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    Momentary throttle cut at WOT

    Hello All!

    this past weekend I was shaking down my new to me Spec E30 at VIR and encountered a slight issue in my very last session. After more or less running flawlessly for 8 sessions, the car would momentarily cut throttle when at WOT (north of 5000RPM) in 4th and 5th gear. This would also seemingly be accompanied by the ABS light flashing.

    In the course of 30 minutes, this happened 6 or so times, with the majority being in 5th gear. As you can see in the video, throttle came right back, but my foot was pinned to the floor the entire time. I would really like to chase this down before it becomes a bigger, more disruptive issue, so I appreciate whatever thoughts you may have on it! I am likely to pull the engine to replace the long block and install a fresh clutch/flywheel while I am at it (190k on the motor with no evidence of any rebuilds and it is down on power from the data I collected).

    here is the video, just 20s in total cut from the various instances in the session

    https://youtu.be/kKHVyjwQNSI
    Last edited by gybe,; 02-24-2020, 06:50 PM.

    #2
    Start by testing the WOT part of the throttle switch. Next would be AFM.

    Out of curiosity, is that the 1984 se30 that just left South Florida?

    Edit: After watching the video, it appears you might be dropping spark. Make sure the ignition coil mount is stable (the welded stud tends to break off, we always drill out and through bolt the race cars at work), zip tie the three relays in on the fender near the fuse box (they can rattle loose and cause a momentary loss of fuel pump and or ECU relay), and get a NEW CPS from BMW, don't use any aftermarket brand for a reliable race car (30yr old CPS tend to break the wire where it meets the sensor, aftermarket fail easily). If the CPS is new, then check the air gap from sensor to reluctor wheel. The two 13mm hex head bolts that hold the sensor bracket to engine have play in when you loosening the bolts.

    Oh, and make sure the CPS wire has not rubbed on the water pump pulley. Many cars have the conduit left out, so all the race car builds we do at work run the CPS wire over the timing belt cover, but may have the remove the female Bosch 3 pin connector from the bracket that holds it to reach.
    Last edited by ForcedFirebird; 02-24-2020, 07:38 PM.
    john@m20guru.com
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      #3
      Thanks, lots of good info that is indicative I was on the right path (my initial thought was CPS or TPS), but that ABS light is throwing me.

      This car is a “fresh” build out of Virginia. Just got safety inspection this weekend so should be able to get a log book in March. This was just my first shakedown of the car, which went pretty well overall. My times would put me in the back 1/3 of the pack, but given the data that shows how far down I am on power (bridge straight at VIR, despite same corner exit speed am 5mph down) and how much of a transition in driving style this is from my normal HPDE vehicle (Alfa Romeo 4C), I am pleased with my initial progress!

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        #4
        Right on! A local friend just sold his car and it went to VA, so looks like some more se30 action. :) I haven't done an se30 sprint race in 2yr, been having more fun in the Champcar builder series where we get to tinker with the cars, and strategize long race pit etc.

        I have heard about issues with "ice mode" faulting in an e30, but never experienced it. Apparently a couple ECU's have a wire that connects to the ABS, but the only one it could possibly be is from an ix.

        You can try unplugging the ABS module and see if the issue goes away, but be weary of the braking zones, you will have to brake harder and earlier to avoid flatting a tire.

        What's the last 3 digit on the ECU? 173 is the preferred computer, but I have found the 153 is actually a little better in peak, but less under the curve. I have a thread here with many Dyno pulls and advice for diagnosing a loss of power.


        https://www.r3vlimited.com/board/for...20-dyno-thread
        ​​​​
        john@m20guru.com
        Links:
        Transaction feedback: Here, here and here. Thanks :D

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          #5
          I had similar cut off at WOT as your video describes. It did not do it all the time. It happened mostly when engine is under heavier load. I replaced the coil, wires, plugs, distributor, rotor, fuel pump, fuel pump relay, main DME relay, CPS, DME and rebuilt AFM, TPS. But the issue lingered. I always got some oil consumption and the plugs got fouled easily. Eventually, I pull the head, refreshed the head and replaced the valve stem seals. The oil consumption went away, the foul plugs went away, and cut off at WOT went away as well.

          For the ABS light, I will check the relay described in the following video.
          https://www.youtube.com/watch?v=9N4ITKWv-6w

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            #6
            We've had master switches (the kind the rules tell you to add) do this, from vibration. The ABS light makes me
            think of that, because the OE relays, MAF, and TPS and also the connector under the intake are all possibilities,
            but the ABS light is odd...

            Hell, if you still have the ignition switch, flakeyness there could do it.

            Get all the bugs out before you spring for the new motor!

            Things like this drove me down the datalogging rabbit hole.

            t
            has yet to spring for the new motor in PRO3
            now, sometimes I just mess with people. It's more entertaining that way. george graves

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              #7
              These issues are usually attributed to failing CPS, AFM and DME. We are staring to see a lot of failing DME's now.
              Owner - Bavarian Restoration
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                #8
                For me it was ICV...after going through "the list."
                How I found it? At one of the track days, I went out for a session forgetting to reconnect ICV wire (don't remember why i had ICV disconnected). The car did not stumble without the ICV. Replaced the ICV and no issues since then.

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                  #9
                  Originally posted by bad3 View Post
                  For me it was ICV...after going through "the list."
                  How I found it? At one of the track days, I went out for a session forgetting to reconnect ICV wire (don't remember why i had ICV disconnected). The car did not stumble without the ICV. Replaced the ICV and no issues since then.
                  Interesting. I do know the ICV moves as the throttle is opened, so it makes sense.
                  john@m20guru.com
                  Links:
                  Transaction feedback: Here, here and here. Thanks :D

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