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W126 High idle problem

Discussion in 'Engine, Drivetrain, Fuel and Exhaust' started by steve beard, Nov 20, 2012.

  1. steve beard

    steve beard Senior Member

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    Your Mercedes:
    w126034 g reg 420
    Hi,
    Im having problems with my 420se 1990.
    I have replaced the following
    Rotor arm
    Distributor cap
    Spark plugs
    Ignition leads

    I have cleaned the idle control valve and when disconnected the idle rises.
    The car starts fine and when warm idles at 1100 rpm unless in drive.
    I cant find any air leaks.

    Any advice would be very much appreciated.
    Values for the sensors would be helpful and a diagram for the vacuum pipes.
     
  2. television

    television Always remembered RIP

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    Your Mercedes:
    2002 SL500, 216 CL500, all fully loaded
    If you pull of the egr pipe does the speed increase
     
  3. star

    star Forum Supporter Authorised Forum Supporter

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    At 1100 rpm, I would check the idle contact switch. it sounds like its not working properly.
     
  4. ianrandom

    ianrandom Senior Member

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    Your Mercedes:
    1986 420SE, 1990 420SEC, 1989 2cv6.
    Might be worth trying a different idle control valve. I just had success with my SEC by changing the OVP relay. No ABS light was on so i had ruled that out, but it turned out to be the cause.

    Rob Parker at Threepointedparts is good for parts.
     
  5. Alex Crow

    Alex Crow Senior Member

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    Your Mercedes:
    W169, W124, w202, W203, KTM 250 EXC, VW T25 camper and a Polo in a pear tree
    As above, idle contact switch first, then the throttle plate/airmass plate potentiometer.
    Beyond that, air leaks, idle valve and ECU (possibly a standalone idle ECU, but I think on this 1990 car it is all in the KE-Jetronic module in passenger footwell), although an ECU fault would be unusual.
     
  6. Gary Hanson

    Gary Hanson Senior Member

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    A Jag. Jags are my favourite.
    I've got a similar problem on a volvo 940, it's intermittent, and usually only happens after a run. I replaced the idle control valve and it made no difference.
     
  7. star

    star Forum Supporter Authorised Forum Supporter

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    Sorry love to help, but we only work on Mercedes and wouldn't like to guess. :)
     
    Last edited: Nov 21, 2012
  8. OP
    steve beard

    steve beard Senior Member

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    w126034 g reg 420
    Thank you for the replies,
    I have looked for the idle switch but could'nt see one.
    The abs light is not on and the ovp relay clicks when the ignition is turned on.
    If I disconnect the egr valve it does nothing unles I suck on the pipe then the idle comes down? Is this just due to mixture?
     
  9. star

    star Forum Supporter Authorised Forum Supporter

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    Definately got one, look for a 3 pin plug, bolted to the manifold, close to the idle control valve.
     
    Last edited: Nov 21, 2012
  10. maska

    maska Senior Member

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    I Think You will find, Most Volvos Of That Period , Use The Same Injection System !! We are All Here To Help !!
     
    Last edited: Nov 21, 2012
  11. star

    star Forum Supporter Authorised Forum Supporter

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    But on a Mercedes forum??!!! The trouble with guessing or giving the wrong information cost time and money!!!!

    I like to do niether.
     
  12. maska

    maska Senior Member

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    Thats a Bit One-Eyed !!:roll: I'm Glad I dont Use Your Services !!
     
  13. ianrandom

    ianrandom Senior Member

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    I discounted my OVP for the same reasons, but just recently found it was the cause, so wouldn't hurt to try another just in case. Mine was a very irregular high idle though, 1100 sometimes, but all different speeds when it felt like it, and sometimes the right speed but not very often!
     
  14. Gary Hanson

    Gary Hanson Senior Member

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    No don't worry, I wasn't asking for solutions. I've had this happen on various cars and know how to cure it. I'd check the TPS as well for a start, and if all else fails, I'd block the airway from the idle control valve to the inlet manifold, and manually set the idle on the throttle stop screw. Slightly more complicated on v engines, but not impossible.
     
  15. star

    star Forum Supporter Authorised Forum Supporter

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    Thats cleared that up, I thought you were asking for a solution, I know lots of manufacturers were using KE around that time but I wouldn't know specific fitments.
    The trouble with fitting a manual throtle idle adjustment is that it would be lacking in several areas, cold fast idle, increased idle when a gear is selected or when any other loading is applied.
     
    Last edited: Nov 23, 2012
  16. Alex Crow

    Alex Crow Senior Member

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    As above, never ever adjust a throttle butterfly on KE to achieve idle speed - a bad idea for many reasons.
     
  17. OP
    steve beard

    steve beard Senior Member

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    I have checked the idle switch and it is working, idles fine when cold but slowly gets higher as it gets warmer. Sits around 1100rpm all the time unless in gear.
     
  18. ianrandom

    ianrandom Senior Member

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    Have you tried a different OVP?
     
  19. television

    television Always remembered RIP

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    Yes the OVP relays made in Austria do get bad soldered joints and can be very intermittent, the Irish versions are much better,,, the only odd thing here is the engine would not go faster as it warms up with an OVP relay,,
     
  20. star

    star Forum Supporter Authorised Forum Supporter

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    Try looking for an air leak especialy around the injector pods.
     

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