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Stumbling problem with piggyback/MAF

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  • Stumbling problem with piggyback/MAF

    I had a local shop do a MAF conversion using the Unichip piggyback system several months ago, and I have been having a recurring problem.

    When the AC is on, the motor will bog/miss horribly off idle, as if it's running on one cylinder. Nothing like having your car sputter and go nowhere at a green light with cars behind you :rolleyes:
    Otherwise, it runs fine without the AC on.

    The shop is fairly lost, so anyone else care to conjecture? I would imagine that the unichip piggyback is not happy with some sort of signal it's getting when the AC is on, maybe something with idle compensation?

    I'd hate to go back to the stock system, since the improvement has been quite nice when it's running well, and I don't have the extra dough to do a full A-N conversion right now (I traded a spare S14 I had to the shop to do the work).

  • #2
    I realize this won't help you, but I was just going to comment that I have found your sort of situation to be a big problem when you switch to the assorted MAF/AN conversions. When you encounter driveability problems, a lot of times you end up having to solve it yourself by trial and error. Unless you can find someone who is using the same system on the same car who may have some insight.

    There are three of us on this board who are using Lenz, and the only really usefull advice I have gotten regarding tuning and tweeking it has come from the other two. Tech support is a HUGE problem for most of these conversions. There just aren't that many of them sold for the manufacterer to justify the expense of having techs available to help with problems.

    And there are pretty much always problems.....

    Not sure why I wrote this. Didn't help at all, did it? :rolleyes:

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    • #3
      Does reving with the clutch in and a/c ON do the same thing? How about being aggressive in the launch, keeping the revs up. Same thing?

      I don't know really how the a/c would fit in with the motoronic, and then the unichip on top of that but a guess, and my opinion of course, would be you do not have enough timing to overcome the load. Can you increase the timing below idle? Or could you raise the rpms when the a/c is on?

      Checking that everything was good with the hardware (plugs, wires, rotor, cap, the basics etc) would be on my list too.

      hth david
      Last edited by dhs-; 08-26-2004, 09:55 AM.

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      • #4
        My guess is that when the compressor goes on, then even though it may send a signal to the DME for higher idle speed, but that signal gets lost somewhere and the idle valves doesn't get the signal. Remember that the idle valve opens up a bit more when A/C is on, or the motor will... stall.


        [email protected]

        1969 2002 racecar
        1989 M3 racecar
        e39 Touring

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        • #5
          What kind of maf conversion are you revering To????
          Moving from a flapper door type VAF to a MAF ( with heated wire).
          If so that would be to much for the dastek

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          • #6
            The standard ECU gets a signal when the A/C is turned on - I believe it raises the idle by around 50rpm.
            Steve
            Scottish Cecotto

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            • #7
              so if i'm reading this right, it only happens when your stopped and then starting to go? Does the car miss at all when you are driving down the road at a constant RPM and the AC on?
              Chris
              90 Corrado/91 M3/01 M5

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              • #8
                When you turn on the AC, the ECU changes maps.

                It will change the fuel & spark curves.

                SO, you may be running WAY rich with the AC on. You will need to put on a WB02 to read what the engine is doing.

                With a MAF, you pull air in a lot faster and some times the Bosh ECU can't keep up just on idle.

                The other problem, is idle, since the ECU raises the idle with the AC on, your piggyback ECU might be getting screwed up thinking you are in throttle enrichment off idle. You can try to raise the idle speed threw the piggyback to what it is the AC on.

                Do not click
                At least it's German

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                • #9
                  Quick update:

                  It seems I was mistaken about it only happening with the AC on, as I experienced the same stumble with the AC off yesterday.

                  So as a quick recap, it will stumble off the line badly, but there is no consistency to when/why it does it. It has happened on a cold motor, warm motor, AC on, AC off, and it doesn't happen at every light. Sometimes it will happen two or three times in a row, sometimes just once in a whole day. It also doesn't seem to miss anywhere else in the RPM range, whether under load or not.

                  The wiring harness for the unichip has been examined and re-soldered, and the MAF was replaced with a new unit.

                  This is just plain infuriating, as it's so inconsistent that it makes it difficult to isolate the problem. :mad:
                  Last edited by vlksdragon; 08-27-2004, 08:34 AM.

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                  • #10
                    Originally posted by vlksdragon
                    Quick update:

                    It seems I was mistaken about it only happening with the AC on, as I experienced the same stumble with the AC off yesterday.

                    So as a quick recap, it will stumble off the line badly, but there is no consistency to when/why it does it. It has happened on a cold motor, warm motor, AC on, AC off. It also doesn't seem to miss anywhere else in the RPM range, whether under load or not.

                    This is just plain infuriating, as it's so inconsistent that it makes it difficult to isolate the problem. :mad:
                    Does the unichip/MAF system have an adjustable throttle enrichment? If not, it is going to be hard as hell to tune it out. The only way the Moronic knows that the gas has been pushed is the TPSwitch, since it is not a TPSensor, it doesn't know how much. By the time the RPMs/MAF/Coolant temp/O2 signals get to the Moronic to add the additional fueling, its too late, it stumbles. If you had a WBO2 on the car you could probably see the lean spike that is making it do this.

                    It could be the ign map also. If the change from idle to tip in isn't dialed in correctly, the engine will fall on its face coming off idle without additional advance and enrichment. Welcome to the tuning game, play on.

                    T

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                    • #11
                      youre probably also modifying the ign. signal with the unichip?

                      If so how did you tie into the cranck sensor signal, did you shield this signal fully???

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