W124 Misfire/Idle Issues

Discussion in 'Engine' started by dylantaylor544, Jan 7, 2019.

  1. dylantaylor544

    dylantaylor544 New Member

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    Hi Everyone

    Heres another W124 mercedes with this very common issue it seems. The car is misfiring at certain RPM/Throttle body positions and sometimes it will stall at idle and then sometimes also battle to keep its idle. Throttle body position makes a difference, if you do a 20% easy pull off it seems ok, with an initial hesitation. Any fast pull of you can notice a heavy shudder. Once it reaches 4000rpm you can feel a noticeable improvement in power and steadiness.

    The wiring loom has been removed and tested, it has been repaired before(previous owner) but it is all good. It has been to a merc specialist garage who tested the codes and the 02 sensor and MAF were replaced. After they were replaced the error codes cleared and retested and the same remained so it was suggested an ECU fault.

    Currently the ECU is on its way for testing /repair but is there anything obvious that it could be?

    Thanks
     
  2. GP801

    GP801 Hardcore MB Enthusiast

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    Certainly could be the ECU. It was when ours had a similar issue. I was told that dodgy coil packs caused the ECU to fail so its probably worth changing those unless their provenance is known
     
  3. grober

    grober MB Club Veteran

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    Other problems worth investigating would be the inlet camshaft adjustment solenoid and the OVP [over voltage protection relay] which supplies the engine ECU- its a straight DIY swap/fix if it happened to be that. A more remote possibility is a fluctuating fuel pressure due to faulty fuel rail regulator, a partially blocked in line filter [ the one beside the pump] or tank outlet strainer or even dodgy fuel if you have recently filled up somewhere different recently.
     
  4. OP
    OP
    dylantaylor544

    dylantaylor544 New Member

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    Thank you for the detail. As soon as the ECU is back in I will update this thread and let you know if that was the problem. Otherwise i will tackle those next!
     
  5. WDB124066

    WDB124066 MB Club Veteran

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    What about those pesky coil pack things, they are known to break down aren't they..?
     
  6. GP801

    GP801 Hardcore MB Enthusiast

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    They are indeed. Ours had dodgy coil packs and HT leads that were leaking current through the insulation. Unfortunately replacing them did not solve the issue and it was then they diagnosed the ECU problem. They felt the faulty coil packs had fried the ECU.
     
  7. grober

    grober MB Club Veteran

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    The way the ECU works is that the coils are supplied by a constant "ignition on" 12volt line. Each coil fires two plugs in two cylinders simultaneously in a "lost spark" system. One cylinder is on the compression stroke [ the operative one] and the other on the exhaust stroke [ no combustion]. The 3 coils are fired by three heat sinked power transistors in the ECU by "earthing" the coil in question causing the field in the primary coil winding to collapse thus inducing a current in the HT part of the coil firing the 2 plugs.
    I descibe a DIY repair for this in a two part thread on this forum.
    All you need to recognise is that the fault in the ignition circuit that fried the ECU transistor/s must be identified and addressed at the same time as the ECU repair- if not it will blow those transistors again.

    HOW TO repair your ECU ignition part 1
    and
    How to repair your ECU ignition part 2
     
    Last edited: Jan 9, 2019
  8. OP
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    dylantaylor544

    dylantaylor544 New Member

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  9. grober

    grober MB Club Veteran

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    Personally I would go for a Merc OEM brand if possible like Hella Beru or Bosch. Some makes seem to come with a long extender others are just the coil.
     
  10. OP
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    dylantaylor544

    dylantaylor544 New Member

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    Hi everyone. So very interestingly the ECU went for repairs and the injector module within had to be replaced plus some output transistors. They can't really tell me if this is age or a short but now that I have a healthy ECU, but. . how would I be able to know if the injectors wont cause this issue again?
     
  11. grober

    grober MB Club Veteran

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    I haven't heard much about repairs to the injection side of the ECU so can't comment sorry. Most ECU failures seem to be associated with engine wiring loom or coil shorts but I guess these can happen on the injector side of things too. Perhaps certain ECU components had been weakened by the wiring loom shorts [ now repaired]as you described. If repaired adequately the problem should not return. Only safe way is to replace the entire wiring loom for a new one. I guess you could test the injector coil windings for resistance and earth leakage with a DVM. Don't have any manufacturer figures for these to hand but if one or two are markedly different from the others???
     
    Last edited: Jan 11, 2019
    WDB124066 and stwat like this.
  12. OP
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    dylantaylor544

    dylantaylor544 New Member

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    The ECU arrived today with UPS and I don't think he was even out the door and I was fitting. Replaced coils packs with OEM Bosch at the same time. The car is now purring like a kitten! Thank you all for your advice and tips, a great success story!
     
    WDB124066, merc85 and grober like this.
  13. GP801

    GP801 Hardcore MB Enthusiast

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    Great result. Another old car lives on!
     
  14. darreningram

    darreningram Active Member

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    I just wonder if this could be my problem? I’ve ordered a new fuel pump relay to try- it’s cheap!

    my E280 has 170000 miles on clock- runs fine, very smooth- then all of a sudden it will just stall- starts again then stalls after20 secs- leave it for a few hours and then it is fine- runs for a bit then same again- it has had new OVP relay, MAF and fuel pump- please help or I’m scrapping it! Any help always appreciated[/QUOTE]
     
  15. GP801

    GP801 Hardcore MB Enthusiast

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    Ours would run fine for ages and then start to misfire - one cylinder, perhaps two were down. Did it once leaving the tunnel from Italy to France and I pressed on with some trepidation as it was snowing hard but it then cleared itself after a few more kilometres and made it back to the UK as if nothing was wrong. A few days later it just would not run at all which was when the fried ECU was finally diagnosed. Coil packs or wiring loom are likely causes too. Don't know if that helps but its not an easy fault to diagnose. With ours it probably took 9 months from initial symptoms to final cure.
     
    WDB124066 and darreningram like this.
  16. paulnw

    paulnw New Member

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    [/QUOTE]

    Hi, sorry for jumping on to your post but I am trying to buy a replacement fuel pump relay. The code on the relay is Hella, 12V 20A, 4RA 007 813-00. I can only find used ones of these relays but ideally I would like to buy new. Did you replace the relay as mine? If so which relay did you buy. thanks
     

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