured by my developer from an opel corsa in real time and this data can be rewritten in realtime too.
    The only problem is HOW to interpreter this data to find where is the MAP/MAF sensor and then update with a new value to the ECU.
    The point of my hardware tool is to trimmer Fuel, when using HHO as a fuel supplement, the tool will adjust fuel delivery and timing to optimize efficiency.

    Volo Performance chip use this system, BUT we have to reprogramming the chip for each Car model.

    Another way ( but not so good) is to use one external MAF/MAP enancher potentiometers connect on MAP/MAF sensor:

    Anyone have a idea how we can resolve this issue?
    Thanks again for the help!

  • #2
    That sounds like you need to do some analog trickery. You would have to intercept the analog signals from the sensors, and feed into one leg of an op amp, add the modification signal on the other and feed the output back to the maf connector?



    • #3
      I believe EFI live can do and also HP tune.

      You will not be able to remap (that I have heard of ) to an external trim.

      You would have to have the entire "tune" being controlled by the OBD port, not just part of it.
      link to my CTS-V project:


      • #4
        Originally posted by rray View Post
        That sounds like you need to do some analog trickery. You would have to intercept the analog signals from the sensors, and feed into one leg of an op amp, add the modification signal on the other and feed the output back to the maf connector?

        Hi Robert
        This already exist:

        I like to make all by OBD port, without cut/edit map/Maf wires.

        Volo chip HHO is working about fine, BUT have a fixed map to reduce ony the 5%.
        If we use a bigger HHO CELL, we like to trimmer by software by OBD can bus, this is good balanced Fuel/HHO and then SAVE the parameter.


        • #5
          Changing the value of the map or maf via the Can-bus is NEVER going to work.
          The value of the map or maf is not present on the can-bus
          Even if this value is present, then it's only for other systems than the engine.

          I don't see you doing it via the OBD port either.
          So maybe you can change the value for airflow that is present in the ecu, but usually that's only used for output to an OBD2 scanner, it's not used in the calculations the ecu makes continuously.
          How are you going to write back this value? That's not part of the EOBD/OBD2 standard.
          Writing back something is only possible if the factory protocol is used and there are provisions made in this factory protocol for writing values.
          And how will you diagnose any problems with the engine? You already have a device communicating with the engine ecu, so how will a scanner (factory, aftermarket or EOBD) connect to the engine and retrieve information?
          Last edited by p2psmurf; 10-01-2013, 01:47 AM.


          • #6
            p2psmurf is right, the best option would be to get an ECU reflash instead of all this work.


            • #7
              LIONSTRACS you are going down a dark road. you need to stop, back up, and listen to the people in this forum. a map / maf sensor reading is a completely different animal and is not directly linked to fuel injector duty cycle or pulsewidth. it affects other areas of the software driving the powertrain. making sensors lie to a controller will never, NEVER, N E V E R end well. these tuning solutions such as efi live, hptuners, sct, aemtuner, hondata, chrome, etc. rewrite the ecu. aside from fueling, changing these values can affect ignition timing which can be very dangerous at wot pulls, it can affect egr and canister purge operations, a/c clutch state, when the pcm is supposed to switch to or off of idle, automatic transmission shiftpoints, on vehicles with electronic throttle control changing map values can and usually will have negative effects and on corvettes the map sensor determines when the butterflies in the exhaust tips open. the only right way to tune an efi vehicle is to log data, plot hystograms, and make changes to fuel and timing maps without changing sensor calibrations. well, I mean, except on some newer vehicles that have "virtual ve" tables that are 5 axis and we can not change, on those we configure histogram to plot long term fuel trim data over maf sensor cells, log some data, then copy and paste special multiplying by %. also on vehicles that we boost we install a 2 or 3 bar map sensor and then we have to re-calibrate map for the new sensor(and switch to a 2 or 3 bar os). when we change sensor values or calibrations it is with the utmost care and precision. by using hystograms and logging data, the vehicle pretty much tunes itself, or rather it tells us where everything needs to be. another big trick to tuning is that it is near impossible to tune an engine away from that engine. and there is no such thing as a good "canned" tune. even if we lived in a perfect world and could produce two exactly identical engines, by the time they both hit 50k they would still be noticeably different because of different levels of wear in different places from different driving habbits. there is no universal power adder that can be measured in a percentage of a sensor change. making the maf report back 5% more air on a Honda might make more power because of a little extra fuel on an engine that is predispositioned to running on the lean side, but that same 5% extra added to the maf might make an automatic mustang act like its tranny is about to fail, or make a gm car with electronic throttle body throw up the dreaded reduced engine power message on the cluster and limit vehicle speed to 20mph.

              and no matter how you make any sensor report anything back to the ecu, the o2 sensors are gonna report back that the engine is outside it's commanded afr and the ecu will trim it back into spec and start throwing codes and bring up the check engine light. and if it is drive by wire, it will throw the car into limp mode.