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Thread: Misfire? M40b16 engine loss of power near 5000rpm

  1. #1
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    Misfire? M40b16 engine loss of power near 5000rpm

    Hi everyone,


    I'm having little problem with my 1990 E30 316i Euro Spec that's starting to pissing me off.


    2 year ago i change a head gasket from a M40B16 engine . Also changed the following parts:


    - Thermostat (Gates -> 87ºC)
    - HT Leads (Magneti Marelli) - Disconnected the Pulse Generator
    - Distributor Cap and Rotor (Bosch)
    - Spark Plugs (Bosch - FGR7DQP)
    - Fuel and Oil Filter (Bosch)
    - Timing Belt and Tensioner (Gates)


    After 3 months the car stalled on the middle of Traffic. After 10 minutes i was able to start it again but just for 30 seconds and it stalled again.
    On the Shop they said it was a bad Fuel Pump. So i purchase a Bosch unit (BMW OE-16 14 1 179 992).


    After they install it they said the Crankshaft Pulse Generator was bad too. So i purchase one (STARK) and also a new Blue Temp Sensor (Bosch) and a Throttle Position Sensor (Bosch). They were all installed and they call it Fixed.


    The car run fine but immediately run into an issue that never happen before. Sometimes pulling 1st or 2nd gear to the max RPM wide open Throttle, the car loses power at nearly 5000 rpm, dropping the RPM like engine don't have strength to pull the car.
    If i immediately change to the next gear the problem stops.
    Also when happens i don't ear any misfire, but the car shakes a little bit, like its working with less a cylinder but i'm not sure.


    This is specialty dangerous when i'm overtaking, since i never know when the car is going to do this.


    The Shop couldn't replicate the problem and said everything was fine. This happens sometimes, like 2 in 10 times.


    Do anyone have a clue what can cause this?
    Thanks in advance.

  2. #2
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    Sounds like the throttle position sensor is not triggering at the wide open position, you could try adjusting it, otherwise find a different shop, preferably one that has a dyno?

  3. #3
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    Hi, thanks for your reply, i would discard the TPS, because before all this, i had a faulty TPS, was relying on the AFM only and never had any trouble reeving all to the redline.

  4. #4
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    Well, in what way was the TPS faulty? Was it not reading idle or wide open throttle?

  5. #5
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    Quote Originally Posted by msservices View Post
    Well, in what way was the TPS faulty? Was it not reading idle or wide open throttle?
    Yes it was faulty. Changed and the idle immediately smooth out and on full throttle pulls a tiny bit more.

    The problem is, i changed so many things at once that its being difficult to find the culprit.

    But it began to happen when i changed the following parts: Fuel Pump, Crankshaft Pulse Generator, Blue Temp Sensor and the Throttle Position Sensor.

    I also re-gaped the
    Crankshaft Pulse Generator to about 1.1mm i think.

  6. #6
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    I know it was faulty. I asked in what way was it faulty. Okay either way your response leads me to understand it was the idle position that wasn’t registering, so you changed it, now I think it could be the wide open position that is not registering, which, IF that’s the case, could be a matter of simple diagnosis/adjustment

  7. #7
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    Quote Originally Posted by msservices View Post
    I know it was faulty. I asked in what way was it faulty. Okay either way your response leads me to understand it was the idle position that wasn’t registering, so you changed it, now I think it could be the wide open position that is not registering, which, IF that’s the case, could be a matter of simple diagnosis/adjustment
    It was faulty either on Idle and on WOT. Now its working fine i suppose, i can check with a multimeter.
    What is weird is that before when it did not work, i never had this issue.

  8. #8
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    No e30s, again :(
    it sounds like a bad connection to something. maybe unplug all those new parts and replug them in. maybe one has a loose connection
    No e30s again.

  9. #9
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    Quote Originally Posted by superj View Post
    it sounds like a bad connection to something. maybe unplug all those new parts and replug them in. maybe one has a loose connection
    That's a good idea, i'm gonna do some pulls with the TPS unplugged. Also i have to check the connection of the Crank Sensor. Thanks for the tip.

    BTW, do your think a bad connection can only happen on X RPM on WOT 1st or 2nd gear? Should a bad connection problem appear on other circumstances?

  10. #10
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    No e30s, again :(
    it could be that the engine is moving a certain way or the wind is making the wires move a certain way from the fan spinning. I had a bad connection at the crank sensor that would only happen if I hit a bump with both front wheels a certain way. I found it by just popping the hood and wiggling the wires, one section at a time, with the car running.
    No e30s again.

  11. #11
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    Quote Originally Posted by superj View Post
    it could be that the engine is moving a certain way or the wind is making the wires move a certain way from the fan spinning. I had a bad connection at the crank sensor that would only happen if I hit a bump with both front wheels a certain way. I found it by just popping the hood and wiggling the wires, one section at a time, with the car running.
    Wow, if that happened to you, its better to follow your advice. This weekend i'm gonna do some testing.

  12. #12
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    There is a write up about an almost identical problem that is Parked at the top of this forum, Mr Jlevie was kind enough to lay it out for us in detail, but most people probably scroll past these threads (as I have done) looking for the latest.
    His symptoms apparently boiled down to wiring/harness issues

  13. #13
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    Quote Originally Posted by msservices View Post
    There is a write up about an almost identical problem that is Parked at the top of this forum, Mr Jlevie was kind enough to lay it out for us in detail, but most people probably scroll past these threads (as I have done) looking for the latest.
    His symptoms apparently boiled down to wiring/harness issues
    Its amazing how similar Mr Jlevie problem is to mine. He even changed the goddamned engine and the problem still persisted.
    Thanks to calling my attention to that post.

    I'm most certain now that the problem will be about the Crank Position Sensor, its gonna be either Magnetic Interference and/or Bad Wiring.

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