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The effects of poor cam timing......misfire

Discussion in 'Engine, Drivetrain, Fuel and Exhaust' started by Arudge, Nov 12, 2017.

  1. Arudge

    Arudge Senior Member

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    Of all the issues i've had with the engine (M111) misfire has been a persistent issue. I decided to investigate what i thought was a cam chain rattle. Foolishly i didn't follow procedure removing the tensioner and upon replacement it skipped a tooth. This forced me to retime the engine, and what i found along the way was shocking.

    Firstly, thankfully, there was no damage to the valve train but i found it impossible to time the engine to the book spec. Basically crank set at 20 deg and the locating pegs in the cams. In reality the chain had stretched to such a degree that the inlet was timing at 30deg and the exhaust 35deg, right on the limit that WIS suggests is acceptable.

    It's regarded that the M111 engine is very robust and cam chains rarely fail, but they do stretch and they stretch a lot, what don't know is the ramifications of this, but misfire keeps cropping up.

    I watched a fella check the timing on an old SL500 and found one bank 17deg out to the other bank, it ran perfectly well but clearly wasn't as smooth as it should be, but it ran.

    I've now resigned myself to having to change the cam chain, which is a pig of a job whichever way you do it.

    Anybody got any thoughts about this?
     
  2. Botus

    Botus Senior Member

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    nice engineering

    10% wrong ! its fairly surprising it starts, let alone expecting it to make sensible power.

    on proper cars they make Vernier cam gears so you can get it spot on when trying to make an engine run properly
     
  3. OP
    Arudge

    Arudge Senior Member

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    We used to spend a lot of time timing cams in when i used to mess with mini based kit cars, and we used to experiment advancing cam profiles to get more top end, sacrificing the bottom end in the process. Back in the day the Piper Magnum was the cam of choice with possibly the 285 the most popular. But what i found in mine takes the biscuit, but what this fella is claiming with this SL 500..........judge for yourself.


     
  4. star

    star Forum Supporter Authorised Forum Supporter

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    We replace a chain on a 420 that was giving an intermitant misfire, the chain had stretched, misfire cured.
     
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  5. Naraic

    Naraic Moderator

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    Is this a matter of splitting the existing chain then attaching the new one to the old to pull the new one through?
     
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  6. Steve@Avantgarde

    Steve@Avantgarde Forum Supporter Authorised Forum Supporter

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    Yes. All MB's have this procedure to replace once the engine has been timed. There is always a master link in the chain.
     
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  7. Naraic

    Naraic Moderator

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    I thought it was a stupid question...but it is that simple.
     
  8. OP
    Arudge

    Arudge Senior Member

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    I was hoping somebody would chime in and say they had a good result from changing the cam chain. Cheers.
     
  9. OP
    Arudge

    Arudge Senior Member

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    Thank you.

    I'd watched several procedures for dragging a split chain around the crank but was nervous because it was drilled into me at an early age not to use split chains.

    That may have been because they didn't use to riviters on the day and hand peaned the master link, which was never going to work.

    But I didn't want to peal the whole front of the engine off to fit an endless chain either.

    Split chain it is.

    I'll order stuff in and give it a go.
     
  10. Steve@Avantgarde

    Steve@Avantgarde Forum Supporter Authorised Forum Supporter

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    There is a special tool for rejoining the chain. Once you have done it, you have the confidence to do it again and again. The tools for doing are so good you can't actually tell the master link from the normal one.
     
  11. LostKiwi

    LostKiwi Senior Member

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    If it helps Andy I had a Saab 9-3 2.0t a few years back (hateful POS but it did go like a rocket!)
    I had to lift the head and that requires the use of locking tools on the cam.
    After I refitted it all the idle was very poor. Tried everything to fix it and in the end took it to a Saab specialist to look at it. Told him what had been done and he re-timed the cam.
    The locking tool I had was defective and didn't hold the cam in the correct place and the resulting mistime caused misfires at low speed. Re-timed the cams and it was perfect.
     
  12. Naraic

    Naraic Moderator

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    My engine is a little rough at 1200rpm...could this be the timing chain?
     
  13. Botus

    Botus Senior Member

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    17 / 360 = 4.7%

    a lot less than the 10% you mentioned

    35 / 360 = 9.7%

    if you can swap out the chain as others state, I'd do that should be pretty cheap?
    What I would like to understand is if the variable valve timing being electronic has the ability to bring stuff back in line to some extent ? I would guess so or we wouldn't get a range where it comes up on the diagnostic kit as OK or not?
     
  14. OP
    Arudge

    Arudge Senior Member

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    The problem i had was that i couldn't re time the cam because the locating holes didn't line up, just like the video. But i couldn't believe the chain could have stretched that much, but it has.

    Star has indicated that he's done a cam chain change and got rid of a mis fire problem, and that's enough for me to go in and do the job.

    You know more than most just how many hours have been poured into this little car, you've helped out yourself. With all the work done on the induction system, the throttle body, the supercharger, the vacuum pump and it's associated vacuum leaks, new MAF and new plugs. The oil we've cleaned out of the inlet manifold and intercooler. We've even pulled the engine ECU apart and cleaned out both oil and the dreaded Verdi gries from the pins. Everything we did appeared to make some improvement but we never seemed to cure the mis fire, never found the root cause.

    If.....just if...the cam chain is the root cause of the mis fire due to poor cam timing, then at the next GTG i'll buy you all a beer.

    Cam chain and cam chain breaker/riveter have been ordered.

    Fingers crossed.
     
  15. OP
    Arudge

    Arudge Senior Member

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    Could be. Hopefully i'll be reporting back in a couple of weeks, see how we get on.
     
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  16. OP
    Arudge

    Arudge Senior Member

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    I can't see where i mentioned a 10% error? What i have got wrong is i stated that the one bank was 17deg out to the other, when the 17deg was the total stretch measured, 6.01 mins into the video.

    Febi chain and master link, £43 or thereabouts. Riveting tool, £20.
     
  17. OP
    Arudge

    Arudge Senior Member

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    I'll be looking a lot closer at how this works. In the timing procedure it states that the variator must be in the 'back' position, not too sure what this means yet. What i do know is that the bit you see on the outside is just an electro magnet, working just the same as a solenoid actuator. I've seen the plunger on the variator that the magnet acts on, but how that adjusts cam i'm not sure yet. The available movement appears quite small, maybe 5 deg, but if your cam chain can stretch 17 deg........

    We'll see.
     
  18. OP
    Arudge

    Arudge Senior Member

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    Whilst i've been typing all this, the lady of the house sat opposite me thinks i've been searching for new fencing. Which is what i should be doing:p:p
     
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  19. Botus

    Botus Senior Member

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    from your first post... "In reality the chain had stretched to such a degree that the inlet was timing at 30deg and the exhaust 35deg" I took the 35 degrees
     
  20. OP
    Arudge

    Arudge Senior Member

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    Arhh.....Maths never was my strongest subject, please forgive me....

    What i meant was.....

    When we time the cams we set the the crank to 20 deg ATDC (After Top Dead Centre). At this point the holes in the cam wheels should line up, mine didn't. I had to rotate the crank a further 10deg to 30deg ATDC for the inlet cam peg to fit in the hole. At this point with the inlet cam pegged, the exhaust peg should go in, it didn't, i had withdraw the inlet peg and rotate a further 5deg to 35deg ATDC to fit the exhaust cam peg. And this is, remarkably, within spec, but only just.
     

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