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is this most likely a bad ECU?? 700 1990

posted by  myoder on  Thu Aug 16 18:27 CST 2007
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last visit: Tue Mar 5 12:40 CST 2013
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I wish I knew more here but I am helping a friend with a problem on his 90 740. He was driving and came to a quick stop and when he started again the engine started running very poorly, and the car would not really rev higher than 1200 rpm. It has been described to me as a boom boom noise when the engine is reved. He took the car to a shop which did extensive testing and so far has not been able to pin point the problem. all the sensors seem to be fine AMM fine etc. THe car runs fine at idle but wont really go faster than 1200 rpm.. The mechanic told me it seems like there is a rev limiter or something. Unfortuantely, I have not actually had a chance to look at the car myself yet, but based on this information is there any suggestions anyone can give?? I guess the only thing left causing this kind of problem is the ECU? I looked a little in the FAQ and there was somthing about a failing module having a symptom of "the engine will not rev above 3000 rpm" so that seems to apply to a certain extent.

Thanks for any help you can give!








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    is this most likely a bad ECU?? 700 1990

    posted by  mapleleafer on  Fri Aug 17 12:39 CST 2007
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    last visit: Wed Oct 9 00:36 CST 2013
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    His mechanic should connect a fuel pressure gauge to the rail and confirm that pressure is rock steady at 3 bars (with vacuum hose off). This would be the first step in determining whether the trouble is fuel delivery (and the phrase "boom boom noise" indicates a lean mixture)... or not. There is no guessing on this point - a proper gauge reading is the only definitive test.

    He could spend from today until doomsday checking sensors, computer, ignition, engine mechanicals and never find the answer if fuel pressure drops under load.








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    is this most likely a bad ECU?? 700 1990

    posted by  someone claiming to be fixit2003 on  Fri Aug 17 02:51 CST 2007 [RELATED]

    "Quick stop" could have dislodged some gunk in the fuel tank and plugged the gas filter. Will it go faster than 1200 rpm in neutral or just when in gear?








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    is this most likely a bad ECU?? 700 1990

    posted by  mgarageu on  Thu Aug 16 19:43 CST 2007
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    last visit: Sun Mar 9 03:34 CST 2008
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    hello has anyone checked the OBD flash codes on that car? if not, i suggest to do it. if no luck, check timing belt, air cleaner duct, fuel pressure...or sth else.
    --
    i prefer non-german cars








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      Another possibility 700 1990

      uid 18083's pic
      posted by  lucid on  Fri Aug 17 01:00 CST 2007
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      last visit: Fri Oct 24 12:31 CST 2014
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      The "came to a quick stop" event could have "unloaded" the Timing Belt tension, if the belt was old or stretched for any reason, causing it to jump several teeth at the crank sprocket.

      The one time I've actually seen this happen (new belt but not properly retensioned at 600 miles), it caused a no-start on a 240). But I suppose the amount of "slippage" could vary, as could the symptoms.

      I would get the #1 piston at TDC compression according to the crank pulley mark (keeping in mind that it can also slip) and then check the first two cam lobes. If the crank and cam shafts are still in correct time:
      #1 should be pointing almost horizontally inboard
      #2 should be pointing outboard, and upward about ~30°.

      If the cams look wrong, The balance/puller mark may be wrong, so double-check that the piston really is at TDC by feeling the piston top with some kind of thin probe through the sparkplug hole, while rocking the crankshaft with a 15/16" socket on the crank pulley bolt.
      --
      Bruce Young
      '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.







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