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1992 RRC 3.9 V8 running problems

Discussion in 'Range Rover' started by JJKM, Aug 3, 2019.

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  1. JJKM

    JJKM Member

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    Hi all - new here. Have always been a member of another forum as a Series owner but since buying my RRC, haven't had much luck in their section!

    After many years of resisting the urge, I finally gave in and bought an RRC that's had a significant amount of the nasty jobs done, mainly involving the rot removal! However, one annoying issue it has is that once it has warmed up, it hesitates/momentarily cuts out under medium power. If you plod along or floor it, it recovers but it does make cruising quite tricky, particularly at mid-range speeds. Very annoying!

    So, it is a 1992 3.9 efi with Lucas 'Limb' system 35DLM8 (ignition module on the side of the distributor). So far, I have done the following:
    • Replaced plugs. Leads, cap and rotor all look new but I haven't ruled them out.
    • Removed and cleaned the MAF - wires weren't touched but actually looked quite clean.
    • Removed stepper motor and cleaned. Again, was quite clean but there's not a lot to see. Am I right in thinking that I shouldn't be able to move this in and out? There was a little sideways movement on the sprung cone shaped part indicating it wasn't stuck. Could anyone confirm what I should look for here? I did unplug it whilst running and the engine revved up much higher so it's clearly doing something.
    • Removed and cleaned the throttle potentiometer. Again, not a lot to see but the central tab seems to rotate easily.
    • Replaced temperature sensor. On this note, this part came from Bearmach and annoyingly, the outer plastic body has detached from the metal threaded core meaning that I can neither remove or tighten it! Fortunately, it is sealed but will be a job for another day. Disgruntled to say the least, particularly as it is little more than finger tight.
    • Replaced ignition module.
    So, I'm stuck. No idea what to try next and that's not to say that I haven't missed something from above. Can anyone shed any light? Another forum user has suggested looking for a code but am currently struggling to find the reader...

    Thanks
     
  2. Henry_b

    Henry_b ̿̿ ̿ ̿'̿'\̵͇̿̿\з= ( ▀ ͜͞ʖ▀) =ε/̵͇̿̿/’̿’̿ ̿ ̿̿

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    Its Full of WIN.....
    check your CO trim on the airflow meter.

    As a base setting, remove the plug from the air flow meter, use a multi-meter and measure the resistance between the two outside pins. The value should be 300 ohms. The value should be changed by turning an allen bolt on the AFM clockwise makes it richer, anticlock leaner. etc etc

    If you are measuring between sensor ground and air flow signal, the voltage will be something like 1.7v at tickover rising to 4.5v at full load. With engine off and ignition on, the value should be between 0.2v and 0.7v.

    However, CO trim value is different and should be 1.0v - 1.5v at tick over.

    assuming you have a 3.9 with a 3AM air flow meter.
     
  3. JJKM

    JJKM Member

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    This is really useful however, I am totally useless with this sort of stuff so may need a bit more help - some of what you said sounds like wizardry to me!
     
  4. JJKM

    JJKM Member

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    Just adjusted the MAF as advised. Was reading 150 so turned clockwise to read 300. If anything, it's made general running less smooth and the issue is still there. I should also say that it can be lumpy when cold. There is a golden period of about 2 minutes when it runs perfectly!
     
  5. Ratae

    Ratae Well-Known Member

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    Cut the plastic away from the temp sensor & you can then get some grips on the brass remains. Fiddly but it works.
    Been there, done that!
     
  6. JJKM

    JJKM Member

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    Rubbish that new parts do that. I wrote to Bearmach with an invoice for a repair
     
  7. Grrrrrr

    Grrrrrr Technician, Bodgit & Scarper Ltd Full Member

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    I won't touch Bearmach or Britpart. Not anymore.
     
  8. JJKM

    JJKM Member

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    I think you're right. Always had joy with Bearmach but crappy and incorrect destructions on one part, followed by the failure of another has left me thinking otherwise!!
     
  9. JJKM

    JJKM Member

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    So, did some testing today and got some queer results from the throttle pot. After that, I simply unplugged it. Surprisingly, it runs like a dream. Why would this happen and why would I replace it?
     
  10. sidewayspete

    sidewayspete Member

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    y i wouldnt touch britpart again. mate had a brand new rad here in Australia fail, and i had one of theirs with failure in the tank within a few mths too. pathetic.
     
  11. sidewayspete

    sidewayspete Member

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    ive been taught to never touch the AFM on these models and i found it to be true. bought a 3.9 off mechaninc who adjusted one and ruined it. maybe try running some injector cleaner thru it. i had awesome results with a product from a motorbike shop called Yamalube. they make a variety of fuel treatments. mate had a bike that was rarely used and always ran bad on first usage. tried the Yamalube and perfect ever since.
     
  12. JJKM

    JJKM Member

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    Thanks for the tips. Where in the world are you? I've changed the MAF back to its previous setting as the base setting made it run worse...
     
  13. JJKM

    JJKM Member

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    So, turns out it's the throttle potentiometer. I've unplugged it and it's running spot on. Not sure what happens if I leave it unplugged. The replacements are damn expensive though and currently, can only find Britpart replacements...
     
  14. JJKM

    JJKM Member

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    So, new throttle pot fitted and following a 100 mile drive, all seems good (touch wood). The Britpart kit is actually surprisingly good quality. The actual throttle pot is a Colvern type, not produced by them but the adaptor is theirs. Some excellent machining, great stainless fittings and strong wiring. Hope it lasts!!
    IMG_20190822_160522.jpg
     

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