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EGR, vacuum leak, rough idle m112

Discussion in 'Engine, Drivetrain, Fuel and Exhaust' started by mattuk, Nov 3, 2012.

  1. mattuk

    mattuk Senior Member

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    Hi

    Due to mud in "fluctuate revs again" thread here is fresh one

    My car c320 petrol 2001, 95k miles


    Car runs faultless apart hunting/fluctuate revs on idle and sometimes vibrations

    I checked all vacuum hoses few times - good condition
    Also cleaned throttle body, maf, sealed breather cover leak, checked everything! Car pass MOT without problems, car feels powerful and accelerate without problem, no codes on diagnostic tool , i/m readiness shows that everything works correctly, o2 sensors work in correct values, no oil leaks

    Today I was checking EGR valve.
    - disconnecting does not indicate check engine light However was pending code. All clear now
    - with correct engine temp vaccum supply was disconnected, no difference on idle, still fluctuation, no vaccum in hose.
    - engine revs at 2000- 2500 rpm, disconnecting vacuum supply does not make any difference to engine, there is vaccum in hose.
    - with engine off vaccum was applied directly to the valve. Valve holds the pressure.
    - engine on, idle - valve is hot but it's tube which goes to manifold is cold.
    - engine on, revs at 2000-2500rpm - valve is hot but tube(pipe) only slightly warm.
    - also I hit valve by screwdriver few times on idle but no changes.

    Egr pipe(tube) - how hot it has to be ?

    I would be very thankful for help
     
  2. Alex Crow

    Alex Crow Senior Member

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    Cannot answer the EGR question, a picture of exactly which parts are hot/cold could help.

    Why the vacuum and EGR focus?
    In other words, what clues point to EGR or vacuum faults?
    I never ever ever had an EGR fault on a 112 engine.

    How about the gearbox, does it still do it in 'P' and 'N'?
    Can we all be sure there are no codes? Often only top end tools like Star will show all codes on MBs...
    How much do revs fluctuate by? 200rpm? 800rpm?
    How high and how low do they go?
    Has it been tried without the polybelt on?

    The 'vibrations' you describe do sound like cylinder shutdown, which for sure throws codes - if this were the case you would have a much better place to start.
     
  3. WG M-B

    WG M-B Senior Member Authorised Forum Supporter

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    Had a couple of egr valves causing rough idle on m112 engines Alex. Although its rare it is not uncommon.
    Op need to look at live data and see if the air intake temp is raised too high at idle. This does not usually log a fault code
     
  4. OP
    mattuk

    mattuk Senior Member

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    What intake temp should be on idle? What temp will indicate that is too high? If engine is fully warm then what AIT? If this is wrong then what will cause it?

    How about egr tube?

    Thanks
     
  5. OP
    mattuk

    mattuk Senior Member

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    Vibrates only when cold, when warms is ok and revs bounce about ~100rpm

    Why egr? Because I checked everything, so egr is left, and I saw somewhere that egr pipe(returning gases back to manifold intake) should be hot but I don't know if this is true
     
  6. WG M-B

    WG M-B Senior Member Authorised Forum Supporter

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    Not sure of exact value range. What do you have???
     
  7. OP
    mattuk

    mattuk Senior Member

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    Can't remember I need to run car

    what temps do u want me to see?

    with eengine off,at 40C, 60C, 90C ?
     
  8. Alex Crow

    Alex Crow Senior Member

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    Has it been checked with the polybelt off?
    I'm wondering if it could be an alternator load problem.
     
  9. OP
    mattuk

    mattuk Senior Member

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    No it has not been checked with polybelt off. What do u mean?

    My first thoughts was alternator, I checked voltage on battery with engine off, start up, revving and remains 14V , no problems with engine starts. First owner changed alternator in MB( I have invoices) in 2007 @ 65k miles

    Is there any way to check alternator directly? What else I did was that I Ioaded all electrics on to see how car will behave and also I turned everything off, still same no difference with revs

    How. About air intake temperature? If u know can u please say what should be?
     
  10. bob13

    bob13 Senior Member

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    Sorry to butt in on this thread but I have a problem that sounds like yours, if it is a problem. As the car drives fine.

    However when in drive waiting at lights etc the engine feels like it is hunting but only very slightly. The revs are at 500rpm but the rev counter moves very slightly. When cold the revs are higher of course around 800rpm and I cant feel any hunting.

    Now that the alternator has been mentioned, I have noticed that the dashboard info display dims for a split sec every now and then, as if a large current has been draw quickly ie the fan or other large current drain.

    As I have said the car drives and runs fine, just this hunting sometimes feels as if the car will stall when starting off, but it never has.

    my car is a 2002 clk 320.

    many thanks rob.....
     
  11. OP
    mattuk

    mattuk Senior Member

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    I have exacly same problem! So now is two of us heh

    Yes you just gave me to think that my dash info display and radio dims as well but in my car it happends only when secondary air intake turns on when engine warms up or hot. While this pump is on revs are smooth and when switch off keeps fluctuate back again. Have u got that same?
     
  12. bob13

    bob13 Senior Member

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    Not 100% sure with mine but it is a strong possibility. Like i have said hasn't been a problem yet, just worried it could be the start of something that leaves me stranded !!

    When in neutral it is as steady as a rock.
     
  13. OP
    mattuk

    mattuk Senior Member

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    Hope that experts on this forum will be able to help
     
  14. Alex Crow

    Alex Crow Senior Member

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    At least try it with the belt off, and see if same or different.
    And pictures please, of the pipes you have queried.

    What about the airmass meter?
    What scanners have been used to check codes?
     
  15. OP
    mattuk

    mattuk Senior Member

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    Unplugged maf pulled codes, and revving was so poor, but on idle still hunting.

    I will upload picture later

    I use autel diagnostic tool

    I'll check belt. How to check belt when on? Try to push down by finger between pulleys how much I can push right? I don't feel confident testing car with belt off, I might get someone for help?
     
  16. Alex Crow

    Alex Crow Senior Member

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    You know, booking it in to your nearest good MB specialist might be cheaper in the long run.
     
  17. tode

    tode Senior Member

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    What voltage did you get for each of these (and at tickover) ?
    14V all the time seems strange.
     
  18. OP
    mattuk

    mattuk Senior Member

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    14v is on idle but I can't remember on startup. I check it again and will let u know


    Alex - how to check if revs still hunt while I got belt off? I mean best visible fluctuating is when engine hot, without belt water pump won't do it's job - I don't want overheat engine during this test.
     
  19. Alex Crow

    Alex Crow Senior Member

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    If run at idle for no more than 30 seconds you can do no harm - this should be long enough, right?
     
  20. OP
    mattuk

    mattuk Senior Member

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    Well you are right, i'll try to get my mate to help me, maybe I will be able to do it

    To check alternator - should I pay attention on rusty wires, ground? Could corroded,dirty ground be a culprit?

    There is main ground from battery to chassis but is there any other grounds which could cause it(if that is it)

    During the week I can't do anything because is dark when I'm back home so I could run it over the weekend.
     

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